Halogen Lamp (Lamp Heater) for Lamp Heating

The light source (filament) reaches 4532ºF (2500ºC) ~ 5432°F (3000ºC). Clean heating is possible without contact if the light can be focused appropriately to obtain maximum temperatures up to 2372ºF (1300ºC) to 2732ºF (1500°C).

Temperature can be controlled accurately from zero to MAX using voltage control.

Lamp heater for halogen spot heater HSH-160 100v-2000w and 100v-2500w are available.The filament is accurately positioned with respect to the aluminum base.	Spot heaters using lamp heaters
	Line heaters using lamp heaters
Most powerful halogen lamp heater 200v-5kw Luminescent length 250 mm Color temperature 2950 K
Various halogen lamps

Halogen lamps used as a heater are called “Lamp heaters”. The halogen lamp is the same lamp that is used for lighting.

Halogen lamps use a tungsten filament that is heated to a high temperature by passing current, and the light (electromagnetic waves in the near-infrared to visible region) that is emitted from the filament is used.

The conversion efficiency to light in the visible range is extremely inefficient at 10% or less, but the efficiency conversion to electromagnetic waves including infrared region is around 90%, which is a very efficient method of heating.

However, while considering the overall thermal efficiency, in addition to the focusing method for the light emitted from the lamp on the object to be heated, percentage of light absorbed (absorption rate) by the object to be heated is important.

These values are approximately as follows.

The efficiency with which the electric power (energy) supplied to the halogen lamp is transferred to the heating target is about 40%.

Since the object heated absorbs heat equivalent only to the absorption rate, the overall thermal efficiency is about 30% even for stainless steel oxidized surfaces and only about 10% for shiny surfaces of stainless steel.

The thermal efficiency is good for this open system. Furthermore, since halogen lamps switch on immediately, power supply can be switched on when required, and is a very efficient heating method because of this reason.

For heating with a concave mirror, lamp shape is different based on “Point heating” and “Wire heating”. Filaments as close as possible to the point heating is preferred, and short filaments with an aspect ratio of 1 to 2 packed in spherical or short cylindrical shaped quartz tubes are used.

For wire heating, a long and narrow filament is enclosed in an elongated quartz tube.

High power is difficult to obtain with point heating and a maximum of 2 kW can be achieved. 10 kW or more can be achieved by using filaments with a length of 1 m to 2 m in case of wire heating. However, there is a limit for the lamp current capacity, and since the limit is approximately 25 A even if a thick wire is used, a rated voltage of 400 V is required for lamps that exceed 5 kW significantly.

Many of the lamps used for point heating have no restriction on the lighting direction, but horizontal lighting is basically used for long lamps used in wire heating Products with special specifications are available for vertical and diagonal lighting. However, please avoid diagonal and vertical lighting as they tend to cause various problems even with suitable measures.

Various characteristics of the halogen lamp changes when the voltage is changed. The major change is to the service life, the service life increases by 3 times when the voltage is decreased by 10%.

The lamp service life is mainly determined by the temperature of the filament (color temperature). The service life is approximately 1000 hours at 3000K, and 200 to 300 hours at 3200K. The calculated service life will be very high when the temperature is significantly lower than 3000 K, but even if the calculated service life of the filament is long, the service life of the lamp may not be as calculated due to various factors. As a thumb rule, the values are 5000 hours at 2600K and 20,000 hours at 2200K.

The filament temperature is not the only factor limiting the lamp service life. If the temperature of the seal is high, this will be the factor that determines the lamp life (see the figure below). When the lamp service life is 2000 hours and the seal temperature is 662ºF (350ºC) or less, then the heat resistance of the seal will not be a limiting factor for the service life. When the temperature is 662ºF (350ºC) or more, the seal will rupture within 2000 hours and cannot be used.

When the halogen lamp (heater) is turned off, the electrical resistance is nearly 1/10 of the value when the lamp is lit. Therefore, a large inrush current flows in a very short time at the instant of the lamp is lit. Since the inrush current has a negative affect on the lamp life, the power supply voltage slows down if the operation involves frequent blinking operations over a long period of time. Slow up time is approximately 1 second. ⇒ Inrush current data

Following are the standard lamps kept in stock. These lamps can be brought off the shelf.

Halogen lamp of standard stock has lead wires on both sides and is used mainly for line and panel heaters
*	Model name	Remarks
(1)	QIR100v-160w/GL100/GD10/CL56/TC2750/LF2000
(2)	QIR110v-500w/GL106/GD10/CL60/TC2900/LF1500
(3)	QIR100v-1kw/GL324/GD10/CL285/TC2550/LF5000
(4)	QIR220v-500w/GL106/GD10/CL68/TC2900/LF1500
(5)	QIR200v-850w/GL106/GD10/CL68/TC3050/LF800
(6)	QIR200v-2kw/GL340/GD10/CL255/TC2700/LF5000
(7)	QIR200v-3kw/GL340/GD10/CL255/TC2950/LF1000
(8)	QIR200v-5kw/GL345/GD18/CL255/TC2950/LF1000
QIR: Cylindrical halogen lamp heater GL: Length of the lamp glass portion GD: Outer diameter of the glass tube CL: Emission length TC: Color temperature of lamp LF: Service life of life (approximate average)

Halogen lamp of standard stock is with one side lead wire type and is mainly for spot heaters
*	Model name	Remarks
(8)	JC10v-80w/GL32/GD8/LCL18/TC2900/LF1200	For HSH-30, and HSH-35
(9)	JC12v-110w/GL32/GD8/LCL18/TC3050/LF800
(10)	JD24v-75w/GL30/GD8/LCL19/TC3000/LF800
(11)	JD24v-150w/GL43/GD10/LCL26/TC3100/LF800	For HSH-60
(12)	JD24v-300w/GL50/GD18/LCL32/TC3000/LF1000	For HSH-60
(13)	JD36v-450w/GL50/GD18/LCL33/TC3200/LF200	For HSH-60
(14)	JCD100v-500w/GL43/GD22/LCL28/TC2900/LF1000	For HSH-120
(15)	JCD100v-1kw/GL67/GD32/LCL48/TC3050/LF800	For HSH-120
(16)	JCS200v-1kw/GL92/GD23/LCL54/TC3000/LF1000	For HSH-120
(17)	JD100v-2kw/GL145/GD43/LCL92/TC3100/LF300	For HSH-120
(18)	JD100v-2.5kw/GL145/GD43/LCL92/TC3150/LF300
JC: Single-side lead halogen lamp (Vertical single coil) JD: Single-side lead halogen lamp (Vertical double coil) JCD: One side lead halogen lamp (Horizontal double coil) JCS: One side lead halogen lamp (C-13D coil) GL: Glass portion length GD: Outer diameter of glass tube LCL: Distance of center optical axis (from lower end of glass) TC: Color temperature of lamp LF: Life of lamp (approximate average value)

Special order halogen lamps

- Products can be manufactured in small batches -

Fintech can also manufacture halogen lamp heaters for special orders in small lots.

Basic fee: 50,000 yen (including design, materials, manufacturing line, and machine adjustment cost) Manufacturing cost: Depends on the product type and quantity, but is approximately 3,000 yen each for a small lamp.

Delivery date: Approximately 30 days after receipt of the order For example, if 10 special order products is to be manufactured, cost will be 50,000 yen + 3,000 yen x 10 Nos = 80,000 yen, so the unit price will be 8,000 yen (including shipping fees, and taxes).

Special order quartz lamps

- Products can be manufactured in small batches -

We can also manufacture quartz products for special orders in small lots.

Price: We will provide a quotation based on your order.

Delivery date: Approximately 2 weeks after receipt of the order. However, please discuss with us for products that are difficult to process.

[Reference]

Physics of Light Heaters Optics of Light Heaters Condenser Type Heater Line Condenser Type Power Supply, Controller

- Knowledge of halogen lamp additional information -

We will explain the details of halogen lamps including manufacturing technology.

Overview of Halogen Lamps
Vacuum and Gas Filled Bulbs
Types of Inert Gases
High Pressure Method to Fill the Electric Bulb with Gas
Information on Halogen Gas
Information on Sealing Halogen Lamps
Problems with Sealing Using Molybdenum Foil
Measures to Increase the Heat Resistance of the Sealing Parts
Halogen Lamp Using Glass Other Than Quartz
Information on Filament Coils
Design of Filament Coil
Manufacturing Method of Filament Coils
Manufacturing Method of Filament with Double Coil
Heat Treatment of Tungsten
Surface Treatment of Tungsten Coil
Information on Quartz Bulb of Halogen Lamps
Surface Cleaning Treatment of Quartz Glass
Quartz Glass Processing
Removal of Distortion after Processing Quartz Glass
Devitrification Behavior of Quartz Glass
Halogen-free Quartz Tungsten Heater
AC, DC, and Low Voltage Lighting
Chemical Reaction and Control Method Inside the Halogen Lamps
Information on Power Supply and Controller for Halogen Lamps

Overview of Halogen Lamps

Halogen lamp has trace amounts (about 0.1%) of halogen element (mainly bromine) in sealed gas that contains inert gas as the main component.

This causes a chemical cycle called halogen cycle. First the tungsten atoms actively evaporate from the incandescent filaments (Material: Tungsten) at extremely high temperatures when the lamp is lit and then combine with halogen at relatively low temperature during lighting. → If there is no halogen, the evaporated tungsten will adhere to the inner surface of the glass bulb and brightness decreases because of blackening.

“Tungsten-Halide” is a substance that evaporates easily, and can maintain a gaseous state above 482ºF (250ºC) The halogen lamp is designed so that the inner wall temperature of the glass bulb is 482ºF (250ºC) or higher at all locations.

For this reason, “Tungsten-Halide” in the gaseous state floats inside the glass tube without adhering to the glass bulb and decomposes thermally when the gas comes close to the filament due to the high filament temperature and is converted back to tungsten atoms and halogen.

The separated halogen is reused for the reaction mentioned above (Halogen cycle) and as the tungsten atom is released very near or close to the filament, tungsten vapor near the filament reaches saturation, and evaporation appears to have disappeared at the macroscopic level.

However, actually evaporation and redeposition occur frequently, and filament surface becomes uneven over time and finally the filament may get cut. For these reasons, service life of the bulb cannot be expected to increase due to the halogen cycle itself, but rather the tendency of deterioration in the service life due to the corrosive action of halogen is a problem.

However, prevention of vaporized tungsten atoms from adhering (blackening) to the glass bulb because of the halogen cycle is a significant advantage. Since there is no blackening, the lamp does not become dim, and the glass bulb can be reduced to the heat resistance limit, which is 1/50 the size of a normal electric bulb. If the the glass bulb is made smaller than a normal electric bulb, there is dense adhesion of evaporating tungsten and the bulb becomes black quickly. The bulb size has to be increased to reduce this effect.

The small size of the halogen lamp is a big advantage, but since the small glass bulb withstands the high pressure of filled gas, the halogen lamps are designed such that they can withstand several atmospheres up to 20 atm (293.91 psi). Normal electric bulbs are designed such that the pressure is around 1 atm (14.696 psi) during lighting. Therefore, the pressure difference is more than 10 times.

Filament evaporation can be suppressed by sealing the enclosed gas with high pressure and life can be increased by 2 times or more compared to a normal electric bulb. Also, this reduction in the bulb volume enables the economic use of high performance inert gases (Krypton and xenon) that are expensive, and there is significant improvement in characteristics.

The glass bulb is coated with infrared reflective film to try and increase the visible light for improving luminous efficiency. The infrared reflective film reflects the infrared rays that is not required for illumination to the filament, and this reduces the heat losses and improves luminous efficiency.

This method is effective because the bulb is small. Effectiveness cannot be obtained unless large bulbs are manufactured with a very high precision. However, an increase of 7% to 12% in luminous efficiency can be expected with this method. Luminous efficiency cannot be increased by several 10 of times or even doubled.

In addition, halogen lamps may be advertised as “Service life improved by more than three times with double the brightness”, but to be exact this should be “Service life improved by more than three times with the same brightness, and brightness doubled with the same service life”. The relationship is that service life will reduce for both halogen and normal electric bulbs if they are made bright. Though halogen lamps are an groundbreaking innovative technology of light bulbs, but the breakthrough is small with luminous flux stability, and improvements in brightness and service life cannot be considered as groundbreaking.

The luminous flux stability of halogen lamps (the rate at which the brightness attenuates while the lamp is in use) reduces by about 5% at the end of the service life, which is about 1/10 that of other light sources (such as fluorescent lamp, and HID).

Halogen lamps are extremely inefficient to obtain visible light, and halogen lamps cannot compete with fluorescent lamps, HID lamps, and LEDs no matter how much they are improved. The luminous efficiency of halogen lamps is approximately 20 Lm/w, considering a service life of approximately 2000 hours and power of 50 W ~ 100 W, on the other hand luminous efficiency of fluorescent lamp and HID lamp is 80 Lm/W and 80 ~ 100 Lm/w respectively and cannot be compared.

The difference reduces if flux stability is considered, but in reality if the life of halogen lamp is set to the life of fluorescent lamp which is 6000 hours, the luminous efficiency will be a maximum of approximately 16 Lm/w only.

At present, luminous efficiency of even commercial LED products has reached (mass produced products 60 to 100 Lm/w) the level of fluorescent lamps. In the near future, light output of each lamp is expected to reach 500 Lm or more with a luminous efficiency of 150 Lm/w or more, and since the service life is at the level of several thousand hours, a major revolution is taking place in light sources for illumination.

* Lm/w (lumens per watt) is the unit of luminous efficiency. The unit represents how many lumens of light is emitted per 1 W. Lm (lumen) is the energy of light multiplied by the sensitivity of the human eye to the wavelength of that light. Since the maximum sensitivity of human eyes is approximately 0.5 µm (green), luminous efficiency can be improved by emitting light of only this wavelength. However, white light is a must for illumination and white light must be distributed in the range of approximately 0.4 ~ 0.8 µm with a good balance (not necessarily continuous spectrum). This limits the luminous efficiency. The typical values described above for the lamps are based on white light.

Based on the above, using halogen lamps to obtain visible light in the future is doubtful. Halogen lamps may coexist if only fluorescent lamp and HID are considered, but if the improvement of efficiency, increase in power output and cost reduction of LED continues, then LED may replace fluorescent lamps and HID, and these lamps may not be used after 10 to 20 years. Specifically, there is a need to move in this direction with the increase in demand for energy conservation in recent years (As a result CO2 emission reduction).

However, for high temperature spot heating there is no other technology that can provide clean heating like halogen lamps, and application of halogen lamps for industrial heating is expected to increase in the future. ⇒ Laser or electromagnetic induction heating occupies a part of the high temperature clean heating field, but still halogen lamps are considered to be superior for most applications based on the overall judgment of safety, ease, effect, and cost.

In addition, halogen lamps are optimum light sources for fields requiring continuous spectrum ranging from the visible light to infrared region. Also, light of halogen lamp gives a feeling similar to the direct sunlight including the warmth effect to humans, and hence halogen lamp is considered to be a light source that can be used easily.

Vacuum Bulbs and Gas Filled Bulbs

The initial bulbs were vacuum bulbs that had nothing inside the glass bulb. Since the vacuum cannot carry away the filament heat, light bulbs can be manufactured with less heat loss based on this point. Vacuum does not have the ability to prevent evaporation of the filament and it evaporates steadily, so the filament gets used up quickly if heated to high temperatures and the wire gets cut after being used for a certain duration.

Therefore, gases that do not react with tungsten such as nitrogen or inert gases (Gases that do not reacts with anything) such as argon were used to fill the bulb. Higher sealing pressure is advantageous, but high pressure cannot be used for large glass bulbs.(Problems of withstanding pressure and safety) Therefore, ordinary bulbs are operated at around 1 atm (14.696 psi) that is normal pressure during lighting. In this case, the pressure at normal temperature reduces.

Carbon was initially used for the filament Though carbon has a very high melting point of 6332ºF (3500ºC), evaporation (sublimation) is severe and cannot be used at very high temperatures. Higher the filament temperature, higher will be the luminous efficiency (maximum efficiency is at around 9932ºF (5500ºC)). Therefore, we searched for materials that can be used at high temperature, first we tried tantalum (melting point 5414ºF (2990ºC)), and then settled for tungsten (melting point 4352ºF (3400ºC)). Tantalum carbide (melting point 7205ºF (3985ºC)) and hafnium carbide (melting point: 7232ºF (4000ºC)) are examples of materials with higher melting points, but processing is difficult because these are nonmetals. This material exists as a special light source with a disc shaped light emitter subject to high frequency induction heating and was probably not used for ordinary lighting and is not likely to be used in the future too.

Types of Inert Gases

When an inert gas is filled in the glass bulb, evaporation is suppressed by the pressure and also evaporated tungsten cannot travel straight because of the collision with inert gas molecules, and the surrounding tungsten vapor pressure rises, and evaporation is suppressed due to this aspect too, prolonging the service life.

The above effect increases for heavy gases; also convection is less likely to occur with heavy gases, and the gas carries away very less heat from the filament decreasing the heat loss. Lamp with higher performance can be manufactured with heavy gas. Starting from light inert gases, the order is helium → neon → argon → krypton → xenon → radon. Argon followed by krypton and xenon (Xe) can be used to manufacture high performance electric bulbs. The difference in luminous efficiency increases by 5 to 10%.

This means that xenon (since radon is a short half-life radioactive gas) should be used for all electric bulbs, but the cost increases by 100 to 1000 times and the gas cannot be used economically.

Based on this point, volume required for halogen lamps is less (1 to 2 cc for a 100 W lamp) and only 10 cc is required even during pressurization, even expensive gases that cost of several hundred to thousand yen per liter can be used economically.

However, electrical insulation may be insufficient with only inert gas. Especially in the filament surrounding, the inert gas also gets hot and can easily pass electricity. On exceeding the limit, an arc discharge occurs at the shortest distance with the filament surface, and will be cut instantaneously.

To prevent this, nitrogen gas is often mixed. Usually a few percent of nitrogen is mixed if the filament is short, if the voltage is 100 V for a filament length of 10 mm, then the proportion of nitrogen may be increased or even 100% nitrogen may be used. Since nitrogen is not an inert gas and hardly reacts with tungsten, this gas can be used as the sealed gas in lamps and considered as an inert gas.

High Pressure Method to Fill the Electric Bulb with Gas

Increasing the pressure of the filled gas is the best way to increase the service life of halogen lamps. High performance of halogen lamps can be said to be dependent almost on the effect of the high pressure sealed gas. Since density of gas molecules increases by pressurization, movement of evaporated tungsten becomes difficult increasing the vapor pressure around the filament, and evaporation is suppressed as saturation is reached. The gas pressure has the effect of suppressing filament evaporation. High pressure is considered to contribute to longer life by suppressing the formation of bubbles due to impurities present inside the filament.(The impurity referred to here is actually potassium that is added intentionally to tungsten)

Increasing the lamp sealing pressure by 2 to 3 times, approximately doubles the life expectancy. Increasing the sealing pressure is supposed to increase the heat loss due to heat conduction, but due to certain a property, there is almost no change in thermal conductivity in the range from approximately 0.1 to several atmospheres, and increase in heat loss caused by pressurization can be almost ignored.

A glass bulb with an exhaust pipe (thin glass tube) is the general method used to seal gas at high pressure in a halogen lamp, the gas is filled and then cooled with liquid nitrogen. The filled gas liquefies with a significant decrease in volume and the internal pressure drops. Once the internal pressure is lower than the atmospheric pressure, a part of the exhaust pipe is softened by heating to a high temperature, the pipe shrinks and closes, and then gets separated. This completes high pressure filling of the bulb. This method can be used to fill with several tens of atmospheres pressure.

Also, with this high pressure gas filling method using liquid nitrogen, the process of emptying the lamp and filling the gas can be performed efficiently with almost no loss of gas. If liquid nitrogen is not used, a large volume of gas remains in the equipment. The cost increases significantly for xenon gas.

Schematic diagram of the “Exhaust-gas filling” process.

Information on Halogen Gas

Basically filled with minimum amount of gas that will not cause blackening (phenomenon in which the evaporated tungsten is not returned by the halogen cycle and the glass bulb turns black). Although this appears to be contradictory, halogen cycle is a necessary evil for halogen lamps, but reducing halogen concentration as much as possible and making the halogen cycle as moderate as possible increases the service life and offers stability. The minimum required halogen concentration varies depending on parameters such lamp type and color temperature, but generally is about 0.1% by molar ratio to the inert gas.

The choice of halogen type used is also important to have a moderate halogen cycle. There are four types of halogen elements, fluorine → chlorine → bromine → iodine, given in the order of moderate chemical activity. Although iodine is excellent in this respect, the manufacturing method for filling iodine into the lamp is difficult because iodine exists as a solid at room temperature. When iodine is filled by mixing with an inert gas, it is generally filled in the form of hydrogen iodide.

However, hydrogen iodide is difficult to handle because of corrosion and toxicity problems, and the presence of hydrogen may lead to excessive suppression of the halogen cycle in the case of iodine. Injecting liquid and solid iodine compounds directly into the lamp and sealing them is more common.

Since iodine is too moderate, sometimes halogen cycle cannot catch up in halogen lamps due to severe evaporation (lamps with high efficiency, high color temperature and short life are designed for automobiles.) causing blackening of the glass bulbs. In other words, iodine is suitable for lamps designed for long service life, but not suitable for high efficiency lamps designed with short service life.

For the above reasons, at present bromine is used as halogen in most halogen lamps. Bromine is stable when mixed with an inert gas in the form of methylene bromide and is easy to handle in terms of production technology. In the case of methylene bromide, the same number of moles of hydrogen as bromine enter simultaneously and this is convenient for bromine based halogen lamps.

This is because if the same number of moles of hydrogen is present, bromine in the halogen lamp mainly exists in the form of hydrogen bromide (HBr), a relatively stable substance compared to bromine gas.

Hydrogen bromide (HBr) thermally dissociates at high temperatures near the filament and the independent bromine that is generated gives rise to the halogen cycle. The dissociated bromine again combines with hydrogen at low temperatures to form stable hydrogen bromide. The advantage of this is control of the negative effect where the halogen cycle shaves off the relatively low temperature parts such as the the filament edges reducing the life span.

Also, filling of methylene bromide introduces carbon that is 1/2 the mole of bromine in the lamp. Carbon has been described to work as a getter for residual oxygen remaining in the lamp, but in reality the getter action cannot be expected. Although the presence of carbon may cause problems such as cuts due to impact because of embrittlement of the filament part that is at a low temperature, but it is still acceptable because carbon is relatively harmless substance.

Bromine in forms other than methylene bromide, a compound with carbon (organic compound) is difficult to handle in terms of corrosiveness, stability and toxicity and its usage is avoided. However, there are examples where compounds (BBr3) with boron have been used though it is a material that is difficult to handle. Boron, aluminum, and silicon have been observed to be effective as a powerful getter (absorbent) of the residual oxygen inside the lamp. Residual oxygen is involved in the water cycle as will be described later and the presence of a large amount significantly reduces the lamp service life.

The mole ratio of hydrogen is sometimes increased 2 to 3 times in halogen lamps that are designed for long life with the objective to further control the halogen cycle. Ethylene bromide is used for such cases. Permeation by diffusion of hydrogen increases for quartz particularly at high temperatures, and the bulb temperature has to be set to a low value to retain hydrogen for a longer duration.

Information on Sealing Halogen Lamps

The characteristic “Ultra-low coefficient of thermal expansion” is accompanied by the disadvantage that sealing is difficult. In other words, a halogen lamp requires a highly airtight structure so that the filled gas does not leak, and at the same time, an electric circuit has to be prepared using metal from the outside to the inside of the quartz bulb for energizing the filament. Preventing the gas from leaking through the gaps between the metal and quartz bulb is difficult. Gas leaks if the rate of thermal expansion does not match with normal insertion.

Achieving air tightness is relatively easy in normal electric bulbs if the insertion is done by matching the coefficient of thermal expansion of the glass and metal used to conduct electricity. However, the rate of thermal expansion is quite different for halogen lamps and achieving airtightness just by a simple insertion is not possible. Therefore, the method used in halogen lamps is to prepare extremely thin foils from molybdenum that has a high melting point and insert into the quartz glass tube, the quartz glass tube is then softened by heating from the outside (approximately 3632°F (2000ºC)) and press crimped to maintain the airtightness.(Pinch seal)

This method is used because of the coefficient of thermal expansion of quartz glass and metal cannot be matched, the metal part is made thinner to induce flexibility so that the thermal expansion of metal matches the quartz glass. For this reason, stress (compressive strain) in the quartz glass increases considerably. Stress increases with the increase in thickness of the molybdenum foil. Very high stresses causes the quartz glass to crack or peel off leading to gas leakage. For this reason, a thinner foil is safer. However, the current-carrying capacity reduces in the case of thin foils, and the thickness of the molybdenum is decided based on the current-carrying capacity value. That is thickness of 25 to 30 µm mentioned above.

In lamps carrying high currents, methods other than the sealing method using molybdenum foils (foil seal) may also used. This method attempts to overcome the difference in coefficient of thermal expansion by joining glasses that have slightly different thermal expansion (step splicing) in multiple steps. This method is used for lamps with several tens of Amps or more, but is hardly used in halogen lamps because of the high costs.

Supplementary explanation

To secure airtightness of the molybdenum foil used for sealing, the cross section is shaped like a knife edge with etching. Tungsten or molybdenum rod is welded to this foil to pass the current. The welding is difficult as metals with high melting point have to be welded. Even if the welding is successful, there is the problem of embrittlement. Sometimes directly welded since the material is the same as the molybdenum rod, but if cost is not a constraint, then problems can be reduced by welding with platinum (Pt) sandwiched in between. The platinum melts during application of heat for pinch sealing, electrical resistance is stabilized by spreading the molten platinum around the welded part and oxidation due to the high temperature is suppressed. However, platinum clad molybdenum foil (1 to 2 mm width) sandwich is used for welding as an intermediate solution because of the exceedingly high cost.

“Overlap resistance Welding” method is used for welding. Welding machines are of two types condenser and inverter, direct welding is also possible, but sometimes the parts are welded after spraying with nitrogen or by applying alcohol to prevent oxidation of the welded parts. Alcohol disassociates due to heat during welding releasing hydrogen, this has a deoxidation action and the parts can be welded by preventing oxidation.

Problems to Seal Using Molybdenum Foil

The molybdenum foil used for sealing gradually oxidizes and expands on exposure to high temperatures of about 662°F (350ºC) or more, and ends up limiting the lamp life by cracking the quartz glass or by burn out of the foil. Temperature has to be maintained below 662°F (350ºC) to use the molybdenum foil (To be precise, the part where the molybdenum foil comes in contact with the outside air) in halogen lamps. If lamps are attached to equipment with poor ventilation, this part becomes very hot even in lamps with a normal design reducing service life.

Sometimes halogen lamps have been designed to withstand temperatures in the danger zone for this part. The temperature of this part is determined based on the heat received from the filament, heat generated by the current flowing in the molybdenum foil, and quality of radiation conditions. Lamps designed with extremely short distance to the filament and lamps with a large current value will cause the temperature of the part to rise excessively. The last condition of heat radiation, is mainly a problem at the user end including the equipment manufacturer.

The current capacity of molybdenum foil cannot be stated unconditionally, but is around 10 A per 4 mm width. Also, molybdenum foil of 4 mm width is widely used in halogen lamps. Halogen lamps with a rated current of 10 A or less rarely cause overheating problems. The heat generated in this part increases in proportion to the square of the current or more (Heat generation increases in proportion to RI^2, that is, increases in proportion to the square of the current, but since the resistance value R also increases as the temperature rises, heat generated will be greater than the square of the current).

This means that a 20 A lamp will generate 16 times more heat than the 5 A lamp for the same molybdenum foil. Therefore, lamps with rated current above10 A tend to cause problems due to the overheating of the molybdenum foil. Wider molybdenum foils should be used for lamps with large current. There are few problems if this can be done satisfactorily, but in many cases using a molybdenum foil of sufficient width is not possible due to restrictions such as lamp size etc. In such cases, whether short life is okay or measures such as cooling is required has to be determined.

In the case of lamps of high current, a wide molybdenum foil has to be used, and the number external lead rods have to be increased based on the current (those heading to the outside of the lamp, mainly molybdenum rods). If a thick rod is used, the current density of the molybdenum foil near the welded part increases and the amount of heat generated increases.

Therefore, a rod that conducts current at a rate of 5 A for each rod is preferred. However, only one current conducting rod may be sufficient up to around 10 A. 5 external lead rods are used in the 200v-5kw lamp of Fintech line heater. → Refer to the photo at the top of this page.

Properties of tungsten and molybdenum → Physical property data of various metals

Measures to Increase the Heat Resistance of the Sealing Parts

We are conducting trials to improve the heat resistance (anti-oxidation effectiveness) of the sealing parts. One of the methods is to plate the molybdenum foil and connect an external lead pin (molybdenum rod) with non-oxidizing metals such as platinum and gold. In most cases, the plating is spread by melting with the heat (approximately 3632°F (2000ºC)) used for pinch seal. Platinum clad rod may be used instead of the molybdenum rod. However, the effect of these antioxidants are limited.

Another method is to apply and fill low melting point glass in the gap between the outer lead pin and quartz glass. While the light is on, the low melting point glass liquefies due to high temperature and fills the gap preventing air from moving towards the molybdenum foil. This is also effective, but is limited.

There are various methods to improve the heat resistance of the sealing part, but there is no perfect anti-oxidation method, and the best method is not allow the temperature of the sealing part to rise above 662°F (350ºC). The recommended value of 662°F (350ºC) or less is for the halogen lamp designed with a service life of 2000 hours. To design lamps with longer service life, recommended values of 572°F (300ºC) or less and 482°F (250ºC) or less is used.

To improve the heat dissipation and reduce the temperature of the sealing part, methods such as heat sinks and cooling by blowing air are extremely effective. HSH 100v-2kw has a copper plate of t 0.5 mm that dissipates heat from the lamp sealing part to the aluminum base.

Also, we have devised the method of filling the aluminum base with magnesia powder to allow heat from the lamp sealing part to escape to the aluminum base for HSH type that has a large capacity. The cooling effect of the sealing part can be confirmed because magnesia has excellent electrical insulation and thermal conductivity is also relatively good. →Data of temperature drop due to the filling of radiation powder.

* Magnesia = magnesium oxide MgO, melting point 5072ºF (2800ºC) and is relatively safe (used as a laxative in the medical field)

Halogen Lamp Using Glass Other Than Quartz

Quartz glass bulbs are not the only bulb materials for halogen lamps. Halogen lamps using normal sealing method without foils made from glass (aluminosilicate glass and a type of borosilicate glass whose expansion coefficient matches the molybdenum) that can withstand high temperatures even though not to extent of quartz glass are also available. These are mass-produced varieties that are adopted to reduce costs. These halogen lamps cannot be used for higher output and is not suitable for small volume production.

amps and lamp heaters that use crystallized glass have also been manufactured. Crystallized glass has thermal shock resistance comparable to quartz glass. Crystallized glass is no longer used due to the fall in prices of quartz glass.

Translucent alumina tube and sapphire glass (single crystal alumina) has the highest heat resistance. These are rarely adopted because of high cost. The coefficient of thermal expansion is large, and even the heat resistance is high, the glass may break due thermal shock (sudden change of temperature).

Information on Filament Coils

Tungsten (element symbol W) is used as the filament. Normally not be used as a single wire and is usually coiled. One of the reasons for this is to maintain the value (100 Ω for 100 V and 100 W, 40 Ω for 200 V and 1000 W) in an easy to use range of voltage-power, the resistance value must be adjusted to the requirement of an electric bulb filament and the length required may be 1 m when calculated. The reason for this is the wire is too long and is coiled to shorten the length. The length of a normal single coil is about 1/10, and much more smaller for a double coil (Double coiled coil).

Filament heat loss (electric energy lost in convection, conduction, but not in radiation) is largely dependent on the filament length, heat loss is significantly reduced by coiling and double coiling, and this leads to an improvement in light emission efficiency.

Another advantage of coil filaments is that the effect of thermal expansion can be absorbed due to its flexibility. If the wire is straight at normal temperature, on lighting, the wire will bend considerably due to thermal expansion.

If tungsten wire diameter is d and coil diameter is MD, MD/d ≒ 3 to 5 is appropriate.Deformation occurs easily with MD/d < 2 and strength decreases with MD/d > 8. If the pitch of the coil winding is P,P/d ≒ 1.5is appropriate.P/d < 1.2 There is a risk of short circuit between the pitches with the above value.P/d > 1.8The heat loss is high and it is disadvantageous for luminous efficiency.

Further, coiling forms a cavity the inside the coil and the light coming through the gaps between the pitches becomes a type of cavity radiation and is close to blackbody radiation. Although this is disadvantageous for lighting but is somewhat useful for heat utilization.(Average brightness rises) This effect is more pronounced for double coils than single coils. Moreover, the thick and short double coil is suitable as a filament of the mirror condensing lamp. (point condensing)

Radiation characteristics of tungsten (spectral emissivity) is relatively high in the visible light region and emissivity tends to decrease as the wavelength becomes longer (this applies for most metals). Therefore, the luminous efficiency is considerably higher than a black body for the same temperature (100% emissivity in all wavelength ranges). This is one of the reasons why tungsten is suitable as a lighting filament material. Even at the same temperature, the carbon filament is close to a black body therefore luminous efficiency is considerably lower.

The advantage is slightly lost by winding the coil as described above, but almost without exception coil filaments are used since other advantages are much larger.

The electrical resistance temperature coefficient of tungsten is quite high. This is also a general characteristic of pure metals and is not to the extent that tungsten can be considered as special.

The change in resistivity with respect to the temperature of tungsten can be approximated with the following formula.

Tungsten resistivity ρ = 1.77 (T/1000)2 + 26.52 (T/1000) - 3.44 [×10-8 Om]

However, T is absolute temperature [K]

As can be understood from the calculations, the resistivity is comparatively high at the filament temperature (2500°K ~ 3200°K) during lighting of the bulb, and the resistivity is not more than 1/10 at room temperature. In other words, the resistance value of a halogen lamp at room temperature is extremely small.

Therefore, a large current close to 10 times flows instantaneously (about 1/10 second) at the moment when current is passed through a lamp at room temperature. (Inrush current = rush current) This instantaneous high current (instantaneous high power consumption) helps temperature of the halogen lamp filament to rise and has the advantage that the lamp lights up instantaneously, but there are many problems such as service life of the switch and lamp is reduced.

⇒ Measured value of inrush current

To mitigate this effect, power supply with soft start may be used. The power supply does not apply 100% voltage immediately by turning ON the power switch, voltage gradually increases from zero to nearly 100% in approximately 1 to 3 seconds and rush current can be suppressed.

Ideally, using a constant current type power supply can be said to be good as a measure for rush current. However, such methods are not commonly used to supply power.

Design of Filament Coil

(1) Check the required specification (voltage, power, and color temperature).

(2) Depending on the required specification, decide whether to use a single coil or double coil, or short or long filament coil.

(3) Decide the color temperature of the filament coil depending on the required life and application.

(4) Decide the enclosure (glass tube of lamp) size. Low temperature parts 482ºF (250ºC) or above and high temperature parts 1472ºF (800ºC) or below

(5) The pressure of the sealed gas is determined by taking the required service life and application into account.

(6) Determine the method and number of intermediate supports for the coil

(7) Determine the length of the coil lead wire. Also determine whether a sub coil is required to be used.

The coil is designed using a software once the above parameters are determined

Design software for tungsten coil filament →PCO

Explanation of technical terms

MG: Units of tungsten wire thickness in mg/200 mm → Mass of 7.87 in (200 mm) length expressed in grams

d: diameter of tungsten wire ∅ in (mm) Relationship with tungsten wire thickness d = (MG/3020)^0.5

MD: Mandrel winding diameter ∅ in (mm) In the case of double coil, primary winding diameter MD1 secondary winding diameter MD2

TT: Number of turns. In the case of double coil, number of primary turns TT1, and number of secondary turns TT2

In the case of double coil, the primary pitch % PP1 = (P1/d) x 100

Secondary pitch % PP2 = (P2/(MD1 + 2d)) x 100

PP: Pitch percent PP = (P/d) x 100 PP = 100% indicates closely wound

Efficiency: Luminous efficiency Lm/w, closely related to color temperature, but the relationship is not simple (impact of heat loss).

Color temperature: Temperature defined from wavelength distribution. In the case of tungsten, approximately 122ºF (50ºC) higher than the true temperature.

Service life: Operating life up to burn-out of the filament. Average value. The time required for the number to reduce to half when multiple lights are lit.

Luminous flux: Lm (lumen) → Amount of light. 1 Lx (lux) is the brightness when 1 Lm of light is incident on 1 m2.

Bright lighting is around 2000 Lx. Tens of thousands Lx is quite dazzling.

Manufacturing Method of Filament Coils

A filament (single winding) is formed by winding a tungsten wire around a core rod or a core wire. In many cases, the coil is unwound after wrapping to remove the core rod or core wire (on release returns naturally by spring back).

If the wire or filament has been subject to heat treatment for dimensional stability, ability to spring back is lost. In such cases the core wire is removed by dissolving with acid. This method requires equipment and cost is incurred to treat gas and dissolved solution released during dissolution.

Coil filament manufactured in this way and designed for durability can be used for lamps, but if the distortion is not removed with heat treatment, the coil will be subject to distortion in the lamp. Hence, the coil has to be inserted into the lamp after completing the process of secondary recrystallization.

Manufacturing Method of Filament with Double Coil

Primary and secondary windings are used in the case of double coil, but the most common manufacturing method used for the primary winding is to wind tungsten wire at a specified pitch on the molybdenum core wire. Then the wire is subject to heat treatment(furnace with hydrogen atmosphere 1832°F (1000ºC) to 2912°F (1600ºC)). The filament will not have spring back ability even if cut to a very short length on winding very closely.

Next, secondary coil is wound on the primary coil. In most cases of secondary winding, the wire is wound at a specified pitch on the core rod and then removed.

Next the ends are formed into specified shapes and then subject to high temperature heat treatment(Heated by hydrogen atmosphere furnace of 2912ºF (1600ºC) to 3452ºF (1900ºC) or by passing current directly). Then dissolve and remove the molybdenum core wire with acid mixture (water 2: nitric acid 2: sulfuric acid 1) to obtain a double coil filament.

In this method, a large quantity of NOx, residual acid solution, and molybdenum salt is released during removal of the molybdenum core wire and equipment cost is incurred for removal and detoxification. If molybdenum is used for the primary core winding and heat treated at very high temperatures, molybdenum infiltrates into tungsten and this has an adverse effect on the halogen lamp.

For this reason, heat treatment can be carried out at around 3452ºF (1900ºC), and secondary recrystallization of tungsten cannot be completed adequately. In this case, secondary recrystallization takes place on lighting the lamp, and the filament may deform.

Therefore, heat treatment is performed at a higher temperature (Approx. 3992°F (2200ºC)) after removing the molybdenum core wire.

One of the methods to manufacture a double coil without the above disadvantage is to use a primary wound coil (removal from core wire complete) to form to a double coil shape using some method and heat to a high temperature (Approx. 3992°F (2200ºC)) to obtain a double coil filament.

The method used to obtain a double coil shape is by forming a tungsten rod that is slightly thinner than the primary core wire into the secondary winding shape (coil shaped rod), and the primary winding coil is inserted into this to form the double coil shape and then solidified with heat treatment. The coiled core rod of tungsten is removed and reused after heat treatment.

However, mechanization of this method is difficult for use in mass production. Some designs of double coils are difficult to manufacture with this method. →Such as coils with MD/d < 3.

Heat Treatment of Tungsten

In the previous section we came across the term “Secondary recrystallization”, this phenomenon is also referred to as crystal coarsening.

Tungsten has a very high melting point, tungsten blocks made by melting is hard, fragile, and difficult to process.

For this reason, tungsten wire is manufactured using the powder metallurgy method. Tungsten powder is solidified and then subject to heat treatment, the manufacturing method is similar to that of ceramics etc.

Tungsten made by this method is an accumulation of fine crystals, and is relatively flexible due to sliding between the crystals and processing such as coil winding can be done.

Tungsten is fragile, manufacturing is better if processed while heating to the range 752ºF to 1292ºF (400°C to 700°C). Fintec hot air heaters of SAH series are suitable for indirect heating. Usually coiling machines are equipped with a function to wind tungsten wire while heating the coil by passing a current, and coiling can be performed safely with this. If the wire is wound without heating, then the wire may get cut easily or in some cases cracks are present inherently even though the wire is not cut and this is a problem for lamp quality.

The tungsten coil formed in this way is a set of fine crystals(Polycrystalline structure). Once the temperature reaches around 3632°F (2000ºC), the fine crystals are fused together growing to tens of thousands of times in one stretch (about 1 second) and becomes large crystals. The coil is very fluid during the process of secondary crystallization and even if a little force is applied to the tungsten coil including its own weight, the coil deforms significantly in that direction. The coil becomes hard and brittle on completion of secondary recrystallization, and the high temperature strength becomes relatively high.

Since the coil is very fluid and deformable for about 1 second until the completion of recrystallization, some type of support has to be provided during this period to prevent deformation. The method is the one described above.

The method of secondary recrystallization requires attention. Even for the same tungsten material, crystalline structure after secondary recrystallization differs depending on rapid or slow recrystallization. The crystal grains are long with slow secondary recrystallization and has good high temperature creep resistance.

High temperature creep is the phenomenon of slow deformation over time in the weight direction at high temperatures. This is mainly caused by slippage of grain boundary and is also called viscoelastic deformation. Since the temperature is very high while the filament is lit, high temperature creep phenomena takes place and the filament sags over time with its own weight. Some models are affected by this phenomena and some are not affected. This is almost determined by the voltage between the supports of the filament, and there is almost no problem if this voltage is 20 V or less. The problem starts when the voltage is around 50 V, and becomes significant around 100 V. Almost no lamps can be made at voltages higher than this.

There are problems with high-temperature creep resistance such as the filament may break due to short-circuit between the pitches if deformation increases, deforms the quartz bulb if the filament is too close to the quartz bulb, and causes blackening due to halogen cycle abnormality. Therefore, the method used for secondary recrystallization is important.

The heat treatment method for making filaments with good high temperature creep resistance is to maintain the temperature at which secondary recrystallization starts for 10 seconds, and heat to +572°F (300ºC) above the temperature at which secondary recrystallization starts and maintain for about 1 second to completely finish the secondary recrystallization.

Supplementary explanation

High temperature creep resistance significantly differs with the form of crystals formed by secondary recrystallization of tungsten wire. Generally, formation of intertwined long crystals along the direction of the wire is considered to be good. The important factor to prepare such crystals is to add trace amounts of potassium in the manufacturing process (doping). The tungsten wire used in halogen lamps is almost without exception doped tungsten wire. Excellent halogen lamps with less filament deformation can be manufactured by using this wire material and paying attention to the method of secondary recrystallization mentioned above.

However, the temperature at which secondary recrystallization starts tends to increase with the increase in doping amount. Tungsten wire that has been excessively doped cannot fully complete secondary recrystallization even at a high temperature of 3632°F (2200ºC) and may start deforming when switched on as a lamp. Both quality and quantity of doping are important for stabilizing performance and quality.

Trace elements such as potassium added for doping creates good tungsten crystal structure (intertwined long crystal) and gathers at the crystal grain boundary forming minute bubbles. This prevents slippage of grain boundaries and suppresses creep of filaments at high temperatures.

However, these minute bubbles formed by doping gradually come together over time and forms large bubbles inside the filament. This is a factor that limits the service life of the lamp, but since the gas filled in the halogen lamp is at a high pressure, the growth expansion of the bubbles (dope hole) is suppressed. Based even on this point, high pressure sealed gas is considered to contribute to a longer lamp life.

The impurities in the bubbles erupt like a volcano in the gas enclosed inside the lamp, collapsing the halide balance of the sealed gas and is the factor for problems such as blackening (Alkali metals such as potassium strongly binds with and inhibits the halogen cycle). This is mentioned as one of the reasons for the occurrence of blackening in the lamp after use for hundreds of hours.

The filament deformation due to high temperature creep is known not to be determined only by the nature of the filament. If too much oxygen and water remain in the lamp, water cycle described later is considered to take place along with the halogen cycle during lighting, this causes repeated intense evaporation and re-adherence of the filament surface, and tungsten wire surface no longer resists deformation resulting in the increase of the high temperature creep phenomenon.

Surface Treatment of Tungsten Coil

The filament coils may be used as they are, but have to be cleaned before they are incorporated into lamps to remove impurities and oxidation. Finally, deoxidation heat treatment is performed in hydrogen.

First, tungsten coil is generally boiled for about 10 minutes in 10% NaOH aqueous solution. If surface etching is required, clean-up is performed using 5% HF (Hydrofluoric acid)treatment and surface corrosion is treated with ferricyanide potassium solution. On completion, the chemical solution is cleaned thoroughly.

Next, the supports are attached to the coil filament(called the anchor and supporter),and molybdenum foil or the external lead rod are welded.

Next oxides are deoxidized by treating with NaOH again or placing in a hydrogen atmosphere furnace (1832°F (1000ºC)). Dry or wet hydrogen is used. Wet hydrogen has the ability to remove carbon.

Surface Treatment of Tungsten Coil

The filament coils may be used as they are, but have to be cleaned before they are incorporated into lamps to remove impurities and oxidation. Finally, deoxidation heat treatment is performed in hydrogen.

First, tungsten coil is generally boiled for about 10 minutes in 10% NaOH aqueous solution. If surface etching is required, clean-up is performed using 5% HF (Hydrofluoric acid)treatment and surface corrosion is treated with ferricyanide potassium solution. On completion, the chemical solution is cleaned thoroughly.

Next, the supports are attached to the coil filament(called the anchor and supporter),and molybdenum foil or the external lead rod are welded.

Next oxides are deoxidized by treating with NaOH again or placing in a hydrogen atmosphere furnace (1832°F (1000ºC)). Dry or wet hydrogen is used. Wet hydrogen has the ability to remove carbon.

Information on Quartz Bulb of Halogen Lamps

Generally quartz glass is used as the material for glass bulb in halogen lamps. Quartz glass can withstand high temperatures.- - - The maximum allowable working temperature as a lamp is 1652ºF (900ºC). However, the design should be such that the temperature does not exceed 1472ºF (800ºC). The bulb will be stable if the temperature is maintained below 1292ºF (700ºC) with less emission of impurities and less variation in bulb service life.

Volume of the halogen lamp bulb can be reduced to about 1/50 that of normal electric bulbs by using quartz glass.

A glass with thickness of about 1 mm can be used. A halogen lamp bulb this small and glass of lesser thickness can withstand up to 50 to 100 atm.

However, considering safety factor, filling pressure is 3 to 4 atm at room temperature for normal halogen lamps and 6 to 7 atm for high pressure halogen lamps. When the bulb is lit, pressure is assumed to be 10 to 20 atm because of thermal expansion of the filled gas.

Quartz glass has an abnormally low coefficient of thermal expansion. The value is only a few tenths that of normal glass or metal. Therefore, quartz glass can resist extreme heat shocks, to the extent that quartz glass at a high temperature of about 1652ºF (900ºC) can withstand sprinkled water. Regular glass can only withstand a heat shock of about 212ºF (100ºC). Pyrex or Tempax can withstand about 356ºF (180ºC).

Quartz glass contains various impurities, but glass containing many hydroxyl groups is not preferred as bulb material for halogen lamps. These seep into the space inside the lamp due to high temperatures when the lamp is lit causing water cycle inside the lamp reducing the service life of the bulb. If the temperature of the bulb is 1112°F (600ºC) or less, then there is no cause for concern.

In the water cycle, water molecules first oxidize the tungsten coil that is at high temperature while the lamp is lit. This forms tungsten oxide and hydrogen. Tungsten oxide sublimates immediately and evaporates from the coil. Tungsten oxide is then deoxidized at the low temperature areas by the hydrogen generated earlier forming tungsten. This releases free tungsten and water. The water formed again oxidizes the coil to produce tungsten oxide and hydrogen. In other words, the water cycle occurs even if water is present in small amounts causing the tungsten filament to evaporate intensely.

In halogen lamps, halogen cycle is the exact reverse of the water cycle, and takes place simultaneously with the water cycle. The filament does not appear to evaporate at first sight, but due to the severe evaporation of tungsten during water cycle and re-adhesion of tungsten during halogen cycle, the coil surface becomes uneven in a short period of time, leading to burn-out of the filament.

To avoid such problems, quartz glass with minimum hydroxyl content has to be selected. Quartz glass manufactured by melting with an acid-hydrogen flame contains 100 ppm or more of hydroxyl groups and is not suitable for high temperature halogen lamps. This value is excellent at below few ppm for quartz glass manufactured by electro-melting (such as GE 214 quartz. Currently GE is selling quartz tube division). For applications requiring lower values, glass containing 1 ppm or less of hydroxyl groups is also available, where OH groups are released using heat treatment.

Vycol (brand name of Corning Incorporated), has almost the same properties as quartz, but is not suitable for high temperature halogen lamps due to the presence of a large amount of hydroxyl groups.

Even if selection of quartz glass as a raw material is appropriate for the problem of hydroxyl groups, care is required as the hydroxyl group can permeate inside the glass while being processed into halogen lamps. For example, the flame of a gas burner contains a large amount of moisture and if this flame is used to process the glass by heating, then hydroxyl groups increase to some extent. The only method to remove the hydroxyl groups completely is by heat treatment at a high temperature 1472ºF (800ºC).

As one of the methods to avoid this problem, adding oxygen getter in the lamp as described above is effective in some cases. However, halogen lamps with high quality and performance can be manufactured without using getters by removal of water, hydroxyl groups, and oxygen to the utmost limit and high purification, and is considered the right method.

As a characteristic phenomenon occurring in the halogen lamp with severe water cycle, needle shaped crystals are observed to grow in the low temperature parts of the filament. The service life of such halogen lamps is extremely short (about 1/5 the rated service life) However, in most cases such a severe phenomena is generally not observed with the degree of hydroxyl group contained in quartz glass, and if there is frequent occurrence with such lamps then we can determine that high purity (exhaust failure → residual moisture or oxygen) has not been achieved at the manufacturing stage of the lamp.

Quartz glass has a dense structure, but as with other substances allows penetration and permeation of various gases. This has a significant relation to temperature, and increases remarkably from around 1472°F (800ºC) for quartz glass. Because of this property, halogen lamp bulb temperature should be lower than 1472°F (800ºC) preferably lower than 1292ºF (700ºC). As the temperature crosses 1472°F (800ºC), various substances inside the quartz glass seeps out, and on the other hand contents from the filled gas soaks into the glass. Such changes in the gas balance inside the halogen lamp causes various problems (such as reduced service life and blackening).

Halogen lamps used for heating applications always use quartz glass, but hard glass (aluminosilicate and borosilicate glass) may be used for low power illumination lamps. The coefficient of thermal expansion of these glasses can match that of molybdenum or tungsten by blending and can be sealed without using foil, and has the advantage that mass production is easy. However, cost reduction is the only advantage of mass production, heat resistance and thermal shock resistance are considerably inferior.

Surface Cleaning Treatment of Quartz Glass

Quartz glass is washed if the glass is dirty or there is a possibility that it is dirty. If the quartz glass tube before processing is dirty and heated without cleaning, the dirt components infiltrate the glass forming glass with poor transparency and reduces strength, this hinders the halogen cycle. Therefore, the tube must be washed before processing.

Foreign material may adhere even during processing. Though this is not foreign material, redeposited silica after evaporation (sublimation) also reduces the transparency and gives a poor appearance.

The glass is cleaned even in this case. Dirt cannot be removed with normal detergent in many cases, usually such detergents removes the dirt by dissolving the surface. For this, surface etching with hydrofluoric acid (HF) is commonly used. However, since HF is a very dangerous chemical, ammonium fluoride that is less dangerous is used in most cases.

Then baked (baked up to 1832°F (1000ºC)).

Accidents may occur while using hydrofluoric acid. One of the risks is that even if hydrofluoric acid adheres to the body, symptoms do not appear immediately and the pain can be felt only a few hours later delaying the discovery. The acid not only affects the skin surface, but also penetrates to a significant depth and may cause necrosis. Lethal when exposed to large amounts, and the mechanism for this is mainly hypocalcemia caused by conversion of calcium in the body to fluoride.

A little adherence at a concentration of 5% or less to the limbs is okay if washed well immediately, but adherence of higher concentrations of hydrofluoric acid to the body must be washed well, and calcium gluconate must be applied to the affected part and requires visit to a doctor. The neutralizing agent for spilled chemicals is calcium hydroxide (slaked lime). Severe symptoms appear even if hydrofluoric acid of 5% adheres to the body.

Quartz Glass Processing

Quartz glass is processed by increasing temperature to approximately 3632°F (2000ºC) using a gas burner and deformed by pressing with a rod such as carbon and metal or by pressing with a metallic mold.

Oxygen-hydrogen flame is ideal as a gas burner. There are 2 types of glass burners, “Root mixing type gas burner” in which oxygen and hydrogen are mixed in advance and blown at high speed from the nozzle for burning, and “Tip mixing gas burner” in which oxygen is blown into the air and then mixed by drawing in hydrogen while burning simultaneously. The latter method is suitable for processing quartz of a large area because the flow velocity of the flame is small.

Since the root mixing type prevents combustion from entering the nozzle with high speed flow inside of the nozzle, basically the flame also flows with a high-speed. This type of gas burner is suitable for heating spots with small area.

If the flow velocity of the nozzle drops in the root mixing type gas burner, combustion enters the nozzle (backfire phenomenon) and the oxygen-hydrogen gas mixture in the gas burner will burn and explode at once generating a large explosion sound. If left in this state, combustion may continue in the gas mixer and damage the mixer surroundings.

Flame obtained by mixing methane or propane gas with oxygen is sometimes used for processing quartz as it is economical. In this case, these fuel gases do not mix with oxygen as quickly as hydrogen and the combustion temperature is also lower, so “Root mixing type gas burners” are mostly used.

Therefore, this gas burner is considerably difficult to handle. A gas burner with many nozzles is used to heat a large area. The heating point is very close to the nozzle (about 10 mm) and flow speed of the flame is fast, so it tends to push and easily deform the glass that has been softened by the heat. In addition, if the gas flow suddenly stops in this gas burner, the flow velocity of the nozzle drops and back fires with an explosive sound.

To avoid this, oxygen has to be stopped slowly before stopping the fuel gas or the fuel gas has to be stopped first to blow off the flame. In either case, flow speed drops and backfire tends to occur, and quick shutoff operation is also not possible. To shutoff the operation quickly, blow off is performed by stopping the combustion gas and blowing air into the mixer at the same time without lowering the flow velocity of the nozzle.

Ignition of the gas burner also requires attention. Normally fuel is released first and then ignited followed by release of oxygen, but a red flame is obtained and cannot be ignited quickly. Frequent ignition can be managed by pre-setting fuel gas and oxygen, allowing simultaneously flow with the flow rate, and igniting with ignition burner (hydrogen flame).

Glass can be processed once it becomes hot and sufficiently softens. Carbon material does not present any problems, but quartz glass may adhere to metal in press working with metal molds. Carbon is effective as a release material to prevent this. Carbon deoxidizes quartz on contact at high-temperature strongly releasing COx. Generally oil is applied to replenish carbon.

Strong heating softens quartz, silica adheres to the periphery and turns turbid white. Quartz evaporates on heating and adheres to the low temperature parts. To prevent this to the possible extent, the long flame of air or gas burner can be used for the parts where silica is likely to adhere.

In addition, evaporation of quartz increases significantly in case of a reducing flame. This is considered to be because quartz is reduced to SiO that evaporates easily. Therefore, silica cannot be formed easily if an oxygen rich flame is used for processing. However, such flames are weak compared to the flow velocity or there is no deoxidization effect, and there are drawbacks such as molybdenum foil is oxidized during sealing work and can get cut easily.

The silica formed will be burned off with oxygen rich flame or removed by the HF treatment described in the previous section. HF treatment cannot be applied to the lamp after sealing.

Removal of Distortion after Processing Quartz Glass

As mentioned above, when quartz glass is processed as described above, we can say with certainty that distortion remains in the quartz glass. Strain is a state in which compressive and tensile force exists between molecules inside quartz. Distortion can be visually confirmed with “Distortion meter” using polarized light.

Residual strain lowers the strength of quartz glass, and the glass cannot withstand internal pressure while the lamp is lit and may rupture or cause cracks leading to initial lamp failure due to leakage of sealed gas. Also, the lamp may crack during replacement even with a small force.

To reduce the residual strain to the possible extent, after processing quartz as described above, immediately re-heat a wide area including the processed part after quartz processing and ensure that the glass is not allowed to cool rapidly. This work also has the merit of burning and closing small cracks that have been generated during press working and renders them harmless. In addition, press working must be completed in the shortest time as possible. Temperature of quartz drops sharply if pressed for a long time causing cracks and severe distortion.

To completely remove the residual strain, temperature should be increased above the annealing point (for quartz, approximately 2192°F (1200ºC)) of glass and stress is removed after sufficient time (15 minutes at the glass annealing point). Then, cool slowly to the strain point (for quartz, approximately 2012°F (1100ºC)) taking as much time as possible to reduce the temperature.

Even if a special strain relief furnace is not available, distortion can be eliminated to the extent that there will be no harm by carefully working with these points. However, removing strain completely to the extent that it cannot be confirmed with a strain gauge is difficult. A furnace is required for complete removal of distortion.

Quartz glass cannot retain high residual strain and such residual strain is difficult to neglect, but not paying attention can cause quality to decrease and occurrence of problems.

In glass other than quartz glass, since cracks will be surely generated if residual strain is neglected and processed, sufficient strain removal process is mandatory. As a result, there will be less occurrence of problems caused by oil leakage rather than quartz.

Supplementary explanation

Glass is weak against pulling force, specifically “Tensile strain” of residual strain is a problem. Note that tempered glass used for automotive windows are intentionally left with strong compressive strain on the glass surface. On applying a bending force, one side is subject to compressive force and the other side to tensile force. As compressive strain is left on the surface of the tempered glass, pulling force works in the direction to eliminate this strain and is canceled.

Glass is very strong against compressive force, and is an extremely strong glass because of this. Since glass has a strong residual strain on the surface as described above, if it breaks even at one place, balance of force collapses and the entire glass breaks into small pieces at once. However, the broken pieces have rounded corners and do not cause injuries.

Removal of Distortion after Processing Quartz Glass

If there is adhesion of salt etc. while using quartz glass at high temperatures, crystallization proceeds with such salts as nucleus, this decreases transparency and strength.

lthough this is certainly a serious problem for halogen lamps, and there has been a trend of widely publicizing this problem across the globe and there are cases where dealers use this reason as an excuse for processing complaints. There are very little chances that the lamp becomes completely useless due to this devitrification phenomenon. Devitrification cannot occur with the adherence of the slight amount of oil (actual harm is caused by salts) from the hands unless the temperature is very high. And most halogen lamps are not designed to become hot enough to be a big problem.

However, even if slight devitrification or burning of dirt occurs that does not render the glass completely useless will cause the brightness and condensing heating ability to decrease, so basically quartz bulb should not be touched with bare hands and in case the glass has been touched then it should be wiped off with an alcohol cloth or tissue paper.

Devitrification phenomena requires attention for lighting in places where there is always a risk of splashing seawater (such as ships). The best option is to protect the lamp by placing in a lighting fixture, if this cannot be done, coat the glass to suppress progression of devitrification or lower the glass bulb temperature (to about 1202°F (650ºC) or lower).

Devitrification Behavior of Quartz Glass

If there is adhesion of salt etc. while using quartz glass at high temperatures, crystallization proceeds with such salts as nucleus, this decreases transparency and strength.

Although this is certainly a serious problem for halogen lamps, and there has been a trend of widely publicizing this problem across the globe and there are cases where dealers use this reason as an excuse for processing complaints. There are very little chances that the lamp becomes completely useless due to this devitrification phenomenon. Devitrification cannot occur with the adherence of the slight amount of oil (actual harm is caused by salts) from the hands unless the temperature is very high. And most halogen lamps are not designed to become hot enough to be a big problem.

However, even if slight devitrification or burning of dirt occurs that does not render the glass completely useless will cause the brightness and condensing heating ability to decrease, so basically quartz bulb should not be touched with bare hands and in case the glass has been touched then it should be wiped off with an alcohol cloth or tissue paper.

Devitrification phenomena requires attention for lighting in places where there is always a risk of splashing seawater (such as ships). The best option is to protect the lamp by placing in a lighting fixture, if this cannot be done, coat the glass to suppress progression of devitrification or lower the glass bulb temperature (to about 1202°F (650ºC) or lower).

Halogen-free Quartz Tungsten Heater

Cannot be distinguished from a halogen lamp heater, even though the heater has a tungsten filament inside a quartz tube the lamp heater does not contain halogen.(K → Kelvin: unit of absolute temperature. Value obtained bt adding 273 to ºC)Halogen filling is not required for lamp heaters with a color temperature 2200 K or below. The evaporation of tungsten is slight within the set service life (5,000 to 20,000 hours) of heaters with color temperature of this range and halogen cycle is not required.(Therefore, the filament is not consumed at all → service life is not limited by this)

During the working of the water cycle, tungsten evaporation becomes extremely large, and the cycle has to be stopped. To stop the water cycle, water has to be completely removed from inside the lamp and the cycle can be stopped by just removing either oxygen or hydrogen.

The substance to be placed in the lamp for this purpose are called getters and there are mainly those removing oxygen and hydrogen. This requires lamps to be manufactured without leaving any water inside the lamp, but practically reducing residual water to zero is not possible. Therefore, using getters is indispensable for manufacturing such halogen-free lamp heaters. A normal electric bulb that is not a halogen lamp also use getters.

Zirconia is widely used for general light bulbs as getters. Using zirconia in heaters with the shape used in halogen lamp heater is difficult, so Tantalum (Ta) is used in many cases. Tantalum is a soft refractory metal that resembles lead and in the dark red heat state of approximately 1292°F (700ºC) can absorb several hundred times as much hydrogen as its volume. This metal is used as a support for the filament to work as a getter.

Of course, even lamp heaters under 2200 K may contain halogen. Halogen acts in the direction that inhibits water cycle and heaters with longer service life can be manufactured if the residual water content is small. However, the reason is mostly due to the lower cost of adding halogen. To manufacture lamp heaters with a reliable design for service life above 5,000 to 20,000 hours, adding getters without halogen is safer than halogen.

AC, DC, and Low Voltage Lighting

Halogen lamps can be lit up by AC or DC. Generally lighting with alternating current is safer. The problem of DC lighting is the behavior when the lamp breaks. The flow of direct current is difficult to stop and the arc discharge may persist in the lamp even after burn-out. This heat softens the glass tube that bends and bulges, or bursts in the worst case.

Therefore, always light the lamp of line voltage (100 V or 200 V) using AC. There are almost no problems with low-voltage halogen lamps and either AC or DC can be used.

While lighting halogen lamp at a voltage significantly lower voltage than its rated voltage, we tend to think that “The halogen cycle does not work because the temperature of the glass tube does not rise, and problems are likely to occur”, but lowering the voltage reduces the filament temperature, so evaporation of the filament is almost eliminated and the halogen cycle is not required. In other words, there is no problem even when the lamp is lit with a low voltage.

Fine filament and direct current lighting may contribute to the growth of needle-like crystals between coil pitches due to the impact of electric field that may contribute to reduced service life. Since alternating lighting is less likely to cause such problems, AC lighting is said to be desirable from this perspective.

Chemical Reaction and Control Method Inside the Halogen Lamps

Various chemical reactions are considered to take place inside halogen lamps.

(1) Halogen-Tungsten Cycle

Tungsten atoms evaporate in the low-temperature part and combine with halogen to form tungsten halide. Tungsten halide has a high vapor pressure (vaporizes at a relatively low temperature of approximately 392°F (200ºC) and maintains the gaseous state) and drifts in space without being deposited and decomposes thermally on reaching near the filament and separates into tungsten and halogen. As a result, tungsten is carried near the filament even on evaporation and vapor pressure around the filament rises suppressing filament evaporation. Halogen generated by heat separation is recycled and is subjected to a cycle (halogen cycle) in which halogen is combined with tungsten at the low temperature part.

(2) Halogen - hydrogen cycle

Hydrogen is intentionally added to the halogen lamp. The added amount is 1 to 3 times that of the halogen molar ratio. Pasts at relatively low temperature, halogen combines with hydrogen and exists in the form of hydrogen halide. This is thermally decomposes due to the high temperature near the filament and is separated into halogen and hydrogen. The cycle of generation and supply of single halogen required for the halogen cycle and disappearance (bonds to hydrogen) at the low temperature part takes place. Since the existence is in the form of a relatively stable hydrogen halide at the low temperature parts, low temperature parts such as the filament lead parts are not corroded.

(3) Carbon Cycle

Most of the time, carbon is present in the halogen lamp. This is because halogen is often added in the form of an organic compound. Since halogen compounds contain about 1000 to 3000 ppm of inert gas, the same number of moles of carbon is also present in the lamp. (Halogen compounds are mostly such as methylene bromide CH2Br2).

Carbon precipitates at the low temperature parts if oxygen is not present in the lamp, this is not seriously harmful but sometimes carbonizes tungsten in specific temperature parts and becomes brittle, and sometimes breaks on impact. If oxygen is present, part of the carbon is present in the form of carbon dioxide in the low temperature parts and as carbon monoxide in the high temperature parts. Since oxygen separates at the high temperature parts, this is related to the water cycle presented in the next section. Although the presence of carbon may possibly capture oxygen temporarily and mitigate the water cycle, but is not effective in stopping the cycle.

(4) Water cycle

Water molecules are present in the lamp and high temperature filament is oxidized and separates into tungsten oxide and hydrogen. Tungsten oxide evaporates easily. This is deoxidized by hydrogen at relatively low temperature and is converted to tungsten and water. A cycle takes place in which this water again oxidizes the filament. This results in a very rapid evaporation of tungsten reducing the lamp service life significantly. In halogen lamps, halogen cycle also takes place simultaneously, macroscopically the filament does not evaporate, but microscopically evaporation and redeposition take place vigorously and lamp service life is significantly short. In the findings of such lamps, corrosion (glittering) is observed in the comparatively low temperature portion of filaments. Growth of needle like crystals is observed in significant cases. The service life of such lamps is shortened from 1/2 to 1/10.

For this reason, stopping the water cycle is the most important issue for halogen and incandescent lamps. Even though halogen lamps are regarded as normal, water cycle takes place to some extent microscopically and in most cases the service life is shortened to some extent, but the impact is not extreme.

In ordinary incandescent light bulbs, oxygen and/or hydrogen can be captured by placing various getters in the lamp so that the water cycle can be stopped. Therefore, general incandescent bulbs have a more stable service life and bad products are considerably few.

In halogen lamps, there is a relationship with halogen and adding getters cannot be added easily. But is not impossible. Using getters is indispensable to manufacture stable long life halogen lamps.

Halogen Lamp With Getter

Water cycle can be prevented if water or one of its decomposition products (oxygen, and hydrogen) can be captured and deactivated by adding certain substances to lamps.

Such materials are called getters. Hydrogen and oxygen absorption types can be considered as getters. The water cycle does not work if any one of the elements is lost. However, since hydrogen has a useful function of preventing low temperature corrosion in halogen lamps, oxygen absorbing type is preferred as a getter.

The property required for the getter material is gasification by gentle reaction with halogen in the lamp and and then transportation to the vicinity of the filament to form a stable solid oxide that is not easily evaporated when bonded with oxygen and should not react again with halogen after deposition.

Boron (B), Magnesium (Mg), Aluminum (Al), Silicon (Si), Phosphorus (P) can be considered as suitable choices from the periodic table. Oxide of B has poor stability (high vapor pressure), the compound is toxic in many cases and is difficult to handle. Mg is too reactive with halogen and difficult to handle as it can ignite easily. Halogenation of P is difficult, and stability of oxide is also poor.

Al is considered first because of the above reasons. Commercially available aluminum foil is cut to a square of 0.8 mm, sucked and held with a syringe needle, inserted from the exhaust pipe and then the usual exhaust gas filling operation is performed. This Al getter is very effective. In some lamps, the expected average service life was slightly below 2 times and minimum service life improved by more than 4 times. One of the features of the lamp using Al as a getter is that tungsten support (near the filament) becomes black if tungsten is lit for several tens of hours. This is considered to occur because of the adherence of carbon particles generated by the deoxidization of CO and CO2 due to the strong reducing action of aluminum. There is no actual harm by this. Al in the lamp is halogenated at low temperature parts and is vaporized, then thermally decomposes around the filament and the vapor of Al is suspended. However, since the method of adding getters is difficult and also strongly reacts with halogen, halogen cycle is inhibited blackening the lamp if the amount is not controlled precisely.

Therefore, Si with relatively weak reactivity with halogen was tried. Adding Si chunks was difficult, and a method of deoxidizing quartz with hydrogen was tried. Hydrogen is added and Quartz deoxidized by raising the temperature of one part of quartz glass (such as the chip cutting part) to near 3632°F (2000ºC) to generate SiO2. This can work alone to serve as a getter that captures oxygen. Also, gradually changes to Si if SiO is not used solely. 2SiO → SiO2+Si

Water is generated during generation of SiO, if filaments are red hot they will be oxidized and oxygen remains in the lamp and use of getters will have no significance. In other words, the place that is heated becomes important for getter generation and the tip cutting part with no metal nearby is a promising location. However, this cannot be applied to models in which a part of the tungsten support is inserted in the tip pipe. The same is applicable for J type with supporters at the tip (this can be solved if an even number of supporters are provided).

Although Si getters do not have a strong effect as Al, they are relatively stable and is difficult to blacken even when the amount is just as required. Workability is good. Si getters can also be applied to automobile balls. Effective for improving the average service life especially for eliminating lamps with an abnormally short service life.

[Reference]

Physics of Light Heaters Optics of Light Heaters Condenser Type Heater Line Condenser Type Power Supply Controller