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Physics of Lamp Heating

Contents Link
1. Reflection, Transmission, Absorption, and Radiation of Light
2. Thermal Radiation Intensity
3. Wavelength of Thermal Radiation
4. Far Infrared Heating and Near Infrared Heating
5. Reflection
6. Data on Sunlight
Product Introduction Halogen spot heater
Halogen line heater

     






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1. Reflection, Transmission, Absorption, and Radiation of Light


As shown in the diagram, part of the incident light on any object is reflected, part of it is transmitted, and part of it is absorbed.

If A% of light is reflected, the reflectance of this object will be A%. Similarly, transmittance is B% and absorptance is C%.

Here, A + B + C will always be 100%.

This is, in accordance with the law of conservation of energy (explained later), the quantity of energy does not increase or decrease even if the form of energy is changed, so the energy of incident light is (transmitted light energy + reflected light energy + absorbed energy).

Next is emissivity. When the object reaches a certain temperature, energy is emitted in the form of light (mainly from microwave to infrared to visible light region), but this is the numerical value of the degree of radiation. The virtual object which is easily dissipates heat is a black body, and emissivity is 100% at all wavelengths. Such an object does not exist in reality.

Important point is that the relation “Absorptance = Emissivity” should be true. If absorption is good, radiation will also be good, if absorptance is zero, there can be no dissipation by radiation.

To be precise, this relationship holds good only for certain wavelengths. Glass has nearly 100% transmittance in the visible light range and its absorption rate is nearly zero, and the transmittance is almost zero in the infrared region (about 3 µm or more), and it becomes an almost perfect absorber. Greenhouses utilize this property, almost all the sunlight penetrates the glass and enters the greenhouse where it heats and warms the interior. From the warm room, heat dissipation is attempted with infrared radiation of long-wavelength, but the transmittance of glass in this wavelength range is zero, and if the insulation performance of glass is good (such as double glass), heat cannot directly go out and remains in the greenhouse.

However, this explanation is absolutely correct. “Greenhouse effect of earth” is a phenomenon in which the atmosphere of earth has the same effect as glass mentioned above, causing the surface temperature of earth to rise. However, the explanation up to “the mechanism due to which the temperature of greenhouses increases above the outside temperature is that sunlight permeates through the window glass (or plastic) warming up the room” is correct, but the statement “Glass prevents escaping of heat by re-radiation” is not correct as it is only a small proportion of the heat. In reality, the main reason for the rise in temperature inside the greenhouse is that the glass or plastic that encloses the air which is heated and prevents it from escaping. Therefore, most greenhouses do not use double glass, even then they are effective as a greenhouse. Even though the same phrase “Greenhouse effect of earth” is used, the mechanism is slightly different.

The radiation (absorption) characteristics changes with temperature even for the same glass characteristics. At room temperature, glass is transparent for the visible light region, when it is heated to its softening temperature (approximately 2192ºF (1200°C)), it is almost opaque and glows with a yellow color. In the case of quartz glass, it is still transparent at this temperature, emissivity of visible light is zero and it does not shine. However, since it is opaque in the infrared region, it radiates infrared light. At 3092ºF (1700ºC), absorptance (=emissivity) of even quartz glass increases for the visible light region, it begins to shine with a bright white light. It is considered that emissivity in the vicinity of red color is low because it feels more whitish than sunlight (6000 K) though the temperature is around 3632°F (2000ºC).

Fig. 2 shows the trend of the emissivity (= absorptance) of metals and nonmetals (ceramics, and plastics) for different wavelengths. This is a general trend, and the values vary significantly depending on individual substances.

Reference data Emissivity (absorptance) of various materials

Absorptance of metals decreases with increase in wavelength, it is not suitable for heating with a far-infrared heater. Halogen lamp heaters (peak wavelength approximately 1 µm) are the best as they are suited for heating at wavelengths close to visible light. Even then, some metallic materials cannot be heated. For example, copper and aluminum have high reflectance even in the visible to near-infrared region, and additionally, they are difficult to heat as they are good conductors of heat. However, these metals can also be heated depending on the surface condition. (When surface is discolored due to oxidation or minute irregularities)

Non-metals (ceramic, plastic, paper, wood, and human body) generally have high emissivity (= absorptance) in the far infrared region as shown above, the far-infrared heaters are suitable for heating non-metals. Far-infrared heaters cannot heat to high temperatures, and a halogen lamp heater is more suitable to quickly raise the temperature even for such objects.

Emissivity (= absorptance) cannot be more than 100% (for any wavelength). “Blackbody” which is a virtual object has 100% emissivity (= absorptance) for all wavelength ranges, and powdered carbon (95 to 98%) is close to a blackbody.

The human body also has an extremely high emissivity of 95% or more in the far-infrared wavelength range (approximately 10 µm) which is radiated at body temperature. Emissivity is generally high except for metals in this wavelength range.

When an object reaches a certain temperature, energy is released in the form of radiation that can be represented by Planck or Stefan Boltzmann's equation. Thermal energy transfer takes place only when there is a temperature difference. For example, though the human body is a very good radiator, the body can radiate heat to the surroundings only when the ambient temperature is lower than the body temperature. You feel “Cool” only when there is a temperature difference with the surroundings and heat dissipation (heat transfer) takes place along with emissivity. (In this case, though heat dissipation by radiation is higher than convection and air, it will not be discussed here)

Heat dissipation cannot take place if the temperature of the surrounding and body temperature are the same, although it can be said that heat dissipation by radiation does not occur if there is no temperature difference, it receives the same energy as the energy radiated to the surroundings from the surroundings and there is no heat transfer due to zero difference. (The latter is more accurate)

If this is considered, the question “Why is there no warmth even though there must be thermal radiation corresponding to the temperature from the cold object?” is answered. It is certain that radiant energy is received from an object at a particular temperature, at the same time heat is radiated to the object based on your own temperature. If your temperature is higher than the other side, more heat is transferred from you and you feel cold as your energy is reduced. If the temperature of the other side is higher than your temperature, you will receive more energy and you will feel warm. At this time, if emissivity (= absorptance) of the other side is high, heat transfer will take place at a faster rate.

Here, the example of “Myself” has been used to compare using the human body so that it is easy understand, the same explanation is applicable even when it is replaced with any object. Replace “Feel warm” with temperature increases and “Feel cold” with temperature decreases.



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2. Thermal Radiation Intensity

When an object reaches a certain temperature, it emits radiation in the form of electromagnetic waves (~ microwave ~ infrared ~ visible light ~). The proportion of heat energy radiated from an object higher in temperature than the surroundings increases with increase in the temperature, and this radiation accounts for a major part of the heat radiated at 500ºC or higher. Below 500ºC heat dissipation due to convection and conduction is dominant.

If the temperature of the object is considered as T [K] and emissivity as 100%

P=5.68×10-12 x T4 W/cm2 (Stefan Boltzmann's equation)

It is important to note that the above value is the amount of radiation under special conditions (when ambient temperature is absolute zero). Ambient temperature cannot be absolute zero and it is usually around 300 K (27ºC).

Therefore, the following equation expresses Stefan Boltzmann's equation in a general form to express the thermal radiation from the object when the ambient temperature is T 0 [K]. A strict definition of the ambient temperature is not possible, and this expression has to be considered as an approximate expression.

P = 5.68×10-12 x (T4 - T04) W/cm2 From this equation, it shows that if the difference between the object temperature and the ambient temperature is 0, thermal radiant energy also becomes 0. If the temperature of the object is lower than the ambient temperature, the value of the formula will be negative. This means that thermal energy flows into the object.

This does not mean that the radiation formula by the first Stefan Boltzmann's rule is wrong. Radiation is emitted from the object at a certain temperature as shown in “Fig. 3”, and even at room temperature, radiation equivalent to absolute temperature of about 300 K is emitted at room temperature. However, unless the ambient temperature is absolute zero, it receives thermal radiation from the surroundings. If the ambient temperature is the same as the temperature of the object, the difference is zero, and energy does not enter or exit from the object.

Also, if the temperature of the object is sufficiently higher than the ambient temperature (since radiation is proportional to the fourth power of temperature), the effect of ambient temperature on the amount of thermal radiation from the object can be almost ignored. As the difference between the object temperature and ambient temperature decreases, the effect of the ambient temperature will be significant.

Measured data of the amount of heat radiation with respect to the object temperature

12 The surface temperature of an actual object, and actual measurement data of the amount of heat radiation from the object is given below. (Room temperature is 25°C)

The object is prepared by tightly winding ∅11 iron-chromium-aluminum wire of sufficient length such that the outer diameter is ∅23.5, and it is oxidized sufficiently and has a dark appearance. It is estimated that the emissivity is about 0.75 to 0.8 and it has almost the same emissivity as the oxidized surface of stainless steel and it is considered that the heat dissipation characteristics is close to many actual workpieces. However, for work pieces such as ceramic and glossy metal, the amount of heat dissipated will be lower.

The amount of heat dissipated with respect to the temperature of the object, in a narrow temperature range is roughly expressed by the following formula.

P ≒ kTn P: Heat dissipation W/cm2 k: Constant

T: the temperature of the object [°C] → For accuracy, temperature difference from the surrounding dT

This relationship is actually measured and summarized in the below graph. nHowever, the index is not constant and it changes with temperature as shown by the blue curve in the figure below. This is because heat dissipation takes place with a combination of conduction, convection, and radiation.n in each case is, for conduction almost n = 1, convection almost n = 2, and radiation almost n = 4. Dissipation of heat due to conduction is more dominant at lower temperatures, as the temperature rises, heat dissipation due to convection increases the rate of radiation rapidly increases from around 200°C to 300°C, and dissipation of heat due to radiation is dominant at temperatures above 500°C. Above several thousand degrees Celsius, radiation will account for more of the dissipated heat. In other words, index n is close to 4 at the high temperature range.

[Fig. 4] The amount of heat dissipated with respect to the surface temperature of an object (Example of actually measured values)

For temperature measurement, an optical pyrometer (CHINO IR-U) at 700°C or higher is used. Though a ∅0.32 thermocouple was used below 700°C, thermocouples are generally used to measure lower temperatures (As the measuring points are cooled by the thermocouple wires), and it was considered that the ratio of the temperatures with the value of optical pyrometer at 700°C or higher is applicable at less than 700°C and the measured values of the thermocouple in the low temperature range was corrected.

This correction value was +12%. From this, it is estimated that the measured temperature is approximately 12% lower when a ∅0.32 thermocouple is used to measure the object surface temperature. This point must be taken into consideration when measuring temperatures with a thermocouple. There is also a method of putting a thermocouple in the heating wire coil, in which case, the measured value will be higher than the true surface temperature.

[Fig. 4] The amount of heat dissipated with respect to the surface temperature of an object (Example of actually measured values)

12 As the object temperature increases, heat dissipation increases rapidly as shown in the above figure.

To maintain the object at the target temperature, it is necessary to continuously supply the same amount of thermal energy as that of thermal radiation and other radiation heat discharge at that temperature.

For example, when heating a thin plate held in the air, its heat dissipation area must be calculated on the front and back sides. Thermal radiation at 1400°C is approximately 44 W/cm2, but heat dissipation of approximately 88 W/cm2 when the back side is added.

However, this is when the emissivity is 100% (carbon plate is close to this), even in the case of the oxidized surface of an iron plate, etc., the emissivity is about 80% and the actual heat dissipation is less than the value given in “Fig. 3”.

However, since heat dissipation is not only due to radiation but also due to conduction, and convection, it is better to consider the value in the above figure, and in many cases the heat dissipation is much higher. For temperatures 1200°C or lower, data of measured values is given in “Fig. 4”, please use these values.

In other words, if you try heating to 1400°C with a spot heater, the energy density will be approximately 90 W/cm2 or more. The absorption rate (= emissivity) should be considered even in case, if the absorption rate is 80%, it is necessary to heat with light condensed to about 1100 W/cm2.

In the case of a spot heater using a halogen lamp, the power density of about 110 W/cm2 is close to the limit, and the upper limit of the temperature that can be heated is also limited to about 1400°C. However, lower the power of the spot heater, greater is the heat dissipation rate due to conduction and convection other than radiation, for example, the 150 W type can only heat up to approximately 1000°C. However, if 2 spot heaters are arranged facing each other and the object is heated from both the sides, upper limit will be a little higher.

When the F value (f/D) which is an elliptical mirror for a standard spot heater is 0.5, power density at the focal position is approximately 64 W/cm2 from P ≈ 0.8 · Q · sin2 θ W/cm2, Q ≈ 160 W/cm2.

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3. Wavelength of Thermal Radiation

Whether the heater mainly emits near infrared rays or far infrared rays is almost determined by the temperature of the heating element, and not its material. For the peak wavelength λ m of the electromagnetic waves that is emitted, if the temperature of the heating element is T°C

Peak wavelength λ m = 2896 / (T + 273) µm Wien’s formula

From the above equation, peak of the emission wavelength at each temperature is obtained as shown in “Fig. 5”. With this emission wavelength as the peak, it is widely distributed both on short wavelength and long wavelength sides. The short wavelength side is up to about 1/3 and on the long wavelength side it is up to about 5 times the peak wavelength. As you can see from Planck's equation, it is actually distributed over a wider range, and it has little significance as the numerical value becomes extremely small. The effective wavelength is about 0.5 to 3 times the peak wavelength → See the figure below

Temperature and color of object

The temperature unit K in the above figure is the absolute temperature. When 273 is subtracted from this, it becomes Celsius ℃. The graph of 300 K is at room temperature 27℃ and radiation (body temperature) from the human body is close to this. The graph of 3000 K corresponds to the radiation of halogen lamp.

Since the radiation intensity is a logarithmic scale, it becomes 1/10 when it is lower by 1 scale from the peak position and becomes 1/100 when it is lower by 2 scales. Therefore, the actual radiation range, ranges from the peak to 1 lower scale.

As shown in the above figure, it does not mean that there is no long-wave radiation from a high temperature heating element. Instead, the radiation of long wavelengths increases at high temperatures too. However, only the percentage is small.

The radiation intensity cannot be higher than the values given in the above graph. For all wavelength regions, the value of the above figure is obtained when emissivity is 100% (black body), and since emissivity is always less than 100% for all substances, the values will be lower than the values given in the above graph for all the wavelengths. Since the wavelength dependence of real substances on emissivity is not constant, the shape may not be quite similar to the mountain shape shown in the graph, but basically it is similar to the above graph, and there is a slight difference depending on the composition of the objects.

The “Thermal radiation intensity and wavelength distribution” mentioned above is obtained by the “Planck's law” as explained below, the above-mentioned graph can be used for practical applications as the calculation is quite complicated.

The radiant energy density W (λ, T) per unit area and unit wavelength of the blackbody is
W (λ, T) = 8phc/λ5/(exp (hc/λkT) -1) W/m2
λ: Wave length m T: Black-body temperature K
c: Speed of light 3 x 108 m/s
k: Boltzmann constant 1.3807 x 10-23 J/K
h: Planck constant 6.626 × 10-34 [J s]
If the above equation is summed up using C1, and C2,
W (λ, T) = C1/λ5/(exp (C2/λ/T) -1) W/m2
C1 (Radiation 1 constant) = 2phc2 = 3.7427 × 10-16 W/m2, C2 (Radiation 2 constant) = hc/k = 0.014388 m·k
Energy of 1 photon is pe = hν = hc/λ
1/(exp (hc/λkT) - 1) is the average photon number

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A more detailed view of “Thermal radiation intensity and wavelength distribution” is given below. The unit of radiation intensity is different from the previous data, it is radiation per unit solid angle [w/cm2/sr/µm], and it must be multiplied by 2p to correct the total radiation to radiation from a plane.

A more detailed view of “Thermal radiation intensity and wavelength distribution” is given below. The unit of radiation intensity is different from the previous data, it is radiation per unit solid angle [w/cm2/sr/µm], and it must be multiplied by 2p to correct the total radiation to radiation from a plane.

Visible light is in the range of approximately 0.4 to 0.8µm. Approximately 0.4µm or less is called ultraviolet rays, and 0.8µm or more is called infrared rays. Sometimes rays above approximately 4 µm is classified as far infrared rays and rays below are classified as near infrared rays, there is not much significance in this classification. Mechanism by which light is converted into heat is also the same. Infrared does not have any function other than changing to heat.



Radiation properties of halogen lamp heater (Red curve on the graph is that of a 2600K black body radiation for comparison)

* Radiation Properties of Halogen Lamp (Calculation expected value) Red curve of the graph is blackbody radiation of 2600K. Temperature 1 is of a 3200K halogen lamp (operating life of 200 hours).

Temperature 1 is of a 2900K halogen lamp (operating life of 2000 hours). Temperature 3 is of a 2600K halogen lamp (operating life of 5000 hours or more). These reflect the emissivity characteristics of tungsten and permeation characteristics of quartz glass bulb for blackbody radiation. Due to emissivity characteristics of tungsten (luminescent material), the total radiation is less than 1/3 of a black body radiation at the same temperature.

In addition, due to the tungsten characteristics, higher the wavelength lower the emissivity and characteristic of quartz glass that it does not allow wavelength of approximately 4µm or more to permeate indicates that it is almost zero for wavelengths of 4µm or more. The software that was used to create the above graph is sold by our company. ⇒ Calculation of thermal radiation

Blackbody radiation



The following figure shows the spectral transmission characteristics of optical materials, silicon and germanium.

Even for quartz glass, transmittance is around 93%, as reflectance is about 7%. Quartz glass absorbs very little in the visible light region. Even other materials also have reflectance of about 7%, this value must be subtracted from absorptance. Pure quartz is used as a communication optical fiber. It is a near complete transparent body to transmit optical signals over a few dozen kilometers.



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4. Far Infrared Heating and Near Infrared Heating

Only the infrared heater that uses thermal radiation will be explained here. Thermal radiation is the electromagnetic waves (light in a broad sense) emitted by a substance when it is heated to a high temperature. Laser heating is another method other than thermal radiation as a light heating method.

The infrared heater is generally a light bulb (halogen lamp), which uses light emitted from a heating element at about 2,000 to 2,800ºC. The peak wavelength is about 1 µm (0.001mm) and is distributed in the range 0.5 to 3µm. It is generally bright as it contains plenty of white light. Measures such as coloring glass are used to reduce visible light. If the temperature of the heating system is set to around 2000ºC or less, it will not be as bright.

The far infrared heater uses light emitted from a relatively low temperature (500ºC to 1000ºC) heating element such as ceramic, quartz, and metal oxide surface. The peak wavelength is 3 ~ 5 µm and is distributed in the range 1 to 15µm. It is not bright as it is slightly glowing red and almost does not contain visible light.

To use infrared heating, the following infrared properties should considered and select either far-infrared heater or near-infrared heater.

Far-infrared rays do not permeate through most objects, and they are heated only on the surface (0.1 to 0.2 mm). When far infrared heaters are used for heating adhesives, the surface is burned even with an adhesive that is nearly transparent, and heat is hardly propagated to the inside. When near-infrared heaters are used, it permeates and heats internally and bubbles are generated from the inside. For this reason, infrared heater should be used for heating adhesives.

When you heat printed paper with a far-infrared heater, it is heated as a whole. When heated with near-infrared heater, the printed letters or photographs are heated strongly while the blank portion is not heated much. That is, with near-infrared heater, there is a significant difference in the ease of absorption depending on the surface condition (color etc.) of the object to be heated, the degree of heating is uneven.

For example, when meat is heated with a near-infrared heater, the areas where burning starts becomes black, the black areas absorb near infrared better heating the same areas intensively in a loop which then gets locally scorched. In other words, even with the same material, burn deposit can be uneven. Far infrared heaters heats evenly.

This is also a disadvantage but depending on the purpose it may also be an advantage. For example, if you want to partially heating, just paint the parts to be heated in black.

Normally, near-infrared heaters can be used in about 1 second after the power has been turned ON, while far-infrared heater requires 30 seconds to several minutes.

Conversion efficiency of supply power to radiant energy by near-infrared heaters is good at 90%. The conversion efficiency for far-infrared heaters is considerable worse at 60 to 70%. Energy that was not converted to infrared is mainly used to heat up the air.

Although some people misunderstand that visible light emitted from near infrared heaters does not contribute to heating, but all the light energy absorbed including visible light is converted to heat.

The absorptance of near infrared rays is generally worse than far infrared rays for relatively light colored objects such as wood, paper, cloth and human body, but the differences are small when the conversion efficiency of heater is considered, and if energy and time loss required for the start-up of the heater is considered, the far infrared heater is not necessarily better in efficiency, and the near infrared (lamp heater) is often more energy efficient.

When a closed space is built with a high reflective material and heated, high overall thermal efficiency can be expected. This is because the light reflected without being absorbed by the first irradiation is also reflected by the wall surface, and the high absorption rate is achieved because it strikes the object which is to be heated and is repeatedly absorbed. In this case, the total thermal efficiency η is defined as Efficiency η ≒ S2 x D2 / (D1 × S1 + D2 x S2) x (conversion efficiency of heater) where the area of the wall surface is S1, absorptance is D1, surface area of the object to be heated is S2, and the absorptance is D2. Even if a closed space is not achieved, for the case where the object to be heated is covered with a large reflector results high overall thermal efficiency and the situation is close to the above.

The density of energy radiated from the heating element of the heater is high for near-infrared heaters and low for far-infrared heaters. The difference is 20 to 40 times. Even if a collector mirror collects infrared rays at one point, it will not exceed the surface density of the heating element in principle, due to which far-infrared heaters cannot achieve a very high energy density. (It is about several w/cm2 for far infrared heaters. It is above 100 w/cm2 for far near infrared lamp heaters.)

Rapid heating, high temperature heating and spot heating is difficult with far infrared rays, and near-infrared heaters are suitable for such applications. Rapid heating is required for most cases of industrial use as there is emphasis on processing speed and energy cost, and near-infrared heaters (lamp heaters) are advantageous for industrial heating.

When there is no problem with rapid heating, power consumption efficiency will improve if the temperature is raised rapidly in a short time with energy density that is high as possible. If the temperature is raised over a long time with a low energy density, there will be a loss due to dissipation of heat from the heated object until the target temperature is reached. The amount of electric power needed for slow heating may be several times that of rapid heating.

Since energy density that can be achieved with near infrared heating is several tens of times compared to far infrared heaters, high energy efficiency can be expected in such cases. When the target temperature is reached, controls by turning off the supply or reducing the power as needed to maintain the target temperature.

Heating rapidly by applying a large energy density may not be suitable depending on the type of the object to be heated. For example, when heating food materials that are thick, sometimes only the surface is heated and internal temperature does not increase adequately. In such cases, it is necessary to heat for a long time with low energy density.

Since the energy density is low for far infrared heating, it necessarily takes long time to heat up and this makes it easy for transferring heat to the inside, but it is necessary to control by lowering the energy density in some cases for near infrared heaters.

The reason for the misunderstanding that, “Penetrates inside the object and heats from the inside” about far infrared rays which is absorbed only by the outer surface, is probably due to low energy density and a long heating time.

From the actual results of our company, near infrared heaters gives better results in most cases, so there are many cases where far infrared heaters are replaced with near infrared heaters as judged by our experiments and experience. First try a simple experiment which will make it easy to understand.

False information and misunderstandings are more in the case of far-infrared rays → Reference False advertisment about far infrared rays



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5. Reflection

The phenomenon of reflection of light (Including infrared rays) is roughly divided into the following 2 forms.

(1) Reflection by metal surface: Reflection by the action of free electrons. A strong directional reflection according to the law of reflection.

(2) Reflection by fine transparent body: Reflection of white paper, white cloth, wheat flour and others. Usually it is diffuse reflection (Diffused reflection)

Reflection by the metal surface of (1) is performed on the top surface. Since reflection takes place according to the law of reflection, it is necessary to make the surface irregular for irregular reflection. The reflectance decreases in most cases, if this is done. If the metal is made into fine fibrous or powder form, the reflection often diffuses between the metal surfaces, and increases the amount of absorption.

For example, even when an aluminum surface with a reflectance of 0.9 is reflected five times, it becomes (0.9)5 = 0.59. Finer the metal, curvature of the surface becomes smaller, since it reflects and repeats reflections within the powder and fibers many times before it reaches the outside, the reflectance decreases and it becomes darker.

The is a significant involvement of free electrons in the reflection from the metal surface, and generally materials with good electrical conductivity tend to have good reflectance. For example, materials with good electrical conductivity such as silver, gold, copper, aluminum are excellent reflective materials. Gold and copper are colored because the reflectance falls in the short wavelength region of visible light, and visible light reflectance is somewhat inferior, but in the infrared region it has excellent reflectance.

Gold-plated mirror are used in optical heating spot heaters for the best infrared reflectance. Copper is also excellent, but it is not used because it easily deteriorates due to oxidation. Silver has the disadvantage that it easily turns black from the Sulphur compounds present in the air.

To digress a bit, the activity of free electrons is significantly involved in the transfer of heat. Hence, materials that have high electrical conductivity also have high thermal conductivity. (Silver, copper, gold, aluminum, etc.)

From the above figure, we can understand that gold plated mirrors are the best for utilizing the radiant energy from halogen lamps. When compared to a gold plated mirror, reflective capacity of an aluminum mirror is about 10% lesser. However, reflectance of the short wavelength (blue range) is poor for visible radiation light and it cannot be used because of its color. Even ultraviolet rays are not suitable. Aluminum mirrors are suitable for short wavelength ranges and range of visible light.

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Reflection of (2) has the opposite tendency compared to the reflection on a metal surface. For example, crystallized sugar is transparent and hardly reflects any light. There is only a slight reflection from the interface surface due to differences in the refractive indices of air and crystallized sugar. However, if the minute aspects are observed, we see that the refraction and reflection of light within these minute particles is complex and light that is returned to the source increases.

In the case of granulated sugar it is not so, and there significant reflections and it becomes whitish, and in the case of white sugar, the reflectance is significantly high and exhibits pure white diffuse reflection (irregular reflection). The same is true for water crystals (ice). Though a lump of ice is transparent, snow, the fine crystals of this precipitation are pure white and show high reflectivity.

When white cloth or paper is observed under a microscope, it is found to be organized as transparent fibers. The principle behind the reflection of white paper is the same as that of sugar and ice described earlier, when light passes through a transparent thin fiber, complex refraction takes place and one part is reflected at the boundary surface, this is repeated within the fibers many times and results in an increase in the light returned, thus increasing reflectivity.

In other words, excluding metals, materials that have high reflectance include transparent bodies, and when they are in fine powder or in fibrous form, repeated complex refraction and boundary reflection occurs in the interior regions, resulting in diffuse reflection of the incident light. The reflectance increases with increase in material transparency, and increase in the refractive index. Therefore, unlike metals, these do not show reflectance with close directionality.

The light transmittance of the fibrous material present in white paper or white cloth is satisfactory in the range from visible light to near infrared light (high transparency), but as the light reaches the far infrared regions, it becomes opaque and even reflection begins to decrease. Infrared heaters are suitable for heating white paper, white cloth, and white pigments, though reflected light and transmitted light decreases, the absorption rate increases based on the above principle.

To digress a bit, reflection based on principles other the above

As explained, diffuse reflection occurs in materials that are transparent, however, in transparent bodies specular reflection, similar to that seen in metals, has also been observed. For example, thin glass sheets show specular reflection of a few % at the interface with air. This is only a slight amount of reflection, however, by stacking many glass sheets, the reflection will get added showing a high overall reflectance.

In nature, scales of fishes is an example from the natural world that has a metallic luster, similar to when thin transparent membranes are stacked on each other. Even insects and worms have metallic luster based on this principle.

Even in industries, technology of using reflectors obtained by stacking many thin membranes are used various fields. Thin films of high refractive index and thin films of low refractive index are alternately coated. With15 layers, the reflectance is comparable to that of a metal reflector and with 21 layers, reflectance which is much higher than that of a metal reflector can be obtained. A higher reflectance can be obtained as the difference in the refractive index increases, titanium dioxide is often used as a material with high refractive index, whereas magnesium oxide, and silicon dioxide (high heat resistance) are often used as materials with low refractive index.

Multilayer film reflectors such as these can achieve not only high reflectance but also selectively reflect or transmit light depending on the wavelength that is controlled by the coating thickness. This is based on the phenomenon of light interference and has applications in many domains. (When the film coating approaches half the wavelength, transmission occurs instead of reflection.)

For example, a film that reflects light in the visible spectrum but transmits infrared light can be selected. This removes only the essential visible light from the light containing infrared rays that is emitted from the lamp, and infrared rays (Increase the irradiation temperature unfavorably) that is not required for illumination can be removed (cool beam lamp). Moreover, by using a membrane that reflects light of only a particular wavelength band, white light can be divided into the 3 primary colors of light. This technology is often used in optical instruments such as video cameras, and video projectors, etc.



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6. Data on Sunlight

The peaks of the wavelength range that can be seen by humans (Visible region) and wavelength distribution of sunlight are approximately the same. This is, of course, not a coincidence, and is the result of humans having used sunlight to gather information and use it as a weapon for survival, in other words, a product of biological evolution.

The reason for not using infrared or ultraviolet region is because it is difficult to focus the lens (crystalline lens) across a wide range of wavelengths, and there are problems connected with the materials that are suitable for the eyes, and is considered to be a disadvantage for survival (Size and weight increases etc.). Of course, trial and error of this kind would have taken place in the course of evolution, and will also happen in the future. For other living creatures, it may not necessarily match with the visible range of humans. It is considered that short wavelengths are more suitable for microminiature eyes. The sensitivity of the eyes of worms and insects shifts to the short wavelengths, and they can see up to the ultraviolet (UV) range.

As can be understood from the above figure, sunlight hardly contains any far infrared rays. The heating effect of sunlight comes from the light within the visible region to near infrared radiation. Naturally, visible light also has a heating effect → The energy from absorbed light is almost completely converted to heat irrespective of the wavelength.

However, in the case of light of short wavelengths, that is in the range blue light to ultraviolet rays, because photon energy is high, and absorbed light energy may be to other forms such as chemical energy with photochemical reaction and accumulated, it cannot be said that there is a 100% conversion to thermal energy. However, since only a small percentage is converted to other forms of energy, it is not a big mistake to assume light energy has been completely converted to heat. In the infrared region, the photon energy is small and photochemical reaction will definitely not occur, and 100% of light energy will be converted to heat.

Since far infrared rays are hardly found in the sun rays and are absorbed by the moisture in the air, they are hardly included in direct light from the sun.

However, since moisture in the air absorbs the rays, this implies that it also radiates the rays, and far infrared rays of long wavelength from the atmosphere and clouds reach the ground in large quantities. From the above figure, it can be understood that the direct rays of light from the sunlight have almost twice the energy, and far infrared rays from the atmosphere and clouds reach the earth's surface in the form of radiation.

Even more energy is radiated from the surface of the earth towards the sky in the infrared range 10µm.

Though this is not felt as “Warmth”, without this radiation, even places with shade will feel cold and nights will feel colder. If these become zero, it comes close to absolute zero (–273ºC).

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