What are the thermal infrared materials available?

Thermal infrared material imaging typically refers to mid infrared (MWIR) imaging at 3-5 μ m and far infrared (LWIR) imaging at 8-10 μ m. In these bands, the focus is on heat sources rather than visible light. There are many different applications of thermal infrared imaging, such as non-destructive testing, infrared cameras that can capture the location of equipment overheating or building heat loss, differences in local body surface temperatures that can be measured in the medical field, rapid identification of heat leakage points in the cooling system of nuclear power plants, and safety protection.

There are many types of glass available for visible light systems, but only a very limited number of materials can be effectively used in the MWIR and LWIR bands. Figure 18.107 shows the transmittance of commonly used infrared transmission materials. These data include the reflection loss on the surface, thus resulting in a relatively high transmittance after the application of an efficient antireflective film. Only a very limited type of glass material can be effectively used in the MWIR and LWIR bands. Table 18.9 lists commonly used thermal Infrared Optical materials and their main characteristics. The Abbe constant V is defined as (n1 λ- 1) /(n1 λ- NH λ), In the equation, nC λ Refractive index at the center wavelength, n1 λ Is the short wavelength refractive index, nH λ Is the refractive index of long wavelengths.

There are several commonly used thermal infrared materials:

Germanium is the most common infrared material and can be used in the LWIR and MWIR bands. In the LWIR band, it is the "crown plate" or positive lens in achromatic dual lenses; In MWIR, it is the "flint" or negative lens in achromatic double lenses. This is due to the difference in dispersion characteristics between the two bands. In the MWIR band, germanium is very close to its low absorption band, so its refractive index changes quickly, leading to significant dispersion. This makes it suitable as a negative power component in achromatic double lenses.

(1) Germanium material:

Germanium is the most common infrared material and can be used in the LWIR and MWIR bands. In the LWIR band, it is the "crown plate" or positive lens in achromatic dual lenses; In MWIR, it is the "flint" or negative lens in achromatic double lenses. This is due to the difference in dispersion characteristics between the two bands. In the MWIR band, germanium is very close to its low absorption band, so its refractive index changes quickly, leading to significant dispersion. This makes it suitable as a negative power component in achromatic double lenses.

Germanium materials have two important parameters: refractive index and dn/dt. The refractive index of germanium is slightly greater than 4.0, which means that shallow surfaces are reasonable and easy to reduce phase differences, which is beneficial for design. The parameter dn/dt is the change in refractive index and temperature. The dn/dt of germanium is 0.000369C. This is a large value, dn/dt=0.000360C for ordinary glass. This can cause a large focal shift that varies with temperature, usually requiring some non heating technique (compensation of the focal point relative to temperature).

Germanium is a crystalline material that is generated in single or polycrystalline form. According to the growth process, single crystal germanium is more expensive than polycrystalline germanium. The refractive index of polycrystalline germanium is not uniform enough, mainly caused by impurities at the particle boundary, which can affect the image quality of FPA imaging. Therefore, single crystal germanium is the preferred material. At high temperatures, germanium materials become absorbent, and the transmittance approaches zero at 200C.

The refractive index non-uniformity coefficient of single crystal germanium is 0.00005~0.0001, while that of polycrystalline germanium is 0.0001~0.00015. For optical purposes, usually Ώ. The resistance coefficient of germanium is specified in cm, and the resistance coefficient of the entire blank is 5-40 Ώ. Cm is generally acceptable. Figure 18.109 shows a typical germanium blank with a polycrystalline area on the right. Please note that the resistance coefficient in the single crystal region behaves normally and slowly changes radially, while the resistance coefficient in the polycrystalline region changes rapidly. If a suitable infrared camera is used to observe the material, strange swirling images similar to spider webs can be seen, which are mainly concentrated at the particle boundaries. This is due to the induced impurities at the boundary. One of the shortcomings of silicon and some other crystalline materials is their brittleness and fragility.

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(2) Silicon material
Silicon is a crystalline material similar to germanium. It is mainly used in the MWIR band of 3-5 μ m, and there is absorption in the LWIR band of 8-12 μ m. The refractive index of silicon is slightly lower than that of germanium, but it is still large enough to facilitate aberration control. In addition, the dispersion of silicon is relatively low. Silicon can be turned by diamond.
(3) Zinc sulfide
Zinc sulfide is a commonly used material in the MWIR and LWIR bands. It generally appears rusty yellow and semi transparent to visible light. The most common process for producing zinc sulfide is called chemical vapor precipitation.
Zinc sulfide made by hot pressing can be transparent to visible light. Transparent zinc sulfide can be used to manufacture multispectral windows and lenses from visible light to LWIR bands.
(4) Zinc selenide
Zinc selenide is similar to zinc sulfide in many aspects. Its refractive index is slightly higher than zinc sulfide, while its structure is not as sturdy as zinc sulfide. Therefore, considering environmental durability reasons, sometimes a thin layer of zinc sulfide is deposited onto a thick zinc selenide substrate. Compared to zinc sulfide, the most significant advantage of zinc selenide is its extremely small absorption coefficient, so zinc selenide is usually used in high-energy CO2 energy systems.

(5) Magnesium fluoride
Magnesium fluoride is also a crystalline material. Its crystal material can transmit the spectral range from ultraviolet to MWIR. Magnesium fluoride can be produced by crystal growth or "hot pressing" methods, resulting in the formation of milky glassy materials. It has good transmission in the MWIR band, but may have unwanted scattering, resulting in a decrease in contrast and off axis stray light. The scattering of particles is inversely proportional to the fourth power of wavelength, so the milky appearance under visible light will shrink by 1/16 at 5um.
C501545f8816da6744a0fe5efc53bb5 Jpg (6) Sapphire

Sapphire is an extremely hard material. It can transmit light from deep UV to MWIR bands. A unique characteristic of sapphire is its low thermal emissivity at high temperatures. This means that materials emit less thermal radiation than other materials at high temperatures. Sapphire can be used to create cavity windows that withstand high temperatures, suitable for infrared band through windows. The main drawback of sapphire is that its hardness makes optical processing difficult. Another similar material is called spinel. Spinel is similar in effect to hot pressed sapphire and can be used as a substitute for sapphire. Spinel stones also have high dispersion. Sapphire has birefringence characteristics, and its refractive index is a function of the incident polarization surface.

(7) Arsenic trisulfide

Arsenic trisulfide is a material that can be used in the MWIR and LWIR bands. It has a deep red appearance and is very expensive.

(8) Other available materials

There are many other available materials, including calcium fluoride, barium fluoride, sodium fluoride, lithium fluoride, and potassium bromide. These materials can be used in the bands from deep ultraviolet to medium wave infrared. Their color characteristics make them highly attractive for wide spectral applications, especially from near-infrared to mid infrared and even far infrared. Many of these materials have some undesirable properties, especially hygroscopicity. Proper coating is required to avoid damage from moisture, and their structure often requires purification with dry nitrogen gas.