The selection of optical materials is a critical steps of the design of infrared optics. Traditional optical materials (optical glasses and plastics) which are used in design of optics for visible radiation can't use used because they have high absorption coefficient for light of infrared spectral range. This means that usage of the special materials with minimal possible absorption coefficient for infrared light is required.
The list of materials are may be used is relatively small and can be divided into three groups:
Crystalline and polycrystalline optical materials;
Semiconductor optical material;
Chalcogenide optical materials (chalcogenide glasses).
Crystalline and polycrystalline optical materials
Crystalline and polycrystalline optical materials have features which makes them very useful for design of some infrared optical devices. Their transmittance is rather high as for radiation of infrared region as for radiation of visible region. This makes it possible to design and produce multispectral optical devices. Such devices can work with infrared radiation and with visible light at the same time or after minute adjusting. Their efficiency will be sufficient for both spectral regions. Assembly and adjustment of the multispectral devices is simpler than infrared devices because visible light can be used for control of these operations.
The downside is that many of thesel materials have low mechanical strength and chemical resistance including, Barium Fluoride (BaF2), Lithium Fluoride (LiF), Silver Chloride (AgCl) and Sodium Chlorides (NaCl).
A second potential disadvantages is low refractive index. It is very rarely may exceed 1.5. As result additional optical elements must be used to achieve of required optical characteristics of the device during its design. This may lead to increasing of complexity of optical system and cost on manufacturing.
The most suitable for production crystalline and polycrystalline materials are Zinc Selenide (ZnSe) and Zinc Sulfide (ZnS). These materials acquire optimal characteristics if they are manufactured according to CVD (Chemical Vapor Deposition) technology.
You can see some optical characteristics of the Barium Fluoride (BaF2) below. They are provided as an example of the crystalline and polycrystalline optical material characteristics.
Semiconductor optical material
The main advantages of the optical semiconductor materials are the very high refractive index and low dispersion coefficient. Such characteristics of these materials make it possible creation optical systems with short focal length which do not require of correction of chromatic aberration. This cuts the number of optical elements required for creation optical device with set-up parameters. As result, price of production decreases.
Semiconductor optical materials absorb visible light. Also their coefficient of absorption is rather high for infrared radiation too. And as a rule it grows with growing of temperature of the material. Hence transmittance of optical devices produced with help of these materials is not too high. A well-designed optical system using this material should avoid thick elements and use minimal number of elements to avoid losses of transmittance.
Optical devices which include semiconductor optical materials are harder to assemble and adjust because visible light cannot be used for this. This process requires special methods and equipment.
Semiconductor optical materials which are usually used in production of the thermal optics are Silicium (Si), Germanium (Ge), Cadmium Telluride (CdTe) and Gallium Arsenide (GaAs).
You can see some optical characteristics of the Germanium (Ge) below. They are provided as an example of the semiconductor optical material characteristics.
Chalcogenide optical materials (chalcogenide glasses)
Chalcogenide glasses have good bandwidth of infrared radiation. But refractive index of these materials is lower than refractive index of semiconductor materials.
The main advantage of chalcogenide glasses is very low temperature coefficient of the refractive index. This makes it possible to usage of chalcogenide optical materials during design of the devices which are required to work in very wide range of temperatures.
Cost of these materials is rather high. Here are names of couple chalcogenide glasses: Amtir-1, IG6, BD2, GASIR-1.
You can see some optical characteristics of the Amtir-1 below. They are provided as an example of the chalcogenide optical material characteristics.