Infrared (IR) optical materials are substances that are specifically designed to transmit, reflect, filter, or absorb infrared radiation, which spans wavelengths from about 700 nanometers (nm) to 1 millimeter (mm). Infrared materials can be broadly categorized as crystals, glasses, semiconductors or metals.
Infrared (IR) optical materials are substances that are specifically designed to transmit, reflect, filter, or absorb infrared radiation, which spans wavelengths from about 700 nanometers (nm) to 1 millimeter (mm). Unlike visible light, infrared radiation is not detectable by the human eye, these materials are essential in systems that operate in the IR spectrum, including various optical components like lenses, mirrors, windows, filters, and detectors.
Infrared optical materials are essential components in many applications, including thermal imaging cameras, advanced sensing systems, and spectroscopic instruments. These materials are designed to transmit or reflect infrared radiation, which is a critical component of many scientific and technological developments, including remote sensing for meteorological, geological, and environmental monitoring.
IR radiation has a longer wavelength than visible light which causes it to behave differently when it interacts with matter. Because of this difference, materials that are transparent or reflective to visible light may not behave the same way when it comes to IR radiation. As a result, infrared optical materials require specialized design and testing.
Infrared materials can be broadly categorized as crystals, glasses, semiconductors or metals. Some typical materials used in infrared optics are described below:
IR grade fused silica (Corning 7980\JGS1\JGS2\JGS3) is an excellent optical material for use in the NIR system, usually along with other materials such as CaF2. It can be used at wavelengths from about 0.25 to 3.5 um, with high homogeneity and good transmission in the VIS and NIR spectral regions. However, it depends on the contained impurities having substantial absorption bands, e.g. about 2.2 um and 2.7 um OH content.
Applications: Visible and thermal imaging, astronomy, laser
Products manufactured: Lenses, mirrors, windows, wedges, beamsplitters, optical filters and prisms
Borosilicate crown glass (BK7, H-K9L) can be used in visible light applications as well as in the near infrared region.
Sapphire (aluminum oxide) is one of the hardest and most durable optical materials (but difficult to work with), with a transmission range of about 0.25 ~ 5 um, and is transparent from the UV up to about 6 um. Sapphire has a rhombic crystal structure and is highly anisotropic, which means that its optical and mechanical properties vary with the orientation of the crystal. It also has excellent thermal conductivity and can withstand harsh environmental conditions. Commonly used in infrared optical systems, operating in the near-infrared (NIR) and milliwatt (MW) spectral bands.
Various fluorides such as calcium fluoride (CaF2), barium fluoride (BaF2), magnesium fluoride (MgF2), and lithium fluoride (LiF) are very common crystalline materials, and some of them are also used for dielectric coatings.
Calcium fluoride (CaF2) and barium fluoride (BaF2) also have high transmittance in the ultraviolet, visible, and near-infrared regions. with a transmittance of more than 90% between 0.25 and 7 um, CaF2 has a high refractive index and low dispersion, making it ideal for applications that require high spectral purity. baF2 has an even higher refractive index and lower dispersion than CaF2, making it ideal for generating high-quality images and reduce chromatic aberration.CaF2 has high thermal conductivity and can handle high-power laser radiation. However, both calcium fluoride (CaF2) and barium fluoride (BaF2) have poor mechanical strength.
Magnesium fluoride (MgF2) is commonly used for anti-reflective coatings. It has higher resistance to mechanical impact and abrasion. It has high transmittance in the ultraviolet, visible, and near-infrared regions, with a transmission range of 0.11 ~ 7.5 um. MgF2 has a lower refractive index and higher dispersion than CaF2 and BaF2, making it well suited for broadband applications such as spectroscopy. However, it has a lower thermal conductivity compared to CaF2 and BaF2. Its birefringence should be taken into account before selecting this material for optical design.
Lithium fluoride (LiF) has the lowest refractive index of all common infrared materials. It has a very low refractive index and high dispersion, making it ideal for use in high-efficiency optics. It is microplastic, which means that when it is subjected to mechanical stress, it does not return to its original shape. LiF has a much lower thermal conductivity compared to other fluorides, which may limit its use in high power applications.
There are also fluorine-containing glasses, such as fluorozirconate, fluoroaluminate, and fluorine fluoride glasses, which have high transmittance in the infrared region and are commonly used in fiber-optic communication systems and infrared optics. They have a lower refractive index, higher dispersion and lower thermal conductivity than crystalline fluorides. However, their mechanical and thermal properties vary widely, making it important to select the right material for each specific application.
Certain selenides such as zinc selenide (ZnSe) and zinc sulfide (ZnS) also have a wide range of transparency; as a chemical vapor deposition (CVD) material, ZnSe is the material of choice for optics used in high-power CO2 laser systems due to its low absorption at 10.6 um.ZnS conventional grades have good imaging quality in the range of 8 ~ 12 um, and it has good transmission in the 3 ~ 5 um band also transmits, but with higher absorption and scattering than Clear ZnS. They can also be used as rare-earth doped laser gain media, especially chromium doped.
Cesium bromide (CsBr) has a transmittance of over 80% in the range of 0.35 to 32 um, and cesium iodide (CsI) has a transmittance of over 80% in the range of 0.42 to 40 um; both are suitable for ultra-long-wavelength applications, such as Fourier-transform infrared (FTIR) spectrometers, laser systems, lens protectors for CO2 laser systems, imaging systems, and analytical instruments.
Potassium Chloride (KCl) can transmit from the ultraviolet to the far infrared; it is one of the widest transmission bands among infrared materials. 0.3 ~ 21 um transmission is above 80%. Sodium chloride (NaCl) is more durable but very fragile. These ionic substances are soluble in water and highly hygroscopic.
There are also sulfur-based glasses that contain substances such as sulfur, selenium, arsenic, germanium, and silver. They are usually sold under specific trade names without specifying the exact chemical composition. The low dn/dT of sulfide compounds makes thermalization of lens systems simpler because it eliminates the mechanical compensation complexity required for high dn/dT optical thermalization. Sulfide series glasses can be generated, polished, diamond turned, magnetorheological finished, or molded. Typically used for MW, LW and sometimes NIR.
Semiconductors such as silicon, germanium, and gallium arsenide, which are completely opaque in the visible region, exhibit good infrared transparency. They have a high refractive index.
Reflective coatings for IR mirrors can be made from a variety of metals, i.e., copper, aluminum, silver, gold, and stainless steel, as well as various metal alloys such as chrome-nickel. They are usually made as the first mirror surface.
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Crystals |
Sapphire, Zinc Selenide (ZnSe), Zinc Sulfide (ZnS), Calcium Fluoride (CaF2), Barium Fluoride (BaF2), Magnesium Fluoride (MgF2), Lithium Fluoride (lix), Cesium Bromide (CsBr), Potassium Chloride (KCl) |
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Glass |
Quartz glass ir grade fused silica (SiO2), borosilicate crown glass (BK7, H-K9L), sulfur glass containing sulfur, selenium, arsenic, germanium and silver; fluorine-containing glass such as fluorozirconate, fluoroaluminate and fluorine fluorine glass |
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Semiconductors |
Silicon, germanium and gallium arsenide |
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Metals |
Various metal alloys such as copper, aluminum, silver, gold, stainless steel and chrome-nickel can be used as reflective coatings for infrared mirrors. |