What optical materials work best in the IR (infrared)?

Custom IR windows are popular because they allow for the transmission of infrared light while providing protection to sensitive infrared sensors and systems. IR windows, stepped windows, lenses, and other precision optics made from materials such as calcium fluoride, germanium, magnesium fluoride, sapphire, silicon, zinc selenide and zinc sulfide, which are highly transparent in the infrared spectrum and possess excellent durability and thermal stability. IR precision optics are ideal for a range of applications, including thermal imaging, spectroscopy, and environmental monitoring. Their ability to withstand harsh environments while maintaining high optical performance ensures accurate and reliable measurements in various industrial, military, and scientific settings.

When dealing with the infrared region, it is important to know this part of the electromagnetic spectrum breaks down into further sub-sections.  

NIR and SWIR together are sometimes called "reflected infrared", whereas MWIR and LWIR are sometimes referred to as "thermal infrared". 

The below chart shows the transmission ranges of the most commonly used IR materials.  

IR fused silica is virtually free of OH-ions providing superior transmittance at the 2.7μm wavelength “water band” region where standard UV grade fused silica absorbs light. The low OH content (<1 ppm) expands the overall usable range of fused silica to 3.6 microns. As with other fused silica designations, IR grade also shares the same outstanding homogeneity, bubble characteristics, low coefficient of thermal expansion, and chemical resistance. 

Sapphire crystal (Al2O3) is an optimal choice due to its mechanical strength, scratch resistance, and its hardness which is second only to diamond. It is used in ultraviolet (UV) and visible wavelengths beginning around 150nm and also performs well in IR to around 5µm. The downside to sapphire is the high material and processing costs.  

Calcium fluoride (CaF2) optics are ideal for a broad range of Ultraviolet (UV), Visible, or Infrared (IR) applications. Its low refractive index reduces the need for anti-reflective coatings. Its application ranges from thermal imaging systems to excimer lasers making it a very versatile material for ultraviolet (UV) to infrared (IR) frequencies.  

Magnesium fluoride (MgF2) is also a crystalline material that transmits well from the UV through the MWIR spectral bands (0.1 to 7.0μm). Magnesium Fluoride has relatively low cost; however, is thermally sensitive and requires special handling considerations. 

Germanium (Ge) is a grayish non-transparent crystalline material and one of the most commonly used infrared materials. It is an optimal material for night vision and thermal imaging systems in the MWIR and the LWIR band. Germanium transmission performs best between 8 and 12µm. Ge has a low optical dispersion and a high refractive index which makes it an ideal solution for a wider field of view lenses. Its crystal structure is similar to diamond and when a DLC coating is applied (Diamond Like Carbon), it becomes very durable against outdoor elements and harsh environments.  

Silicon (Si) is a crystalline material like germanium primarily used within consumer electronics for microchips, as well as, extensive use in the semiconductor industry. Silicon is an excellent choice for windows and lenses in the 3μm to 5μm MWIR spectral bands for use in imaging, biomedical and military applications. Silicon optics are more heat resistant than germanium ones, as operating germanium in temperatures higher than 100°C leads to reduced optical properties.    

There is another group of materials that perform well in the IR but require a high amount of safety precautions during manufacturing due to their harmful material makeup. Chalcogenide glass is a glass containing one or more chalcogens (sulfur, selenium and tellurium, but excluding oxygen). Such glasses are covalently bonded materials and may be classified as covalent network solids. Chalcogenide glass remains amorphous while exhibiting optical transparency over the full IR region of 2-20µm. 

Zinc selenide (ZnSe) is another common material that is used in both visible and IR (MWIR & LWIR) from 0.45 to 21μm.  It is a light-yellow solid compound comprised of zinc and selenium. It is very similar to zinc sulfide, but has a slightly higher refractive index and is structurally weaker 

Zinc sulfide (ZnS) performs best between 8 to 12µm region. Although a lower cost relative to ZnSe, it does not have the longer transmission range. As a strong and stable material, ZnSe has high resistance to particulate abrasion making it an ideal solution for IR windows on aircraft platforms.  

In conclusion, there are many material options available for infrared capabilities and whether you need a IR windows, lenses, or freeform optics, we are available to help you find solutions to your objectives. Our manufacturing team can fabricate windows, prisms, lenses, aspherical lenses, and prototypes all in-house. Please reach out to us on your next project today at sales@EscoOptics.com.