Selecting an infrared window material for thermal imaging, surveillance, targeting, range-finding and LiDAR systems involves assessing the transmission properties of different optical substrates. Knight Optical examines the characteristics of Sapphire, Silicon, Zinc Selenide (ZnSe) and Calcium Fluoride (CaF₂), highlighting their suitability for applications operating across the short-wave (SWIR), mid-wave (MWIR) and long-wave (LWIR) infrared regions in use on autonomous and unmanned systems.
Each material has a distinct spectral transmission range and refractive index, making material selection an important consideration when specifying optical windows for infrared imaging, sensing and surveillance equipment.
Comparing Infrared Window Materials
The four materials provide different levels of spectral coverage, with transmission extending from ultraviolet wavelengths through to various regions of the infrared spectrum.
These spectral characteristics help establish which substrates provide transmission within the wavelength ranges required by different infrared optical systems.
Material Characteristics and Infrared Transmission
Sapphire for Visible and Infrared Optical Windows
Sapphire provides transmission from approximately 0.17 µm to 5.5 µm, encompassing ultraviolet, visible, SWIR and MWIR wavelengths.
This spectral coverage allows Sapphire windows to be used in applications requiring transmission across both visible and infrared regions. Typical applications include protective windows, viewports and sensor cover glass.
Silicon for SWIR and MWIR Applications
Silicon transmits across approximately 1.2 µm to 7.0 µm, covering wavelengths within the SWIR and MWIR bands. Its specified transmission characteristics also include the 25–300 µm region.
Different optical coating configurations are available for Silicon windows used in infrared applications. Knight Optical’s range includes anti-reflection (AR) and diamond-like carbon (DLC) coated Silicon windows, including AR/DLC-coated options covering the 7–14 µm LWIR waveband.
Zinc Selenide for Broad Infrared Spectral Coverage
With a transmission range extending from approximately 0.6 µm to 16 µm, Zinc Selenide (ZnSe) offers broader infrared coverage than the other materials examined.
Its spectral range encompasses the SWIR, MWIR and LWIR regions, making it applicable to optical systems operating across different infrared wavelengths.
ZnSe windows are available with coatings designed for specific spectral requirements. Common coating configurations include 2–14 µm, 2–15 µm, 8–12 µm and 10.6 µm designs. Knight Optical supplies both AR-coated and AR/DLC-coated ZnSe windows for infrared applications.
Calcium Fluoride for UV, Visible and Infrared Transmission
Calcium Fluoride (CaF₂) offers a transmission range of approximately 0.13 µm to 10 µm, extending from the ultraviolet region through visible wavelengths and into the infrared spectrum.
This range provides transmission across multiple spectral regions, including SWIR, MWIR and the lower portion of LWIR.
Matching Window Materials to Infrared Wavebands
Infrared applications are commonly categorized according to three atmospheric transmission bands: SWIR (0.7–3.0 µm), MWIR (3.0–5.0 µm) and LWIR (8.0–14.0 µm).
The transmission characteristics of each substrate determine which portions of these bands it can cover.
- SWIR (0.7–3.0 µm): Sapphire, Silicon, ZnSe and CaF₂ all provide transmission within this region, although their overall spectral transmission ranges differ.
- MWIR (3.0–5.0 µm): All four materials provide transmission throughout the specified MWIR band.
- LWIR (8.0–14.0 µm): ZnSe transmits across the complete LWIR range, while CaF₂ extends to approximately 10 µm and may be suitable for applications operating toward the lower end of the band.
These distinctions are relevant when comparing materials for infrared systems with different operating wavelengths. In particular, ZnSe provides full spectral coverage of the stated LWIR band, whereas the other substrates have different transmission characteristics and limitations.
Custom Infrared Windows and Optical Coating Options
Beyond spectral transmission, infrared window specifications can involve dimensional requirements, optical surface characteristics and coating selection.
Knight Optical supplies stock infrared windows and manufactures custom optical windows according to customer specifications. Available customization options include bespoke dimensions and thicknesses, as well as specified flatness, parallelism and surface quality.
Additional manufacturing options include CNC edging, drilled holes and specialist optical coatings, including DLC coatings. Custom windows can also be manufactured using a range of specialist infrared materials beyond those compared here.
For infrared imaging and sensing system development, Knight Optical provides technical support in identifying suitable window substrates, coatings and optical specifications based on the requirements of the application.
Read Infrared window materials compared: Sapphire, Silicon, ZnSe and Calcium Fluoride on the Knight Optical website.
