Wavebands used for remote sensing, especially in stratospheric and orbital applications, are determined by several factors, including atmospheric transmission, detector capabilities, and the transparency of refractive materials. Atmospheric transmission varies with wavelength but is significantly limited outside the range of 0.3 to 14 µm. Detectors are available for specific wavebands, and various optical materials transmit within different wavelength ranges. The combination of these factors defines the standard wavebands used in remote sensing.
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For stratospheric and orbital imaging, atmospheric transmission is a critical consideration. For instance, attempting to capture images in the 6–7 µm range is impractical due to complete atmospheric absorption. The chart above illustrates atmospheric transmission from an altitude of 50 km. It was generated using the online version of MODTRAN, available at http://modtran.spectral.com/modtran_home, with default parameters. The atmosphere transmits effectively in several bands: 0.3–1.3 µm, 1.4–1.8 µm, 2.0–2.5 µm, 3.4–4.2 µm, and 8–14 µm (with numerous narrow absorption bands). These bands correspond to the ultraviolet (UV, 0.3–0.4 µm), visible (0.4–0.7 µm), near-infrared (NIR, 0.7–1.0 µm), short-wave infrared (SWIR, 1.0–1.8 µm), mid-wave infrared (MWIR, 3–5 µm), and long-wave infrared (LWIR, 8–14 µm) regions.
The choice of detector also influences the usable waveband. For example, silicon detectors are effective in the 0.35–0.95 µm UV, visible, and NIR bands. Indium gallium arsenide (InGaAs) detectors cover the 0.4–1.7 µm range, excluding the UV but including the SWIR. Indium antimonide (InSb) detectors are commonly used for MWIR, and mercury cadmium telluride (HgCdTe) detectors are employed in the LWIR. While we have not used them, strained-layer superlattices (SLS) are reportedly tunable to wavelengths from 1 to 14 µm. Microbolometer-based image sensors are primarily used in the LWIR. Our expertise lies in designing lenses for these sensors, not in sensor technology itself. For detailed information on specific detectors, please consult specialized resources.
While mirrors offer broad spectral coverage, functioning across wavelengths from X-ray to far-LWIR, lens-based systems are constrained by material transmission characteristics, demanding specific material selection for optimal performance in each waveband. Specifically, UV, MWIR and LWIR lenses require different materials than visible, NIR and SWIR lenses.
In the near UV, visible, NIR and SWIR, optical glass is the material of choice for lenses. Schott, Ohara, CDGM, and Hoya are the major manufacturers of these glasses. Each makes at least 100 different types of glass which are used by lens designers like us to optimize the image quality produced by the lens. You can get a sense of why this is so difficult by checking out our page on optical aberrations.
Citation: Schott’s TIE35: Transmittance of Optical Glass (US)
Optical glass transmission diminishes to near zero outside the range of 0.35 – 2 µm, necessitating the use of crystalline materials or chalcogenide glasses. Crystalline materials are the only choice for UV, while chalcogenide glasses also work well for MWIR and LWIR lenses. Examples of crystalline materials, with their transmission ranges in parentheses, include sapphire (0.25–5 µm), calcium fluoride (CaF2, 0.2–8 µm), silicon dioxide (SiO2, quartz/fused silica) (0.2–4µm), magnesium fluoride (MgF2) (0.11–7.5µm), zinc selenide (ZnSe) (0.6–21µm), silicon (Si) (1–7µm), and germanium (Ge) (2–15µm). For a more complete list, check out https://www.crystran.com/optical-materials.
Recently, Meta introduced silicon carbide (4H-SiC) (0.37–5.6µm) as an optical material with good transparency in the visible range and a refractive index of 2.6. Chalcogenide glasses, typically composed of germanium, arsenic, and selenium, offer excellent properties for infrared lenses, despite their toxicity. For more information on Schott’s chalcogenide glasses, please visit: https://www.schott.com/en-us/products/infrared-glasses-and-materials-p1000261/technical-details.
If you have a remote sensing project and are interested in learning more, please contact us. We would be happy to discuss the details of your project.