Infrared sensors in enhanced vision systems come in two varieties: cooled and uncooled. They operate at wavelengths that correspond to "windows" in the spectrum where atmospheric absorption of infrared energy is minimised - mid-wave (3-5µm) and long-wave (8-12µm).
Cooled EVS sensors operate in mid-wave infrared (MWIR), while uncooled systems operate in long-wave infrared (LWIR). Cryogenically cooled MWIR sensors have higher spatial and thermal resolution and - so far - are the only devices certificated for low-visibility approach guidance, but uncooled LWIR systems are smaller, simpler and cheaper.
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CMC's M-series uncooled microbolometer shows LWIR's scene imaging strengths |
EVS sensors use different types of infrared detector. The CMC and Kollsman cooled systems use indium antimonide, a quantum detector that converts photons directly into an electrical signal.
Uncooled sensors produced by Max-Viz and FLIR Systems have solid-state microbolometer detectors using vanadium oxide, a material that changes resistance with temperature. Forward.Vision and L-3 use barium strontium titanate, an older ferroelectric material that works like a capacitor, infrared energy creating a localised electric charge.
To be used for low-visibility approach guidance, EVS sensors must be able see the runway approach lights, but incandescent lighting emits short-wave infrared (SWIR) at 1-2µm. CMC and Kollsman handle this by extending the detection range of their MWIR sensors down to SWIR wavelengths. Max-Viz adds a short-wave (indium gallium arsenide) sensor to the LWIR system and fuses the images.
European airports are introducing energy-saving LED runway-edge and taxiway lighting that emits only visible wavelengths and is invisible to infrared sensors. As a result, Max-Viz is adding a third sensor, a charge-coupled device camera, to its EVS. LED lamps have yet to be approved for use in runway approach lighting.
Source: Flight International
