All objects with an absolute temperature over 0 K emit infrared (IR) radiation. Infrared radiant energy is determined by the temperature and emissivity of an object and is characterized by wavelengths ranging from 0.76 (the red edge of the visible range) to 1000 μm (beginning of microwaves range). The higher the temperature of an object, the higher the spectral radiant energy, or emittance, at all wavelengths and the shorter the peak wavelength of the emissions. Due to limitations on detector range, IR radiation is often divided into three smaller regions based on the response of various detectors
IR radiation has small energy when compared to Visibile or UV rays (energy is inversely proportional to wavelength), these detectors are cooled down to cryogenic temperatures in order to increase infrared detection efficiency/sensitivity. Cooling methods include Stirling cycle engines, liquid nitrogen and thermoelectric cooling (). Cooled thermal imaging cameras are the most sensitive type of cameras to small differences in scene temperature.
Quantum detectors react very quickly to changes in IR levels (response time order of μs), however they have response curves with detectivity that varies strongly with wavelength.
Cooled quantum detector materials include – InSb, – InGaAs, – PbS, – PbSe, – HgCdTe (MCT).
Short-wave infrared (0.9 to 1.7 µm): mainly InGaAs detectors cover this region
Mid-wave infrared (3 to 5 µm): covered by Indium antimonide (InSb), HgCdTe and partially by lead selenide (PbSe)
Long-wave infrared (8 to 14 µm): this region is covered by HgCdTe and microbolometers
COMMON INFRARED (IR) MATERIALS
Zinc Selenide (ZnSe)
Zinc Sulfide (ZnS)
Zinc Sulfide MultiSpectral (ZnS MS)
Gallium Arsenide (GaAs)
ANTI-REFLECTIVE AR COATING
Anti-reflective (AR) coatings are thin films applied to surfaces to reduce their reflectivity through optical interference. An AR coating typically consists of a carefully constructed stack of thin layers with different refractive indices. The internal reflections of these layers interfere with each other so that a wave peak and a wave trough come together and extinction occurs, leading to an overall reflectance lower than that of the bare substrate surface. Anti-reflection coatings are included on most refractive optics and are used to maximize throughput and reduce ghosting. Perhaps the simplest, most common anti-reflective coating consists of a single layer of Magnesium Fluoride (MgF2), which has a very low refractive index (approx. 1.38 at 550 nm)
HARD CARBON ANTI-REFLECTIVE HCAR COATING
HCAR is an optical coating commonly applied to Silicon and Germanium designed to meet the needs of those applications with optical elements exposed to harsh environments, such as military vehicles and outdoor thermal cameras. This coating offers highly protective properties coupled with good anti-reflective performance, protecting the outer optical surfaces from high velocity airborne particles, seawater, engine fuel and oils, high humidity, improper handling, etc.. It offers great resistance to abrasion, salts, acids, alkalis, and oil. Continue reading →