最后,热成像摄像机章节将热成像摄像机定义为利用红外(IR)辐射生成图像的设备,其原理类似于普通相机利用可见光形成图像。 Instead of the 400-700 nanometre ( nm ) range of the visible light camera, infrared cameras are sensitive to wavelengths from about 1,000nm(11,000 \mathrm{~nm}(1 micrometre or mum\mu \mathrm{m} ) to about 14,000nm(14 mum)14,000 \mathrm{~nm}(14 \mu \mathrm{~m}). In opinion of the author this definition is wrong for two reasons. First, it is not mentioned that thermal radiation is to be used to create output image. Second, it suggest that even non cooled SWIR cameras sensitive in 1-1.7 mum\mu \mathrm{m} band can be treated as thermographic cameras in situation when such imagers cannot see targets of typical Earth temperatures.
Optical radiation (light of wavelength from about 100 nm to about 1 mm ) is typically divided according to wavelength on different spectral ranges: infrared, visible and ultraviolet. The infrared range is further divided into: near infrared NIR ( 0.78-1mum0.78-1 \mu \mathrm{~m} ), short wave infrared SWIR ( 1-3mum1-3 \mu \mathrm{~m} ), mid-wave infrared MWIR (3-6 mum\mu \mathrm{m} ), long-wave infrared LWIR (6-15 mum\mu \mathrm{m} ), and far infrared FIR ( 15-1000 mum15-1000 \mu \mathrm{~m} ).
Thermal radiation emitted by targets of typical Earth temperatures from about -40^(@)C-40^{\circ} \mathrm{C} to about 80^(@)C80^{\circ} \mathrm{C} dominates in medium-wave infrared and long-wave infrared (the spectral range from about 3mum3 \mu \mathrm{~m} to 15 mum\mu \mathrm{m} ) over radiation emitted by sun, moon, stars, and sky. Totally inverse situation exists in visible, near infrared range and short infrared range (VIS-SWIR band - wavelengths below 3mum3 \mu \mathrm{~m} ) where thermal radiation emitted by Earth targets is almost not existing.
目标靶标发射的热辐射在靶标与热像仪之间的大气中发生衰减。 Atmosphere transmission depends on many factors (distance, humidity, temperature, altitude, weather conditions) but for any case it can be said that atmosphere transmits relatively well only in two so called atmospheric windows: MWIR window (about 3-5 mum\mu \mathrm{m} ) and LWIR window (about 8-2mum8-2 \mu \mathrm{~m} ). It should be noted that inside MWIR window there is an internal absorption band (about 4.154.35 mum)4.35 \mu \mathrm{~m}). Typical transmission spectrum is shown in Fig. 2.
Further on, it should emphasized that average spectral exitance of typical targets is much higher in LWIR spectral band comparing to MWIR spectral band. In detail this ratio is approximately 24 times for targets of typical lab temperature 20^(@)C20^{\circ} \mathrm{C}. This ratio clearly indicates that even if both atmospheric windows can be used for thermal imaging then MWIR imagers of much higher sensitivity are needed to compensate much weaker radiance in this spectral band.
从历史上看,红外热像仪一般分为三代。
First generation are two directional scanning imagers. They are built using an image sensor with so low pixel number that two-direction scanning is needed. The image sensor can be in different forms: discrete detector, simple non-multiplexing photo-conductive linear arrays (typically PbSe,InSb\mathrm{PbSe}, \mathrm{InSb} or HgCdTe ) of elements number not higher than about one hundred, or the SPRITE detectors. These imagers usually operated in 8-12 mum8-12 \mu \mathrm{~m} spectral range, use the optics of F//2-F//4\mathrm{F} / 2-\mathrm{F} / 4 number, and are characterized by temperature resolution NETD about 0.2 K . Nowadays, first generation camera can be treated as totally extinct group of thermal imagers.
Second generation thermal imagers are one directional scanning imagers. They are built using a linear image sensor with so high pixel number that only one-direction scanning is needed. In detail, Gen. 2 scanning cameras are built using linear arrays of line-elements number higher than about 100 but lower than about 612. Temperature resolution NETD of these cameras is improved up to the level of about 0.1 K.
Thermal cameras built using improved multi-linear FPAs can be treated as a subgroup of Gen. 2 imagers and are called Gen 2+2+ imagers. Temperature resolution of Gen 2+2+ can be improved up to the level of about 0.05 K . Typical examples of these systems are HgCdTe multilinear 288 xx4288 \times 4 arrays fabricated by Sofradir (presently Lynred) both for 3-5mum3-5 \mu \mathrm{~m} and 8-10.5 mum8-10.5 \mu \mathrm{~m} bands with signal processing/enhancement in the focal plane (photocurrent integration, skimming, partitioning, TDI function, output preamplification and some others). Gen. 2+2+ imagers are characterized by smaller weight and size and improved reliability.