Aviation Electronics Technician 1 - Historic Naval Ships Association

The total energy emitted by an object at allwavelengths is directly dependent upon itstemperature. If the temperature of a body is increased10 times, the IR radiation emitted by the body isincreased 10,000 times. If the energy emitted by ablack body and its wavelengths is plotted on a graph,a hill-shaped curve results (fig. 6-4). By looking atthis graph, you can see that the energy emitted byshort wavelengths is low. As the wavelengths getlonger, the amount of energy increases up to a peakamount. After the peak is reached, the energy emittedby the body drops off sharply with a further increasein wavelength.Infrared OpticsMany of the materials commonly used in visiblelight optics are opaque at IR frequencies. For thisreason, they cannot be used in IR imaging systems.The optical material used in IR imaging systemsshould have a majority of the following qualities:. Be transparent at the wavelengths on which thesystem is operatingl Be opaque to other wavelengths. Have a zero coefficient of thermal expansion toprevent deformation and stress problems in opticalcomponents. Have high surface hardness to prevent scratchingthe optical surfaces. Have high mechanical strength to allow the useof thin lenses (high-ratio diameter to thickness)l Have low volubility with water to preventdamage to optical components by atmosphericmoisture. Be compatible with antireflection coatings toprevent separation of the coating from the opticalcomponentNone of the materials currently used for IR opticshave all of these qualities. However, silicon,germanium, zinc selenide, and zinc sulfide have manyof them.Infrared DetectorsThe detector is the most important component ofthe IR imaging system. There are many types ofdetectors, each having a distinct set of operatingcharacteristics. Bolometers, Golay cells, mercurydopedgermanium, lead sulfide, and phototubes arethe most commonly used types of detectors.Detectors can be characterized by their opticalconfiguration or by the energy-matter interactionprocess. There are two types of optical configurations—elementaland imaging.ELEMENTAL DETECTORS.— Elementaldetectors average the portion of the image of theoutside scene falling on the detector into a singlesignal. To detect the existence of a signal in the fieldof view, the detector builds up the picture bysequentially scanning the scene. The elementaldetector requires time to develop the image becausethe entire scene must be scanned.Figure 6-4.-Black body radiation.IMAGING DETECTORS.— Imaging detectorsyield the image directly. An imaging detector isconsidered a myriad of point detectors. Each of thedetectors responds to a discrete point on the image.Therefore, the imaging detector produces the entireimage instantaneously. A good example of animaging detector is photographic film.6-4