INFRARED IMAGING SYSTEMS

Aviation Electronics Technician 1 (Organizational)
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Figure 6-5.-Photoconductive detector circuit. Energy-Matter Interaction There are two basic types of energy-matter interaction. They are the thermal effect and the photon effect. THERMAL EFFECT.— The thermal effect type of energy-matter interaction involves the absorption of radiant energy in the detector. This results in a temperature increase in the detector element. The radiation is detected by monitoring the temperature increase in the detector. Both the elemental and imaging forms of detectors use the thermal effect. PHOTON EFFECT.— In the photon effect type of energy-matter interaction, the photons of the radiant energy interact directly with the electrons in the detector material. Usually, detectors using the photon effect use semiconductor material. There are three specific types of photon effect detection. They are photoconductivity, photoelectric, and photo- emissive. 1. Photoconductivity. Photoconductivity is the most widely used photon effect. (See figure 6-5.) Radiant energy changes the electrical conductivity of the detector material. An electrical circuit is used to measure the change in the conductivity. 2. Photoelectric (also referred to as photo- voltaic). In the photoelectric effect (fig. 6-6), a potential difference across a PN junction is caused by the radiant signal. The photocurrent (current generated by light) is added to the dark current (current that flows with no radiant input). The total current is proportional to the amount of light that falls on the detector. 3. Photoemissive. The photoemissive effect (fig. 6-7) is also known as the external photo effect. The action of the radiation causes the emission of an electron from the surface of the photocathode to the surrounding space. The electron is photoexcited from the Fermi level above the potential barrier at the surface of the metal. INFRARED IMAGING SYSTEMS An infrared imaging system has the following components: detectors, a scene dissection system, front end optics, a refrigeration system, and an image processing system. Detectors Detectors convert the IR radiation signal into an electrical signal that is processed into information used by the operator. Detectors can be arranged in many different configurations for their use in an IR imaging system. DETECTOR ARRAY.— Only a small portion of the image scene is taken by a detector (or detectors) to Figure 6-6.-Photoelectric effect. Figure 6-7.-Photoemessive effect. 6-5

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