The design and analysis of infrared systems requires a consideration of the targets, the background, and the intervening medium, as well as all the components of the equipment itself, the optics, the detectors, the electronics, and the display or processing.
This chapter, a rather mathematical one, derives some simple expressions for the SNR in terms of the target, background, transmission, and components. Although the results have certain approximations and idealizations, they are a useful starting point in the iterative design process. The steps in the derivation are just as important. The observant reader can then adapt the processes to his own application.
The infrared system consists of the target, the background, the properties of the intervening medium, the optical system, the detector (transducer), the electronics, and the display. These are all shown in Fig.6-1. Every infrared system has all of these, although some may be much more elaborate than others. It is tempting to think of the system as just the equipment and not the source, background, and medium. Yielding to this temptation is a sin that will be redeemed in failure. All these components, substances, and physical entities must be considered. The target is what you want to see; the background is what you don't. One man's target is another man's background. Consider the case of people walking in front of a cornfield. If you are designing a remote sensing system, the cornfield is the target and the people are background, even though they are in the foreground. If you are doing night driving, the people are the target(s) and the cornfield is the background. Note that sometimes targets are to be hit and sometimes they are to be avoided. The atmosphere may reduce the level from both the target and background by transmission losses that arise from both absorption and scattering.
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