14-24] SYSTEM CONSIDERATIONS 795 



field element, contrast disappears. There is no detection of the target from 

 this altitude or higher. The determination of this altitude is dependent on 

 the radiative characteristics and size of the target as well as on detector 

 sensitivity and altitude. Therefore, a maximum altitude is determinable 

 only for each individual target with its characteristics. 



The optical system must have sufficient aperture to assure enough 

 energy into the sensor for a usable signal-to-noise ratio. In general, infrared 

 optical systems are reflective and thus spectrally nonselective. If refractive 

 elements are used, their transmission characteristics must be compatible 

 with the detector's spectral response. Since for most detectors (P-type 

 gold-doped germanium is a notable exception) the detectivity varies 

 inversely as the square root of the sensitive area, small cells should be used. 

 This permits small instantaneous fields of view with relatively "fast" (low 

 / number, or small diameter-to-focal4ength ratio) optical systems, thus 

 contributing to compactness. 



With typically small instantaneous fields of view, either spherical or 

 parabolic primary reflectors may be used. More important is the assurance 

 that the circle of least confusion is sufficiently small for the resolution not 

 to be limited by the optics. This, of course, becomes impossible as an 

 approach of within a factor of 2 to the Rayleigh resolution limit is made. 

 High-speed scanning requires careful evaluation of scanning element 

 deformation. Slight deformation will seriously reduce resolution and 

 picture quality. The method of scan must be such that the instantaneous 

 image always falls at a fixed point in image space. This permits the sensor 

 to be fixed in space, an extremely important feature as far as noise is 

 concerned. In conjunction with the scanning operation, it is necessary to 

 assure constant aperture for all scan angles and to maintain constant 

 optical efficiency. Otherwise, nonuniform intensity across the picture 

 results. 



A basic consideration in the selection of detector characteristics is that of 

 spectral response. It is necessary to determine the spectral distribution of 

 the energy entering the system. Water vapor and carbon dioxide are the 

 two constituents of the earth's atmosphere that are most effective in 

 absorbing infrared radiation. By far the greatest amount of absorption 

 occurs between ground level and about 30,000 ft. Fig. 14-28 shows a 

 typical transmission curve for the atmosphere. While conditions change 

 considerably and thus affect such transmission curves seriously, neverthe- 

 less the effect is in absolute value rather than spectral distribution. The 

 absorption must be taken into account to the best approximation that 

 available data and expected weather environment will permit. 



The parameters thus far determined are basic in determination of the 

 "best" infrared detector to be used. Conversely, final choice of the cell 

 limits the range of values these parameters may assume. The spectral 



