14-25] MAJOR SYSTEMS FEATURES 799 



may be compared in two basic ways. First, a more or less ideal response 

 may be calculated when only the system parameters are known. Thus 

 system comparison on a theoretical basis may be accomplished with system 

 evaluation in terms of chosen parameters resulting. This type of evaluation 

 is useful as a preparation for final systems design. However, the comparison 

 value is open to question because of the latitude in assigning values to the 

 numerous parametric quantities involved, and because of the complexity of 

 preparing a concise form of comparison. The comparison at best provides 

 an indication of what the systems involved should do under various circum- 

 stances rather than what they actually can do. 



The second basic method of comparison is that of experimentation. In 

 this method each system would be subjected to experimental evaluation, 

 with the system itself as the measuring instrument. The entire system, 

 including display and recording units, is therefore included. Experimental 

 comparison leaves no room for doubt as far as the difference between theory 

 and practice is concerned. 



The choice of parameters for the proposed evaluation scheme is justified 

 by the following line of reasoning. The ordinate value is the reciprocal of 

 the product of the minimum measurable temperature difference and the 

 instantaneous field of view. The latter parameter is a measured value and 

 can be readily determined. In itself it has little meaning, but its value 

 together with bandwidth determine in part the minimum measurable 

 temperature difference. In view of this, and because of the direct relation- 

 ship with detector size and focal length of the optics, the instantaneous 

 field of view (A<^) is a basic scanning-system parameter. Being a geomet- 

 rically measured quantity, there can be little more than negligible error 

 associated with its determination. The choice of minimum measurable 

 temperature difference (AT) is clear. Given any number of systems, each 

 with fully acceptable characteristics (resolution, etc.) for a particular 

 application, the best system will be the one capable of the smallest AT 

 measurements over the temperature range in question. Effective scan rate 

 in conjunction with the ordinate parameters determines the velocity and 

 height limitations under which the system will be effective. 



The experimental procedure would be as follows: 



Two black body apertures are considered to lie in a uniform and much 

 cooler object space. Each subtends a fixed angle at the optics; probably 

 0.25 milliradian is a good choice. This value is chosen so that the instan- 

 taneous field of view will almost assuredly be larger than the black body 

 apertures. The black body apertures are placed so that they may be readily 

 resolved by the scanner. Then one black body is adjusted to a specific 

 temperature while the other is adjusted to a temperature just sufficiently 

 higher than the first for the scanner recording system to identify a change 

 in gray level between black bodies. This AT, or difference in black body 



