6-8] 



INFRARED SYSTEMS 



343 



100 



Ji? 40- 



Background 



Predominantly 



Scattered 



Sunlight 



Mbsent at' 



V Night 



+ 



Background 

 Predominantly 

 Black Body 

 Radiation 

 Night and^ 

 Day 



U Energy Distribution 

 ^ From Black Body 

 at 600°K 



4\ 



rx Transmission of 

 10 mi. of Sea - Level 

 Air Containing 6 cm 

 Vapor 



Jb Cloud and Heavy 

 Haze Background 



(^Energy Distribution' 

 From 300°K Black 

 vBody 



4 X Exaggerated 



123456789 10 



WAVELENGTH (MICRONS) 



Fig. 6-35 The Useful IR-Frequency Spectrum. 



course, force a different choice. The only really flexible parameters for the 

 designer, then, are the areas of the collector mirror and of the cell. These 

 are in turn influenced by the requirements of the scanning system. 



Scanning System Characteristics. The choice of a scanning system 

 generally represents a compromise between the requirements of the system 

 and the mechanization advantages of rotary optics (particularly for high- 

 speed scanning) and fixed detector elements (which simplify cooling 

 problems and maintenance of cell sensitivity). As an example of the type 

 of analysis which must be performed to assess a given scanning technique, 

 the rosette scanning pattern previously discussed will be analyzed to 

 determine the interrelations among scan time, resolution, coverage, and 

 detection element characteristics. 



The important parameters of the scanning system are: the instantaneous 

 field of view a (radians) square; the whole field of view which is here (see 

 Fig. 6-34) a function of the half-scan angle 7; and the time T required for 

 the whole field to be covered. In order to completely cover the field the 

 number of spokes in the rosette pattern is equal to the number of instan- 

 taneous fields required to cover the periphery of the whole field, or 



—r—. spokes. (6-56) 



a/ sm 7 



