340 



GENERIC TYPES OF RADAR SYSTEMS AND TECHNIQUES 



Infrared 



Transparent 



Dome (IR Dome) 



Fig. 6-33 Typical IR System. 



mirrors by the angles a and /3 away from the perpendicular to the optic axis 

 and then rotating them about the optic axis in opposite directions. If the 

 mirrors turn at equal speed, the effect is to move the instantaneous field of 

 view along a straight line, the length of which is determined by the two 

 angles a and /3 and the intermirror distances. The velocity with which the 

 line Is scanned varies sinusoidally; it is most rapid in the center of the line 

 and is slowest at the ends where scan reversal occurs. If one mirror turns a 

 little more slowly than the other, the line scanned ih space rotates slowly 

 about its center, resulting in the rosette scan pattern shown in Fig. 6-34. 

 With this pattern the surveillance capability is greatest in the center of the 

 field, which is crossed on each spoke of the rosette, and diminishes toward 

 the edges, a property which may or may not be desirable. 



In considering the appearance on the scope of a scene scanned by an 

 infrared device, it should be remembered that while our eyes see almost 

 everything by reflected light (i.e., a "semiactive" process similar to some 

 types of radar), the infrared scanner sees mainly thermal radiation emitted 

 by the observed objects themselves. This is particularly true if the radio- 

 meter is filtered to be sensitive only to radiations of wavelength greater 

 than 3 microns, since reflected or scattered sunlight beyond 3 microns is 

 generally negligible compared to emitted radiation. Therefore, a "hot" 

 object such as a city, or one with a high emissivity such as a cloud, will 

 appear bright. The clear sky, bright blue to the eye, will appear black, 

 since the air molecules do not scatter infrared as they do visible light. 



