6-8] INFRARED SYSTEMS 339 



Conversely, infrared does not have the all-weather capabilities of radar, 

 its ability to penetrate haze, fog, and clouds being only slightly better than 

 that of visible light. Background clutter considerations are also more 

 serious, since everything in a typical tactical environment is to some degree 

 a source of infrared radiation, i.e., a potential source of interference. 

 Passive infrared systems — like passive radar systems — also do not possess 

 the capability for measuring range in the direct and convenient manner of 

 active radar systems. 



The use of infrared for detection and tracking is now new, having been 

 vigorously exploited by Germany during World War II. In this country, 

 where reliance was placed more heavily on the development of radar — 

 with obvious beneficial results during the war — serious consideration of 

 infrared systems has been more recent and stems from four facts: (1) 

 modern targets are better sources of infrared radiation than their predeces- 

 sors and in many cases poorer radar targets; (2) many important targets 

 are encountered at high altitudes where attenuation and absorption of IR 

 energy are minimized; (3) infrared is more difficult to countermeasure than 

 radar, or at least the art is not so advanced; and (4) infrared technology has 

 made significant advances since World War II. 



This will be a short discussion of the application of infrared to airborne 

 surveillance and tracking systems. The fundamentals of infrared science 

 are ably covered in an earlier book of this series {Guidance, Chapter 5, 

 "Emission, Transmission, and Detection of The Infrared") and a knowledge 

 gained by reading that discussion will be assumed. 



Basic Principles. Airborne infrared systems generally use mirrors 

 rather than lenses. Lenses are possible but usually not practical because of 

 limitations imposed by the properties of available materials (see Paragraph 

 5-7 in Guidance). The infrared system is composed, then, of a mirror which 

 collects radiation from the target and focuses it on the detector, a means of 

 modulating the radiation striking the detector in order to produce an a-c 

 signal, and a means of discriminating against spurious targets and back- 

 ground radiation. Frequently, modulation and discrimination are ac- 

 complished in the same process. 



Consider, as an illustration, the simplified system shown in Fig. 6-33.^^ 

 Radiation from the target, background, and intervening air enters through 

 a dome of transparent material (Irdome) and is focused on the detector 

 after reflection from the two folding mirrors. The instantaneous field of 

 view is determined by the size of the detector and the focal length of the 

 main collecting mirror. Scanning is accomplished by tilting the two folding 



I'This arrangement is chosen only to illustrate the significance of the system parameters and 

 not for its desirability or efficacy. It is not an example of a system in actual use, since most 

 such systems are classified and cannot be discussed here. 



