4. Optical and Electrooptical Techniques 



a. Description 



This category of sensors includes the traditional visual, 

 aided visual, and photographic techniques, as well as the more sophis- 

 ticated, recently developed methods of electrooptics typified by low- 

 light-level television and infrared imaging systems. Here, the optical 

 regime is construed not as the limited spectral range of the eye (i.e., 

 visible light) but as the entire range from the long-wave infrared to 

 the near ultraviolet (see Figure 2) from submillimeter wavelengths (less 

 than 1000 \i) to about 0.25 n (2500 A). Longer wavelengths (in the milli- 

 meter range) are more properly considered microwave; shorter (UV) wave- 

 lengths are of little interest in the present context since such energy 

 is absorbed by the atmosphere and is therefore not useful for long-range 

 observation. 



Extending the sensitivity of sensor systems beyond the 

 visible can provide several advantages, well known to military recon- 

 naisance specialists: 



• Many surfaces exhibit very different reflectivities 

 at nonvisible wavelengths than in the visible; thus, 

 objects that would be difficult to distinguish from 

 the visual background may become more readily de- 

 tectable at near-infrared wavelengths. 



• Sunlight (and moonlight/starlight) contains much 

 near-IR energy; thus, a detection system that uses 

 both visible and IR wavelengths obtains an improve- 

 ment in illumination and, therefore, in sensitivity. 



• While there is very little long-wave infrared (LWIR) 

 energy in sunlight and starlight (e.g., in the range 

 from 8 to 15 |i), objects at "room" temperature 



(~25 C, 75 F) and above emit self-radiation -.n pro- 

 portion to their temperature, the so-called "thermal 

 signature." Thus objects, such as ships, which are 

 warmer than their surroundings (the sea) stand out 

 in LWIR images, developed by TV-like scanning systems 



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