SOURCES

Sources include self-emission from hot objects that generally follow a black-body radiation curve depending on the temperature of the source, modified by the spectral emissivity of the object. Alternatively for passive sensors, the reflection or transmission modification of ambient sources can be detected. During daytime, the dominant source in the visible is the sun. There is a significant night glow in the spectral region around 1.5 μm that makes short-wavelength infrared (SWIR) imaging an alternative to visible image intensifier night vision goggles for some night vision applications.3,4 The semiconductor absorbance ranges that enable passive night vision and the nightglow irradiance spectrum are illustrated in Figure 2-1. The peak of the room temperature blackbody curve is at about 10 μm in the infrared.

TRANSMISSION

Spectral Regions

Over the years a number of designations for spectral regions have become somewhat standard, but there is significant overlap and it is useful to define the regions used in this report to assist the reader (see Table 2-1). The transitions between these regions are not sharply defined and the designations are to be interpreted loosely; the detection mechanisms, the transmission, and the dominant noise sources all vary across these bands. These definitions help in discussing those variations cohesively.

Electromagnetic sensors cover the entire range from 200 nm to 20 μm and beyond; this taxonomy is intended merely to provide a nomenclature for the most frequently used bands for long-range imaging.

Atmospheric Transmission

Atmospheric transmission is an important aspect of any terrestrial remote sensing application. Figure 2-2 shows the atmospheric transmission across the 0.2-20 μm region (~1 km horizontal path length at sea level, temperature = 15°C, with 46 percent relative humidity) along with the wavelength bands defined above.

While Figure 2-2 is representative, the transmission curve will vary with atmospheric conditions, as well as the path taken through the atmosphere; for example,

3

T.R. Hoelter and B.B. Barton. 2003. Extended short wavelength spectral response from InGaAs focal plane arrays. Proceedings of SPIE 5074:481-490.

4

Available at http://www.sensorsinc.com/downloads/paper_HighSpeedSWIRImagingAndRangeGating.pdf. Last accessed March 25, 2010.



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