wavelength region they are 340, 410, 650, and 850 GHz. Above 1 THz, the primary window of interest is centered at 1.5 THz.

FIGURE 2-3 Atmospheric attenuation under various environmental conditions from 10 GHz to 10,000 GHz.

If an imaging or spectroscopic system were to be employed in an air-conditioned facility such as an air terminal, the curve for the clear air would be appropriate. Figure 2-3 clearly shows that lower frequencies transmit through the atmosphere with less attenuation than higher frequencies for all three conditions shown.

While atmospheric attenuation by itself is not a measure of possible performance, it does give insight into the difficulty of performing imaging and spectroscopy at various distances. As the attenuation increases, contrast will be reduced for a given distance. In a passive system that relies on natural illumination or emissions from objects, attenuation exceeding 20 decibels (dB) (100:1) will reduce contrast of typical natural scenes to the point of being indecipherable. The decibel is a means of describing relative power. It is defined as:

where P1 and P2 are two power levels that are being compared. So 10 dB is equivalent to a 10:1 ratio, 20 dB is equivalent to a 100:1 ratio, and so forth. With active systems, if transmitter power could be increased an unlimited amount, contrast could still be maintained, assuming the atmosphere does not contain particulate matter that also might reflect energy. To a first order, it can be stated that as the atmospheric attenuation increases, the range at which good image quality can be maintained will decrease.

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