Passive Sensing

Spectrometers are designed and used to measure radiation over a specified spectral bandwidth. All objects at a temperature greater than absolute zero emit radiation according to Planck's Law.

By operating the spectrometers with cooled detectors and in the 7 to 14 µm (IR) spectral band, the self emission of backgrounds and targets can be exploited. In the IR spectral band, all objects radiate some level of photon flux proportional to the objects temperature (above absolute zero) and its emissivity. It is the detection of this self radiation (emission) due to its temperature that is termed Passive Detection.

In the figure below, the sensor is viewing a background at temperature TB through the atmosphere at temperature TA, and also a chemical cloud at temperature TC . The goal of passive sensors is to detect the presence of the chemical cloud using only the naturally occurring (passive) radiance.

For passive detection to be successful there must be a difference in temperature between the target cloud and the background. The temperature differential can either positive (cloud warmer than background) or negative which results in the cloud being seen in emisssion or absorption. The temperature difference can be as small as a fraction of a degree. The larger the difference (delta temperature), the easier the detection process.

As seen from the figure, the final signal that must be evaluated is a complex combination of radiation from the background, the chemical cloud and atmosphere.

In contrast, an active measurement requires some kind of hot source placed behind some of the cloud and within the spectrometers field of view. This is not always convenient or possible. Successful passive detection generally demands very high sensitivity from the spectrometer which requires a very high quality detector (for 7-13.5mm, a HgCdTe detector cooled to 80K), whereas an active system can use an uncooled detector combined with a very hot source to get the desired signal-to-noise or sensitivity.