The Radiometer Operation Principle


This paper was written for the executives and sales representatives who had to explain the BIS_WDS, a microwave passive concealed object detector, to security experts and other potential buyers.


The purpose of the BIS_WDS is the detection of objects that subjects under surveillance could be concealing beneath their clothes. The principle for this detection relies on the fact that these hidden objects are always cooler than the skin in 4 o more degrees Celsius.  All surfaces with temperatures above 0 Kº radiate in all frequencies, since the spectrum of this radiation depends on its absolute temperature, measuring this radiation in a convenient band can differentiate a hidden object from the skin in the background.


Radiation spectra for different temperatures can be seen in Fig. 1.  The wave length at which the radiation peaks grows as temperature decreases. When it gets down to body temperature, this peak goes into the infrared with a wavelength of 9500 nm (~10 mm), a region to be found way beyond  the right limit of the Fig 1. This is the zone where thermal imaging cameras operate; producing the all too familiar pseudo color images that Hollywood loves to show off in their action movies. The problem with using infrared for detecting concealed objects is that clothing may not be transparent enough to the infrared.  However, cloth is perfectly transparent to microwave and, at body temperature, there’s also some microwave radiation, but only with one thousand times less power.


The chances for detecting a particular radiation depend, not only its power, but also on the background noise level. Microwaves over 200 GHz are polluted with CMB or Cosmic Microwave Background (famous for generating the Big-Bang theory about the origin of the Universe).  Below 10 GHz it gets polluted again, this time with atmospheric and galactic backgrounds and last, but not least, the ever growing human communication interferences.  Between these two limits the microwave bands are very clean making bodily radiation in this band detectable.


There are few choices for clean Microwave bands,




Lower bands not only exhibit less radiation power, but require bigger horn antennas that complicate the optical and mechanical parts of the unit. On the other hand, microwave components for the D band could make the unit pricy and uncompetitive. So that’s why the choice for the BIS_WDS is around the E and W bands.

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