Scene simulation for passive and active millimetre and sub-millimetre wave imaging for security scanning and medical applications

The benefits of moving up in frequency from the millimetre wave region towards a frequency of I THz are those of smaller systems and better diffraction limited image resolutions. Limitations will be examined, considering effects such as the absorption in the atmosphere, various materials, the human body and fundamental radiometric noise limitations. The physics behind these considerations will be examined and results and artefacts presented using scene simulation. Conclusions are that above 500 GHz high atmospheric absorption severely limits imager to subject distances to a few hundred metres. The effect of absorption is poor subject illumination and high signal attenuation between the subject and the imager. These limitations may be over come partially, for short imager to subject distances (less than a few hundred metres), by using active illumination with narrow bandwidth radiation sources. However, transmit powers rise steeply with imager to subject distance and radiation frequency, lying typically between I mW and 1 W over the frequency band 500 GHz to I THz, for a radiation bandwidth of I GHz and an imager to subject distance of 20 in. Similar systems analysis for medical applications indicates that the high attenuation in human tissue limits probing or penetration distances of the radiation. Radiometric photon noise, electrical properties of human tissue and irradiation power restrictions (I mW/cm(2)) limit maximum diagnostic depths to between two and one millimetres between the frequencies of 100 GHz and I THz.