Sampling passive millimetre wave imagery

摘要

This paper describes the compromise necessary between image sampling and thermal sensitivity in the design of passive millimetre wave imaging systems. The use of linear arrays of receivers in fast (f/0.5) conical scanned systems, for real time imagery at 35GHz and 94GHz, presents particular difficulties. Analysis is presented which shows that it is not possible, with a single row of receivers, to achieve both high optical efficiency which equates to thermal sensitivity and good sampling of the image. Two methods are discussed for overcoming this limitation. In the first method, good efficiency and sampling are achieved simultaneously with multiple rows of receivers; however this method uses more receivers than predicted by basic theory. In the second method, the focal length in image space is increased by introducing an additional mirror in a Cassegrain configuration. This allows a single row of receivers to sample the image at the Nyquist rate, but results in an efficiency loss. The choice of which method to use will depend on the application. If the application requires the spatial resolution to be maximized and there is a high contrast scene, the Cassegrain method could be chosen. However, if high efficiency and resolution are both required a multiple row solution would be preferred. A 90GHz imager which uses 5 rows is discussed and contrasted to an alternative design that uses the Cassegrain approach.