Near-Millimeter Wave Radiometric Imaging

A unique feature of the far infrared/near millimeter wave spectral region is that many dielectric and organic media that are opaque in the visible and infrared become transparent [1-4]. Thus it is possible to image these materials or “see through” them for the purpose of inspection, non-destructive testing or concealed object detection. The resolution is degraded relative to the visible and infrared imagery, however, it is still adequate for many applications. To demonstrate the potential of far infrared/near millimeter wave imagery, a simple imaging apparatus was constructed in our laboratory several years ago [3, 4]. Small samples, placed on a mechanical stage negotiating a raster scan, were interposed between an FIR laser and a liquid helium cooled GaAs detector. The amplified detector output modulated a light emitting diode affixed to the stage, and the photographic recording of the diode signal provided an optical rendition of the far infrared transmission (or reflection) image of the object. While this early system demonstrated many of the useful features of FIR/NMMW imagery, the restrictions of active illumination and helium cooled detection, and the ability to examine only small objects limited the utility of the approach.