Outdoors at Relatively Low Frequency
Indoors there are typically not large thermal differences across a target and signal margins can be small. As a result, many of the SMM/THz passive images take advantage of cold sky illumination out of doors. Since the atmospheric transmission to space is best at relatively low frequency, these images are often made at 94 GHz [1]. Figure: Passive Image shows one of the best known examples.
Figure: Passive Image. Passive 94 GHz images
with cold sky illumination (From R. Appleby).
References
- Passsive millimeter wave imaging Microwave 4, 39-50 (2003). Google Scholar