We investigate the spectral response of a THz imaging system based on ultrawideband cryogenic microbolometers.
The bandwidth if this system, nominally 0.2 - 1.8 THz, is broad enough to span large variations (>10 dB) in
clothing transmittance and diffraction-limited spatial resolution (factor of x8), factors that are presumably partly
responsible for the unusually high quality of the images taken with it. The chief tools we have used for this are a
simple THz monochromator based on a specially designed frequency selective surface, and a specially designed
blackbody source that provides an accurately known power spectral density over the full bandwidth of the imager.
Two completely independent measurements of the microbolometer's spectral response, in the first case using a
filtered blackbody and in the second using an ultrabroadband, THz photomixer, referred to a Golay cell, agree
within 5%. Evidence of frequency-dependent scattering from ordinary clothing material, distinct from simple linear
attenuation, is presented from an idealized laboratory experiment. However, the scattering is relatively weak, and
unlikely to have a significant effect in practical THz imaging scenarios, particularly with ultrawide bandwidths.