Pulsed terahertz bi-directional reflection distribution function (BRDF) measurements of materials and obscurants

We report a pulsed method to measure reflection and scattering from several samples with different degrees of surface roughness and material properties at terahertz frequencies. Reflection from a flat gold mirror shows that the full width half maximum (FWHM) of the terahertz beam angular spread is < 4 degrees for frequency range 0.2 THz to 3 THz with signal-to-noise of 65 dB. Measurement of a paper index card, used as a low scattering sample, shows that the reflection/scattering properties are essentially similar to the system signature response except for multiple reflections between the front and back surfaces of the sample. Sixty-grit sandpaper shows multiple scattering events with almost no signal reflected from the flat backing paper surface. Corduroy cloth shows periodic reflections in the time domain, which correspond to diffraction lobes in the spectral domain.