  Due to high dynamic range and ease of use, continuous wave terahertz
spectroscopy is an increasingly popular method for optical characterization of
components used in cosmic microwave background (CMB) experiments. In this work,
we describe an optical testbed that enables simultaneous measurements of
transmission and reflection properties of various radiation absorbing
dielectric materials, essential components in the reduction of undesired
optical loading. To demonstrate the performance of the testbed, we have
measured the reflection response of five absorbers commonly used for such
applications: TKRAM, carbon- and iron-loaded Stycast, HR10, AN72, and an
in-house 3D printed absorber across a frequency range of 100 to 500 GHz, for
both S- and P-polarization, with incident angles varying from 15 to 45 degrees.
We present results on both the specular and scattered reflection response of
these absorbers.
