  Mitigating the far sidelobes of a wide field-of-view telescope is one of the
critical issues for polarization observation of the cosmic microwave
background. Since even small reflections of stray light at the millimeter-wave
absorbers inside the telescope may create nonnegligible far sidelobes, we have
developed a method to measure the reflectance of millimeter-wave absorbers,
including diffuse reflections. By applying the planar near-field measurement
method to the absorbers, we have enabled two-dimensional diffuse-reflection
measurements, in addition to characterizing specular reflection. We have
measured the reflectance of five samples (TK RAM Large and Small Tiles and
Eccosorb AN-72, HR-10, and LS-22) at two angles of incidence in the frequency
range from 70 GHz to 110 GHz. Compared with conventional horn-to-horn
measurements, we obtained a consistent specular reflectance with a higher
precision, less affected by standing waves. We have demonstrated that the
angular response and diffuse-to-specular reflectance ratio differ among various
materials. The measurements also imply that some absorbers may affect the
polarization direction when reflecting the incident waves.
