  We recently developed and characterized an absorber for millimeter
wavelengths. To absorb the millimeter wave efficiently, we need to develop the
low reflection and high absorption material. To meet these requirements, we
tried to add polystyrene beads in the epoxy for multi-scattering in the
absorber. The typical diameter of polystyrene beads corresponds to the scale of
Mie scattering for the multi-scattering of photons in the absorber. The
absorber consists of epoxy, carbon black, and expanded polystyrene beads. The
typical size of the expanded polystyrene beads is consistent with the peak of
cross-section of Mie scattering to increase the mean free path in the absorber.
By applying this effect, we succeeded in improving the performance of the
absorber. In this paper, we measured the optical property of epoxy for the
calculation of the Mie scattering effect. Based on the calculation result, we
developed the 8 types samples by changing the ratio of absorber material. To
compare 8 samples, we characterized the reflectance and transmittance of the
absorber in millimeter length. The measured reflectance and transmittance of 2
mm thickness sample with optimized parameter are less than 20% and 10%. We also
measured the transmittance in sub-millimeter wavelength. The measured
transmittance is less than 1%. The shape of absorber can be modified for any
shape, such as chip and pyramidal shapes. By using this absorber, we can apply
for the mitigation of stray light of millimeter wave telescope with any shapes.
