Performance of a low-pass filter for far-infrared wavelengths

We describe a low-pass filter that provides high in-band transmittance and excellent rejection at ν > 100 cm(-1). The transmittance of the filter components was measured at liquid-helium temperatures from 10 to 10,000 cm(-1). The total transmittance is >50% for ν < 50 cm(-1) and is calculated to be <10(-9) for ν > 300 cm(-1). The filter was successfully used in a liquid-helium-cooled, rocketborne, far-infrared absolute photometer.