Measurement of the absorptance of a cryogenic radiometer cavity in the visible and near infrared

We report the results of the measurement of the absorptance of a LaseRad cavity used as an absorber in the cryogenic radiometer in the Physikalisch-Technische Bundesanstalt. The measurements were carried out at several laser wavelengths in the visible and near infrared (NIR), with a He–Ne laser operating at 633 nm and with two tunable laser sources operating between 1280 nm and 1360 nm and between 1480 nm and 1620 nm. The absorptance of 0.999 885 ± 3.0 × 10 −6 measured at 633 nm matches very well with the value of 0.999 879 ± 1.0 × 10 −5 reported by the manufacturer (Cambridge Research & Instrumentation, Inc.). In the NIR the absorptance is approximately 1.1 × 10 −4 ± 3 × 10 −6 lower than at 633 nm, which is significant for high accuracy measurements. The absorptance varies 19 × 10 −6 in the wavelength range from 1280 nm to 1620 nm: this is almost negligible for this wavelength range.