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 mm and between 1480 nm and 1620 nm. The absorptance of 0.999885 +/- 3.0 x 10(-6) measured at 633 nm matches very well with the value of 0.999879 +/- 1.0 x 10(-5) reported by the manufacturer (Cambridge Research & Instrumentation, Inc.). In the NIR the absorptance is approximately 1.1 x 10(-4) +/- 3 x 10(-6) lower than at 633 nm, which is significant for high accuracy measurements. The absorptance varies 19 x 10(-6) in the wavelength range from 1280 nm to 1620 nm: this is almost negligible for this wavelength range.