Preliminary Characterization Study of a Gold-Coated Concentrator for Hemispherical Longwave Irradiance Measurements

We report the preliminary characterization results of a gold-coated concentrator used for longwave irradiance measurements^[1]. Throughput measurements of the concentrator are conducted at 1.562 μm and 10.15 μm using two different approaches, one is referred to a transmittance measurement using the Complete Hemispherical Infrared Laserbased Reflectometer (CHILR) ^[2] of NIST, and the other one is a direct throughput measurement using an existing thermopile detector for longwave irradiance measurement. For the transmittance measurement using CHILR, two diffuse gold references are selected to generate a Lambertian source by shining a laser on them. Spatial variations of transmittance of the concentrator are also investigated by scanning the laser beam across the opening area of it with a gold diffuser. For the direct throughput measurement, a small diffuse gold integrating sphere of 25.4 mm in diameter is utilized to produce an ideal diffuse source. The thermopile detector measures the radiation passing through the concentrator from the small integrating sphere. The incoming irradiance is determined from signal outputs of the thermopile detector and ambient temperature changes. Comparing with the results from the two approaches, a consistent throughput of the concentrator is obtained about 91 % to 92 %. The error sources and uncertainty in the two measurements are also discussed.