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The conical black cavities are used in the Solar Irradiance Absolute Radiometers (SIARs) of the solar constant monitor aboard on the SZ spaceship and of the Solar Total Irradiance Monitor (STIM) aboard on sun-synchronous polar orbit weather satellites. A low reflectance measurement instrument which is used for making the integrating measurement on the reflectance of the conical black cavity in a high precision within the full hemisphere (include the entrance of the Ulbrichtsphere) has been constructed. The characteristic of the instrument is the employment of a semi-transparent mirror, which is mounted in an inclination angle of 45 degrees in front of the entrance of the Ulbrichtsphere. The incident beam is reflected to the black cavity or white board by the semi-transparent mirror. A portion of those, which is made diffused reflection through the black cavity or white board, is measured by the detector located in the side face of the Ulbrichtsphere, and the other portion which is reflected to the entrance is measured by the other detector, after passing through the semi-transparent mirror, and then being focused on by an ellipsoidal mirror. The two measurement data are added up to get the integrating reflectance within full hemisphere. The presentation and verification on the measurement result of the reflectance of the black cavity and its precision are described in this article. The absolute accuracy can reach 0.012%.
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