Abstract: The automatic groundbased allsky imager developed by the Institute of Atmospheric Physics, Chinese Academy of Sciences is able to obtain visible gray images of the sky radiance in red, green and blue channels. The paper introduces this allsky imager system and its capabilities. Radiance, a key parameter for meteorological researches, is recorded as gray value by the allsky imager and can not be used directly for meteorological study. In order to extract the radiance from allsky images, this paper shows some necessary calibration experiments on the imager system. On the basis of the calibration results and the projection principles of the allsky imager, an algorithm to retrieve the relative radiance from the allsky images is then presented. Then using the images obtained from the allsky imager and the radiance at 440 nm wavelength from the CIMEL instrument at Xianghe Observatory in Hebei province （39.75°N，116.96°E）, we make a intercomparison between the retrieval radiance and the measured one. The comparison results show that the retrieval algorithm is feasible. This work sets up a relationship between the radiance and the image data and it is useful for the image data to be made comparison with the numericalsimulation results and to be applied into numerical model simulation studies.