When a multi-channel radiometer is calibrated using the sun as calibration source, the error in the calculated calibration constants may be large when solar zenith angles increase. A multi-regression model has proved to be suitable to derive narrowband irradiance from broadband irradiance, ozone column and solar zenith angles (SZA). In this paper, a modification of this model is being proposed to improved the multi-channel instrument sun calibration against spectroradiometers, considering a channel of a multi-channel radiometers as a broadband instrument. The errors in the GUV irradiance, compared to the spectroradiometer irradiance, diminished considerably at all channels for SZA larger than 50a, then, this technique could be particularly beneficial to calibrate radiometers installed at high latitudes, where SZA during winter, even at noon, are larger than 50a.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.