A Lambertian CCD-camera method is convenient to measure concentrating radiation fluxes, where a crucial factor, a calibration factor, always varies with spectra and brings errors. In this paper, a new calibration method is proposed based on spectral normalization calculation and tries to reduce spectral errors in Lambertian CCD-camera measurement. The calibration factor for AM1.5 is standardized over a transmittance range by matching gray values of photos to readings of calorimeter. A spectrum is calculated by SMARTS (simple model of the atmospheric radiative transfer of sunshine) according to the local time, latitude and longitude. A calibration factor is adjusted by calculated spectral offsets accordingly. Therefore an absolute radiation flux distribution is obtained by a gray value captured by the CCD-camera without calorimeter. Calculated results indicate that spectral irradiance between 700 and 800 nm dominates gray values on the target for solar radiation flux measurement. The offsets are increasing continuously from AM1 to AM5, which are validated by experimental results. The difference between measured and calculated calibration factors is 11%, which fits to the results of error estimate. These indicate that the improved method was feasible and reliable to measure concentrating radiation fluxes easily. (C) 2013 Elsevier Ltd. All rights reserved.