For experimental setup to calibrate spectral radiance by applying a diffuse plane, the measured spectral radiance can be obtained through integral of spectral radiation flux entering the spectrometer. Calculation shows that the measured spectral radiance by the spectrometer is not equal to the spectral radiance on diffuse plane surface, there is a deviation varying with actual calibration condition. The effect caused by variation of distance between spectrometer and diffuse plane, angle from the light axis of the spectrometer′s entrance slit to the diffuse plane′s normal, distance between standard lamp and diffuse plane are analysed, and optimized conditions for spectral radiance calibration are obtained. Finally, the idealized spectral radiance calibration, spectral radiance limit when the viewing area shrinks to a point, and actual spectral radiance calibration by applying a diffuse plane of known bidirectional reflectance distribution function at optimized conditions are discussed.