The main aim of this research is to assess the variation in phototherapy radiometer responsivities toward the optical power of the phototherapeutic devices and hence estimate the spectral mismatch correction factor f1'. Two conventional phototherapy devices were studied to verify their spectral irradiance and three different phototherapy radiometers were studied for their responsivity at the range 400-500 nm. The results show the dependence of total irradiance measurement of phototherapy sources on matching between phototherapy radiometer band responsivity and spectral irradiance of the phototherapy sources. The spectral mismatch values vary from 13% to 47% of the total measured irradiance for the three radiometers. These mismatch values could be added to the measured irradiance as a correction.