The polychromatic point spread function is obtained by integrating the monochromatic intensities along the spectrum taking into account the spectral emission of the source and the spectral response of the detector. In this work we study the influence in the integration accuracy of the number of wavelengths considered in the calculations. The accuracy in the computation of the illuminance and chromaticity distributions of the aberration-free system is considered. We have compared two integration methods: the Gauss quadrature and the Simpson rule. We also show that a higher number of wavelengths is needed to obtain the same accuracy when a system with chromatic aberration is considered. © 1994, Taylor & Francis Ltd.