The absolute spectral response of solar cells is reported in the 400–1000-nm spectral region. Measurements were performed using two different types of monochromatic sources: amplitude-stabilized cw laser lines and interference filters with an incandescent lamp. Both types of calibration procedures use electrical substitution radiometry as the basis of traceability to absolute SI units. The accuracy of the calibration is shown to be limited by the nonideal characteristics of the solar cells themselves, specifically spatial nonuniformities and nonlinearities induced by high light levels. © 1979 Optical Society of America.