Spectroradiometer improves accuracy of photovoltaic detector by integrating a white-bias source into a scanning monochromator. A detectors spectral response is measured by exposing the detector to monochromatic light that is scanned through the wavelength range of interest. The bias source is provided to illuminate the test device with approximately 1 solar constant to simulate the actual operating environment. The bias source is shuttered when performing the system calibration with the reference detector and then used to bias the test detector and PV solar cell during the measurement. Since the bias light is DC and the monochromatic light is AC, the detector response can be measured by locking it to only the AC portion of the detector signal. Adding an assembly to measure the specular and diffuse reflection components such as the OL 750-71 provides measurements of both power response and reflectivity of the detector.