The absolute diffuse reflectance factors of white standard reference materials have been measured in d/0 geometry (Sharp-Little method) over the visible spectral range using a silicon-photodiode array, This method reduces the measuring time to a few seconds to obtain complete spectral reflectance factor data from 380-780 nm in the visible range. The effects of the openings and the wall thickness at the sample port onto the spectral reflectance factors were considered to get more accurate results. The precision of the diffuse reflectance factors in our system was 0.1% in the wavelength region longer than 550 nm and 0.4% in that shorter than 400 nm. We have obtained the absolute diffuse reflectance factors in the visible range of two kinds of barium sulfate, and of pressed polytetrafluoroethylene (PTFE) at three different densities.