An optical sample is mounted on a spatial-acquisition apparatus that is placed in or on an enclosure. An incident beam is irradiated on a surface of the sample and the specular reflection is allowed to escape from the enclosure through an opening. The spatial-acquisition apparatus is provided with a light-occluding slider that moves in front of the sample to block portions of diffuse scattering from the sample. As the light-occluding slider moves across the front of the sample, diffuse light scattered into the area of the backside of the light-occluding slider is absorbed by back side surface of the light-occluding slider. By measuring a baseline diffuse reflectance without a light-occluding slider and subtracting measured diffuse reflectance with a light-occluding slider therefrom, diffuse reflectance for the area blocked by the light-occluding slider can be calculated.