The increasing manipulation of stone by cleaning and the application of preservatives creates the need for a more precise method to measure the degree of discoloration of a stone surface quickly and simply both in the field and in the laboratory. The small color chips of the Rock or Soil Color Charts are insufficient. Of the three Munsell color attributes, hue, value and chroma, the value or reflectance is easiest to measure. This can be done with precision with a through-the-lens photographic light meter of a single-lens reflex camera. Calibration against ambient light conditions is done with a Kodak Test Card, with 20 per cent absolute light reflectance on the gray side and 90 per cent on the white side. With the help of a graph charting the lens opening versus the reflectance, the reflectance of a stone surface with different surface finishes can be readily quantified. Using this method, one can measure the differences between natural, weathered and chemically treated stone. One can also use the method to measure the albedo of the stone for suitability as a solar absorption panel.