Simple methodology of determinig the reflectance of light and its application to some natural stones used in architectural practice. The methodology is based on the interpolation of the value lying between two etalons for which the reflectance of light was precisely measured under laboratory conditions. For field measurements there were two illumination chambers constructed; in these chambers indirect daylight was transmitted to a stone surface. Reflectance of light from the stone surface was then measured by a luxmeter in a predetermined constant height and the result was interpolated between the values of both etalons. In copending case study 300 measurements on fifteen samples of stones was performed in this manner and resulting coefficient of reflectance of light was then calculated. Resulting coefficient is applicable for the calculation of daylighting of buildings according to the standard CSN 73 0580.