A system and method for determining a brightness factor associated with a window shade fabric is disclosed. The brightness factor incorporates the openness factor of the fabric, visible light reflectance of the fabric and visible light transmission of the fabric. The brightness factor helps to determine the optimum window shade fabric for a particular room, building or other location. The brightness factor information may be used to select the correct fabric based on desired interior function, light level, the relative surface brightness, and/or any other desired interior characteristic. The fabric selection may affect the building envelope by, for example, facilitating the optimization of daylighting, reduction of artificial electric lighting needs, and minimization of glare conditions.