Federal specifications restrict glare of reflected sunlight from certain automotive-trim items by calling for a maximum specular (mirrorlike) reflectance of 2% (40 gloss units), thus reducing glare by decreasing the luminance of the reflection. This approach, which requires the use of matte surfaces, has disadvantages. Observations showed that glare of sunlight reflected from bright surfaces could be reduced as effectively by demagnifying the sun’s image by use of increased surface curvature. A glare comparator was constructed, by which glare control by restriction of luminance was related visually to control by restriction of angular area. About 1000 observations were made during the course of more than 150 tests. Results showed that glare was regulated to the same extent by a proportional change of either luminance or angular area.
ROGER L. SAUR, "Influence of Luminance and Geometry on Glare Impression," J. Opt. Soc. Am. 58, 847-848 (1968)