The perceived dark areas are the visual effects of the strain pattern. The intensity of these dark areas is accentuated by a reflective coating and would be inherently less for thinner glass and non-coated glass, all else being equal. The pattern of the strain may vary from one manufacturer to another, however, the effect cannot be eliminated in heat treated glass.

This phenomena is recognized by the American Society for Testing and Materials. ASTM C 1048 Heat Treated Flat Glass - Kind HS, Kind FT Coated and Uncoated Glass contains the following paragraph:

7.5 Strain Pattern - In heat strengthened and fully tempered glass, a strain pattern, which is not normally visible, may become visible under certain lighting conditions. It is a characteris-tic of these kinds of glasses and should not be mistaken as discoloration or non-uniform tint or color.

Often the intensity of the strain pattern in the glass of one building is compared to that of another. These comparisons may involve two entirely different types of glass and two different viewing conditions (daylight vs. surface of adjacent building; highly polarized daylight vs. moderately polarized daylight; sunlit exposure vs. shaded exposure, etc.). Conclusions may be misleading since the factors that influence the intensity of the strain pattern may vary widely between the two cases.

Valid comparisons can only be made when the two glasses being compared are exactly the same and are viewed in transmission with daylight as the light source. The degree of polarization and other characteristics of the daylighting must be approximately the same.