5.1 Gloss3 is associated with the capacity of a surface to reflect more light in some directions than in others. The directions associated with mirror (or specular) reflection normally have the highest reflectances. Gloss is best seen and analyzed when the surfaces studied are illuminated by a light source that provides strong contrasting patterns of light and dark. Such a light source is described in this test method.

5.2 The simplest concept of gloss is that it corresponds to the mirror-like reflectances of surfaces. However, the distributions and intensities of this surface-reflected light are (for real materials) highly variable and affected by a variety of factors: surface smoothness and contour, refractive index, absorptance, angle of incidence, and (to a generally small extent) wavelength. From the great variety of surface-reflection patterns met in materials of commerce, it has been possible to identify seven surface-reflection criteria or "types of gloss" regularly used by skilled technologists for intercomparing and rating their products for gloss. Six of the seven criteria, or "types of gloss," are identified in the section on definitions. The seventh, luster or contrast gloss, is seldom of concern to the coatings industry.