Gloss is a measure of object’s surface property, e.g. shiny, metallic, or matte. It measures the percentage of spectral reflection.
As shown in the above figure, gloss is measured by a Glossmeter which measures the spectral reflection intensity compared incident intensity. The glossmeter is calibrated as GU=100 by a highly polished, plane, black glass with a refractive index of 1.567 ( at 589nm or Sodium D line) for each geometry.
GU is related to both object’s surface properties and incident angle (e.g. 20, 60, 85 in ASTM D 523 for plastic film industry). Angle selection rule is:
The GU of a perfect matte surface is 0; the standard is 100. Typically the GU is between this range. However, for shiny surface, e.g. metal and transparent film where multi-reflection at different interfaces occurs, GU can be higher or even much higher than 100.