ISO 2813 and ASTM D523 (the most commonly used standards) describe three measurement angles for the measurement of gloss across surfaces of all levels.
The standard gloss unit (GU) is used, this is traceable to standards held at NIST.
Universal Measurement Angle- 60º
All gloss levels can be measured using the standard measurement angle of 60º. This is used as the reference angle with the complimentary angles of 85º and 20º often used for low and high gloss levels respectively.
Low Gloss- 85º
For improved resolution of low gloss a grazing angle of 85º is used to measure the surface. This angle is recommended for surfaces which measure less than 10 GU when measured at 60º.
This angle also has a larger measurement spot which will average out differences in the gloss of textured or slightly uneven surfaces.
High Gloss- 20º
The acute measurement angle of 20º gives improved resolution for high gloss surfaces. Surfaces that measure 70 GU and above at the standard angle of 60º are often measured with this geometry.
The 20º angle is more sensitive to haze effects that affect the appearance of a surface.