When reading product specifications for coatings, you can always tell when the marketing department got a hold of the specifications without the presence of an engineer—much gets lost in the translation. After reviewing several paint and coating specs where a gloss rating was provided, it quickly became apparent that a brief tutorial on the measurement of gloss was in order. Armed with this basic knowledge will place you in the 98th percentile of the technical coatings world.
The Physics of Gloss:
Gloss is simply the efficiency and effectiveness with which light is reflected off of a surface. When a light beam is directed at a near-perfect reflective surface like a mirror at a specific angle, the light will be reflected nearly in its entirety and at an equal angle. Imperfections in the surface will affect the reflected angle and the light’s dispersion. Rough surfaces will scatter the light over both a broad area and a wide set of angles.
To measure gloss a light of known intensity is directed at a surface at a specific angle. The light thus reflected at that same angle is quantified as a percentage of the incident light.
Depending upon the material type to be measured, different angles of light incidence are specified. The commonly used angles are 20, 45, 60, 75, and 85 degrees as measured from a perpendicular to the surface.
The reflected light intensity is dependent upon the combination of material type and angle. Non-metals such as coatings and plastics increase their intensity of reflected light as the angle from the perpendicular increases. At smaller angles, much of the light is absorbed into the coating or diffusely scattered depending upon the color. Metals, on the other hand, have a greater reflection intensity in general and at all incident angles.
Incident Angle = 20º from perpendicular
Used for high gloss auto paints, plastics, polished metals and polished stones.
Incident Angle = 45º from perpendicular
Used for anodized aluminum and for ceramics.
Incident Angle = 60º from perpendicular
The most universal measurement angle used for paints, metals, plastics.
Incident Angle = 75º from perpendicular
Used for paper and cardstock materials
Incident Angle = 85º from perpendicular
Commonly used in military specifications and aviation. Most applicable to matte paints and surfaces and auto interiors.
Gloss Measurement Units:
To properly specify a gloss level requires two numbers; the angle of incidence, and the reflected intensity as a percentage of the incident light. Thus, a typical measurement might read as “90% at 85 degrees.”
Sometimes the percentage is replaced with the term, “GU” or “Gloss Units.” So it would be 90 GU at 85 degrees.