Short Introduction in Gloss Measurement for several Industries
Gloss is a visual perception that occurs in reflection of surfaces. The gloss perception is much more pronounced, depending on how directly the light is reflected.
High gloss: On flat, completely smooth, polished surfaces mirror images can be seen clearly. The incident light is reflected on the surface only into the main reflection direction. The reflection angle is equal to the incident angle.
Matt to medium gloss: Rough surfaces do not only reflect the light into the main direction of reflectance. Rough surfaces also reflect the light diffusely into other directions. Thereby the depiction quality of the surface is reduced: a reflecting object is no longer depicted brilliantly, but blurred. The more equably the light is scattered into the room, the lower the intensity of the directed component and the more matt the surface is.
Reflectometer for gloss measurement: The principle of reflectometers is based on the measurement of directed reflection. For this purpose the intensity of the reflected light is measured in a narrow field of the reflection angle. The intensity of the reflected light depends on the material and the incident angle. Non-metals (paint, plastic) reflect more light when the incident angle increases. The rest of the irradiated light enters into the material and is, depending on the hue, partly absorbed or diffusely scattered. Compared to non-metals the reflective faculty of metals is significantly higher and it doesn’t depend that much on the angle.
The measuring results of the reflectometer do not refer to the quantity of the irradiated light but they refer to a black, polished glass standard with defined refractive index. In this case the measured value conforms with 100 gloss units (calibration).
Materials with larger refractive index can have a higher measured value than 100 gloss units, for example foils. In the event of a multiple reflection the measured value of transparent materials can be additionally increased. Due to the high reflection more than 2000 gloss units can be measured on metals. It is a common practice to refer the measurement to the incident light and to express as percentage. In order to get comparable measuring results, the reflectometers and their handling were internationally standardized. The value of the reflectometer is highly influenced by the incident angle. In order to differentiate surfaces from high glossy to matte, 3 geometries (3 measurement areas) were standardized: High gloss in 20° geometry, medium gloss in 60° geometry and matt gloss in 85° geometry. The 20° geometry is used when under 60° more than 70 GE are calculated. The 60° geometry is used when under 60° 10 to 70 GE are calculated. The 85° geometry is used when less than 10 GE are calculated. The main application areas for these 3 geometries are paints, plastics and metals.
Important standards for gloss measuring:
For the plating and coating industry the control angles and the procedure for gloss testing is defined in the DIN EN ISO 2813.
The graphic industry also uses instruments with a 45° geometry (DIN 54502) and a 75° geometry (Tappi T 480). These two geometries are applicable for example in ceramic, foil, paper and vinyl.