On rough surfaces the light is diffusely scattered in all directions. The image forming qualities are diminished:
A reflected object no longer appears brilliant, but blurred.
The more uniform the light is scattered, the less intense the reflection in the main direction and the surface will appear duller.

Intensity is dependent on the material and the angle of illumination. In case of non-metals (coatings, plastics) the amount of reflected light increases with the increase of the illumination angle. The remaining illuminated light penetrates the material and is absorbed or diffusely scattered dependent on the color.


Metals have a much higher reflection and are less angle dependent than non metals.

Measurement results of a glossmeter are related to the amount of reflected light from a black glass standard with a defined refractive index, and not to the amount of incident light. The measurement value for this defined standard is equal to a less than 100 gloss units (calibration).


Materials with a higher refractive index can have a measurement value above 100 gloss units (GU), e.g. films. In case of transparent materials, the measurement value can be increased due to multiple reflection in the bulk of the material. Due to the high reflection capabilities of metals, values of up to 2000 GU can be reached. For these applications it is common to document the measurement results in % reflection of the illuminated light.

Gloss Measurement

Gloss is a visual impression resulting from surface evaluation. The more direct light reflected, the more obvious the impression of gloss will be.

Glossmeter

A glossmeter measures the specular reflection. The light intensity is registered over a small range of the reflection angle.