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Monday, July 22, 2013

75° or 20° Gloss?

When light strikes a sheet of paper, three types of reflectance can occur. In one extreme case, all of the light that strikes the surface is diffusely reflected in all directions equally (A, above).  In another extreme case, all of the light is reflected in an equal but opposite angle of its incident direction (C, above).  Paper is neither totally specular nor totally diffuse, it is a combination of the two (B, above). However, only the specular component is measured as gloss.

The two specular angles which are most commonly used in the paper industry are 75° and 20° (measured with respect to a perpendicular to the surface of the sheet). These angles were selected on the basis of best correlation between measured values and visual assessment of gloss.  The image above shows that for intermediate ranges of gloss, numerical gloss values measured at a specular angle of 75° provide a nearly linear relationship with visual gloss assessment.  For very low or very high gloss  materials, however, large observable changes in gloss result in very small changes in the measured numerical values.  For measurements of high gloss materials such as cast coated papers, high gloss inks, etc. which have a 75° gloss value greater than 85, a steeper angle such as 20°, may provide better discrimination.  As seen above there is a better spread of numerical gloss values in the high gloss region for 20° measurements than for 75° measurements. Sixty degree gloss is commonly employed in other industries for products such as paint and plastics, but it is seldom encountered in the paper industry.

Related standards:
  - 75° gloss: TAPPI T480, ISO 8254-1, PAPTAC E.3 [intermediate ranges of gloss]
  - 20° gloss: TAPPI T563, ISO 8254-3 [high gloss range]