Measuring Specular Gloss Changes during Standard Paperboard Rub Tests
Measuring specular gloss changes before and after standard rub tests quantifies surface coating durability using exact optical angle decay metrics.
20 Degree Gloss
Specular Directionality
Optical measurement geometry utilizing an incidence angle of twenty degrees from the perpendicular axis of a surface determines the specular reflectance of high-gloss print coatings and lacquered paperboard substrates. The application of twenty degree gloss measurement is reserved for highly reflective surfaces where a standard sixty-degree angle fails to distinguish fine variations in surface smoothness. When a surface exhibits a gloss value greater than seventy units at sixty degrees, the narrower twenty-degree angle is employed to resolve subtle scattering effects.
This geometry suppresses the scattering contribution from deeper layers, isolating the reflection to the topmost boundary of the varnish or film.
Gloss Differentiation
Surface appearance on a high-gloss boxboard or cast-coated paper relies on the sharpness of the reflected image rather than total light return. Measurement at this steep angle isolates surface haze and orange-peel defects that degrade visual contrast in premium packaging. Low-angle scattering from micro-topography reduces the twenty degree gloss value while leaving wider-angle measurements unaffected, providing a sensitive test for print varnish curing and film levelling.
A twenty-degree sensor holds a strict tolerance of two gloss units on high-speed folding carton lines.
Coating Evaluation
Performance criteria for high-gloss food packaging demand consistent finish across different printing runs. Measurement with the twenty-degree geometry guides the addition of slip agents and the adjustment of calender roll pressure during board manufacture. The resulting data allow converters to predict how the printed stock will behave under intense retail lighting, preventing a washed-out appearance on the shelf.