Optical Angle
Optical testing protocols specify the distribution of reflected light from paper surfaces to define their visual properties. The standard procedure known as ISO 2813 gloss measurement quantifies the specular gloss of non-metallic materials by directing a light beam at a designated angle and measuring the intensity of the returned ray. This procedure utilizes different geometric angles, typically sixty degrees for intermediate surfaces and twenty degrees for high gloss finishes, to capture surface shininess.
High gloss surfaces require the narrowest angle to resolve fine details.
Calibration Standard
High accuracy instruments rely on highly polished black glass with a defined refractive index as their baseline reference point. For ISO 2813 gloss measurement, the zero point is set against a matte black surface and the upper limit is calibrated to the master glass tile. Photodetectors must receive a linear response across this entire interval to guarantee that measurements taken at different points in a production run remain comparable.
The stability of these tiles affects the reliability of the daily measurements. Dust and surface scratches on the reference plate degrade the calibration values, which can lead to false readings on printed packages.
Application Boundary
Specialized print finishes often exhibit directional patterns that challenge standard optical evaluation methods. When performing ISO 2813 gloss measurement, operators must align the sensor with the grain direction of the paperboard because fiber orientation influences how light scatters across the surface. This test does not measure the overall brilliance or metallic sheen of foil laminate boards.
It remains restricted to flat, non-diffracting coatings that distribute light through simple reflection.
