Fibre Scattering
Optical measurement of paperboards and papers is influenced by how deeply light enters the fibrous network before scattering back to the sensor. The phenomenon of sub-surface light penetration occurs when incident light passes through the surface layer and interacts with the internal cellulose fibres and mineral fillers. This internal scattering path determines the perceived brightness and opacity of the sheet.
It affects both visual appearance and sensor readings.
Print Quality
In high-resolution color printing, the depth of light travel within the paper structure leads to a phenomenon known as optical dot gain. When light enters the unprinted paper near the edge of a halftone dot, it can scatter beneath the ink and be absorbed, making the printed dot appear larger and darker than its physical boundaries. This sub-surface light penetration reduces the contrast and sharpness of fine text and detailed images.
Controlling this effect requires the use of mineral coatings that block the light from entering the base sheet. High-opacity coatings with calcium carbonate or titanium dioxide scatters the light quickly at the surface, maintaining high print fidelity on the carton board.
Measurement Calibration
Spectrophotometers must account for this light travel to avoid errors in color measurement. If the sensor aperture is too small, a portion of the scattered light escapes outside the detection area, leading to inaccurate density readings. Standardized test methods specify the required measurement geometry to ensure consistency across different paper mills.
