Subsurface Scatter
Light behavior within a paper matrix affects how printed images and text are perceived by the human eye. The optical lateral light scattering phenomenon occurs when light enters the sheet, scatters sideways within the paper fibers, and exits at a different point. This lateral movement alters the perceived density of printed areas.
Optical Expansion
Physical dot gain is caused by the spread of ink on the paper, but this optical scattering creates a secondary, apparent dot gain known as optical dot gain. The light that is scattered under the printed dots is absorbed by the ink, making the printed areas appear larger and darker than they physically are. This optical effect can reduce the contrast and sharpness of fine details in printed images.
Controlling this scattering is important for high-resolution graphics.
Substrate Evaluation
Measurement of this phenomenon is conducted by analyzing the light profile of a projected line or point on the paper surface using a CCD camera. This analysis determines the point spread function of the sheet, which reveals how far light scatters sideways through the fiber structure. Formulators can reduce this scattering by adding pigments with a high refractive index, which scatter the light more directly toward the surface and limit lateral travel.
Regular testing of these scattering properties helps mills design paperboards that support sharp, high-contrast printing on high-speed digital and offset presses.
