Imaging Technique
Wavefront sensing methods utilize multiple images captured at different focal planes to reconstruct the three dimensional topography of a surface. Applying phase diversity to the inspection of coated papers allows for the separation of surface roughness from subsurface irregularities. This approach eliminates the need for complex hardware by using computational algorithms to solve the phase retrieval problem.
Surface Analysis
Optical systems looking at a moving web benefit from the ability to correct for aberrations introduced by heat or vibration. The use of phase diversity enables high resolution mapping of the gloss levels and micro-contour of the substrate. By capturing one in focus image and one slightly out of focus image, the system calculates the exact shape of the reflected wavefront.
Data obtained identifies areas where coating coverage is insufficient or where the calendering process has failed to smooth out the fibers.
Quality Control
Precise measurements of ink film thickness and dot gain are possible when the imaging system can account for the scattering properties of the paper. Implementation of phase diversity in a digital press monitoring unit ensures that the color management system receives accurate data regardless of minor mechanical misalignments. The technique is particularly effective for holographic or metallic foils where standard densitometry fails.
It provides a level of detail that traditional bright field microscopy cannot achieve in a high speed production environment.
