Coherent Interference
Laser-based measurement of paper and board surfaces is affected by the chaotic scattering of coherent light. The phenomenon of speckle noise arises when a laser beam reflects from the microscopically rough surface of a substrate, creating a random interference pattern of bright and dark spots. This graininess is not a feature of the paper itself but is an optical artifact of the illumination source.
It obscures the fine details of the surface structure.
Surface Analysis
Precision inspection systems use lasers to detect surface defects like voids, scratches, or coating skips on packaging boards. However, the presence of speckle noise degrades the signal-to-noise ratio, making it difficult for automated software to distinguish a real physical defect from a bright spot in the interference pattern. This limitation is particularly challenging when analyzing highly recycled boards, where the inherent fibre variation already creates a complex surface texture.
The optical system must filter out these random patterns to avoid triggering false alarms that halt the production line. Accurate analysis of printed text also suffers if the laser scan is corrupted by these intensity fluctuations.
Mitigation Strategy
Reducing this interference involves using multi-wavelength laser sources or rotating diffusers that disrupt the coherence of the light beam. These methods average out the random patterns over time, leaving a clearer image of the underlying paper structure. This optical smoothing is necessary for high-speed inspection.
