Surface Uniformity
Digital image analysis quantifies small-scale spatial variations in optical density across printed substrate surfaces. During high-speed offset or flexographic printing on coated paperboard, micro mottle index measures ink absorption fluctuations occurring at spatial frequencies between one and five millimetres. Uneven binder distribution or porous coating structures cause localized differences in ink vehicle penetration.
These micro-scale density variations reduce print gloss and image crispness on consumer packaging.
Optical Measurement
High-resolution scanner systems capture grey-level intensity values across printed solid patches under calibrated lighting conditions. Image processing algorithms calculate the micro mottle index by applying fast Fourier transforms or wavelets to isolate specific spatial frequency bands. The resulting numerical value reflects variance in lightness values within the defined spatial scale.
Base paper formation defects and uneven calendar nipping exacerbate these localized optical variations. Coated board mills adjust latex binder ratios and blade coating pressure to minimize variance in surface pore size distribution. Converting lines monitor these optical values to maintain consistent print fidelity across production runs.
Process Boundary
Assessment metrics cease to provide meaningful surface quality data when print coverage falls below solid ink density levels. Halftone screen patterns and low-coverage tint areas require distinct mottling metrics due to dot gain interference. Macro-level color variations exceeding ten millimetres in spatial wavelength fall outside this specific index.
Micro mottle index applies strictly to small-scale absorption density variations on fully inked surfaces.
