Mottle - Measurement

Measurement

The irregular lightness variations caused by mottling can be objectively measured with specially made instruments. These instruments simulate visual evaluation under different observing angles and characterize clouds / mottles by their size and visibility. Small to large mottles are measured under three observing angles, in which the scan length can usually be varied from 10 to 100 cm. The measurement results are independent of color and curvature of the surface and thus can be considered objective.

The specific measurement process for one such instrument is as follows. It first optically scans the surface and measures the lightness variations. The specimen is illuminated with a white light LeD at a 15° angle and the lightness is detected under three viewing angles to simulate visual evaluation under different observing conditions: 15°, 45° and 60° measured from the specular reflection. The mottling meter is rolled across the surface for a defined distance of 10 to 100 cm and measures the lightness variations point by point. The measurement signal is divided via mathematical filter functions into 6 different size ranges and a rating value is calculated for each angle and mottle size. The higher the value is, the more visible the mottling effect. The measured values are displayed in a graph showing the mottle size on the X-axis and the rating value on the Y-axis. Thus, target values for small and large mottle sizes can be established for paint batch approval as well as process control.