Surface Metric
Statistical variation of intensity in a laser interference pattern provides a measure of surface roughness. Calculating the speckle contrast ratio allows a quality control system to quantify the finish of a board without making physical contact. Methodological use of this value helps monitor delicate coatings during the drying process.
Measurement Physics
Laser light reflecting from a rough surface creates a pattern of bright and dark spots due to the interference of waves with different path lengths. A high speckle contrast ratio indicates a surface with prominent height variations, whereas a low ratio suggests a very smooth or glossy finish. The calculation involves dividing the standard deviation of the intensity by the mean intensity of the captured image.
This value is sensitive to both the refractive index of the coating and the distribution of the mineral pigments. Precise calibration against known standards ensures that the measurement remains consistent across different production runs and mill locations.
Application Boundary
Factors such as the angle of incidence and the wavelength of the light source influence the resulting data. When using the speckle contrast ratio for online monitoring, the system must account for the speed of the web to avoid motion blur. Technique limitations involve the saturation of the detector and the coherence length of the laser.
It works most effectively on surfaces where the roughness is similar to the wavelength of the light used.
