Improving the Repeatability of a Color Sensor by Integrating an FSM for Scanning-based Areal Measurements

Abstract

This work presents the design, alignment algorithm and validation of an optical scanning color sensor system capable of precisely aligning itself to a sample without additional external sensors, improving the repeatability of the color measurement. The proposed system design enables to scan the optical path of the color sensor by a fast steering mirror (FSM), such that the angle of incidence can be varied within ±3°. A color characterisation is performed to determine the parameter of the CIELAB color space, which can be used to correctly align the color sensor with respect to the sample. An alignment algorithm automatically determines the correct measurement distance and angle of incidence, based on the measured brightness value L. Experimental results show that the alignment algorithm can precisely align the sensor with a deviation of 9.3 μm and 8 mdeg. To evaluate the repeatability of the system, ten measurements with various start positions are performed, resulting in a maximum color difference of 0.0741 between the measurements. Additionally, the measurement data acquired during the alignment process can be used to characterize the surface finish.