Characterization and Optimization of a Customized Color Reproduction System

Color Laser Shield (CLS) technology is used for printing personal photos on ID cards as a measure of security. The tasks within this internship are to define a standard way to measure colors, guarantee their consistency, and optimize an approach to obtain the best colors possible within the available materials and equipment. A standard way of measuring colors was defined, based on the CIE 1964 L*a*b* color space values, using a spectrophotometer. But the spectrophotometer can measure only a large spot of one uniform color (i.e., it can’t measure all color details of a picture). A flatbed scanner, calibrated with the spectrophotometer as reference, has therefore been used. Thanks to computational calculations (least square method), good calibration results were obtained and color measurements from both devices were close: an average CIE 2000 ΔE of 1.04 units was obtained, where the ΔE metric can determine how close color measurements are, which in simple words represents the distance between 2 colors in the L*a*b* color space. A method was developed for calculating the ΔE value between color patches or pictures using the scanner. After having a robust approach of measuring and controlling colors, work can be done on optimizing the colors. Using lasers for marking does not give pure primary colors and many factors are in play in the coloration of the materials; the colors values do not vary linearly with lasers powers. Thus, an approach was needed to link the desired colors and the four laser powers, which led to the creation of a Look-Up Table (LUT). The images that need to be printed are digital, which means they are in the sRGB color space, but since the technology color gamut is smaller than the one of the sRGB space, a specific algorithm was needed to be built by coding to reduce the gamut from sRGB to the technology’s. Good results were achieved, i.e., both gamut shapes became visually indistinguishable. After gamut reduction and some optimizations, the laser-marked images were in good overall quality. After that a continuous 4-gradient color chart (RGB and K) that is designed to cover all the sRGB space was printed on a card, the card was scanned and complete data was obtained, an LUT for printing was built using 3D interpolation, of the data that we obtained, some pictures were printed using this LUT and the results were rather accurate.