Digital camera calibration for color measurements on prints

Flatbed scanners and digital cameras have become established and widely used color imaging devices. If colorimetrically calibrated, these trichromatic devices can provide fast color measurement tools in applications such as printer calibration, process control, objective print quality measurements and color management. However, in calibrations intended to be used for color measurements on printed matter, the media dependency must be considered. Very good results can be achieved when the calibration is carried out on a single media and then applied for measurements on the same media, or at least a media of a very similar type. Significantly poorer results can be observed when the calibration is carried out for one printer-substrate combination and then applied for measurements on targets produced with another printer-substrate combination. Even if the problem is restricted to the color calibration of a scanner or camera for different paper media printed on a single printer, it is still tedious work to make a separate calibration for each new paper grade to be used in the printer. Therefore, it would be of interest to find a method where it is sufficient to characterize for only one or a few papers within a grade segment and then be able to apply a correction based on measurable optical paper properties. However, before being able to make any corrections, the influence of measurable paper properties on color characterizations must be studied and modeled. Fluorescence has been mentioned as a potential source of error in color calibrations for measurements on printed matter. In order to improve paper whiteness, producers of printing paper add bluish dye and fluorescent whitening agents (FWA) to the paper. In this study, the influence of FWA in printing paper on the color calibration of a digital camera for color measurements on printed targets is discussed. To study the effect of FWA in the paper, a set of papers with varying additions of FWA but otherwise identical, were produced on a smallscale experimental paper machine. Firstly, the impact on the color calibration when the amount of FWA in the paper varies was studied. Secondly, the situation where the printed substrate has FWA-content, and illuminations having different contents of ultraviolet (UV) light were used in the camera and reference spectrophotometer measurements respectively. The results show that for some combinations of illuminations used in the calibration, very large errors are induced by the variation of FWA in the printed substrate.