Soft proofing of colours on substrates with optical brightening agents

Paper Fluorescent Whitening Agent combined with differences in relative Ultra Violet levels between instrument Illuminants, and real world viewing Illuminants, can be a significant source of error in characterising a printing process, and hence in the ability to accurately reproduce coloured images in print and proof. The appearance and measurement of fluorescent substrates depends strongly on the UV-amount in the source illuminating a fluorescent sample which varies in the different viewing booths available. The appearance of colours printed on substrates with optical brightening agents has been studied with help of a colour matching experiment where the observers matched a colour patch displayed on a LCD monitor, by adjusting its L*a*b* values, to another color patch printed out on paper viewed under varying amount of UV content in lighting condition in the viewing booth. A customised viewing booth was built for this purpose and substrates with varying amount of optical brightners were considered for the study. A model based on CIECAM02 and a scaling technique has been developed to predict the perceived colour match on a LCD display, of colours printed on substrates with optical brightners and viewed under the viewing booth with varying amount of UV content in the viewing illumination. According to the obtained results, the appearance of the colours printed on substrates containing optical brightners varied with variation in the amount of UV content in the viewing illumination. The developed model gave good prediction of the XYZ tristimulus values for the perceived match on the LCD display from the XYZ tristimulus values of the printed colours on the substrate with acceptable ▵Eab . This shows that CIECAM02 can be effectively used for soft proofing