The Effect of OBA in Paper and IlluminationLevel on Perceptibility of Printed Colors
A research was conducted to study the perceptibility of color difference of color pairs, caused by OBA differences in paper substrates, and its relationship with quantitative measurement metrics. Based on the psychometric experiments conducted, the results show the utilization of the visual difference index (VDI), from 0 (no difference) to 3 (noticeable difference), to rate 27 color pairs with each pair prepared by the same colorants but different OBA amount in the substrates. The findings indicate that (a) printed colors are affected by the presence of OBA from no difference to noticeable difference, (b) ∆E00 has a stronger linear correlation with visual color difference than ∆E*ab does, (c) there is no significant association between illumination levels and visual color difference. This research introduces the metric, OBA, per ISO 15397 (2013), as the CIE-b* difference in color pairs under M1 and M2 conditions. It also defines ∆OBA as the OBA difference between any color pairs, including substrates. The results show that there is linear correlation (1) between visual difference and ∆E00 which describes the color difference, and (2) between visual difference and ∆OBA which describes the criticalness of M1 lighting to realize the color match.
Effect of Paper Containing OBA on Printed Colors
Paper makers use OBA to make substrates brightener and whiter. According to Lee, Shen, and Chen (2001), “fl uorescent materials have higher lightness and saturation compared with traditional non-fl uorescent materials” (p. 1). Papers with OBA require ultraviolet radiation of wavelengths of illuminate below 400 nm for their excitation and then emit light in the blue region. It brings the peak refl ectance wavelength at about 457nm. As a result, a shift of the CIELAB b* coordinate towards the blue by about 1 to 10 units will be found (iso 13655:2009, Appendix G). Th e use of OBA results in some problems in printing conformance to ISO 12647-2:2007 Graphic technology–Process control for the manufacture of half-tone color separations, proof and production prints–Part 2. iso 12647-2 recommends fi ve typical kinds of papers with b* values from –3 to 6 and tolerances of ±2. Th erefore, if paper contains OBA, its presence would make color measurement of the paper out of iso print standard. Widely used today, optically brightened papers look whiter and brightener due to added optical brightening agents during paper making. Optical brightening agents (OBA) are chemical additives that correct for the natural yellow color of pulp. It works on absorption of ultraviolet energy and emission of short-wavelength energy. paper, optical brightening agents, color diff erence Keywords
Spectral estimation and color appearance prediction of fluorescent materials
In this paper, we present a method to estimate the reflected and fluorescent spectral radiance factors of a fluorescent object based on spectrophotometric data without using a monochromator. We use truncated Fourier series to approximate both two spectral radiance factors. Then, based on the measured spectral obtained from a spectroradiometer, the coefficients of the truncated Fourier series are estimated using an weighted least square algorithm. The weighting function is defined as the sum of the CIE standard x, y, and z color matching functions. With the estimated reflected and fluorescent spectral radiance factors, we can predict the color appearance of a fluorescent object under other sources such that the color difference is minimized from viewpoint of human vision.