Detailed Analysis of the UV-Adjustment Techniques Used in Paper and Graphic Industries
Li Yang* Innventia AB, Drottning Kristinas V € ag 61, Stockholm 11428, Sweden
Received 6 October 2015; revised 2 December 2015; accepted 4 December 2015
filtering, have been studied. V C 2016 Wiley Periodicals, Inc. Col Res Appl, 42, 19–26, 2017; Published Online 13 January 2016 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/col.22015
Abstract: Many commercial materials (papers and boards) contain optical brightening agents also known as fluorescent whitening agents. Adequate adjustment of the UV content of a measurement device (e.g., spectropho- tometers) is essential for accurate color measurement. As specified in the ISO standards, the UV content is adjusted against an assigned value of an international reference transfer standard, for example, CIE whiteness (D 65 /10 8 ) for the CIE illuminant D 65 or ISO brightness for the C illuminant. Because of the simplicity, these approaches have gained great popularity in papermaking industry. Yet, there has been little evidence indicating how accu- rate the total spectral radiance factor corresponding to the single assigned value is reproduced. Hence, we pres- ent a method that quantitatively evaluates the accuracy of the UV-adjustment technique, through comparing the total spectral radiance factors obtained from UV adjust- ment with the assigned ones. This method has been applied to three second-level international reference transfer standard illuminated by three standard illumi- nants, D 65 , C, and D 50 . We found that the major differen- ces between the assigned spectra and those obtained from the UV adjustments occur in the blue band where fluorescence is strong. At a few wavelengths, the differen- ces may be up to 4–5%. Nevertheless, their color differ- ences corresponding to the assigned spectra and those obtained from the UV adjustments are still smaller than unity (1 D E *) for all of the illumination conditions. Two instruments using the representative UV adjusting techni- ques, for example, the conventional UV-adjusting with an adjustable (GG395) UV filter and the numerical UV- *Correspondence: Li Yang (e-mail: li.yang@innventia.com) Contract grant sponsor: European Metrology Research Programme (EMRP; jointly funded by the EMRP participating countries within EURAMET and the European Union) and Research Institute of Sweden (RISE).
Key words: color measurement; fluorescence; reflectance; UV adjustment; optical calibration
INTRODUCTION
Whiteness and brightness are important properties of paper products. CIE whiteness is a quantity depending on the total spectral radiance factor in the entire visible spec- tral range, whereas brightness is only in the blue band between 400 and 510 nm with the top contribution around 457 nm. There are four possible ways 1 to improve paper’s whiteness: bleaching, adding fillers, using shading dyes, and adding optical brightening agents (OBAs). Brightness can also be improved by bleaching, adding fillers, and OBAs but not by using shading dyes. Unlike bleaching or using fillers, which reduces light absorption or enhances light scattering, OBAs convert invisible UV radiation to visible blue fluorescence and improve total spectral radi- ance factors especially in the blue spectral band. By this method, the paper gains a blue tint that is perceived as white. OBAs occur in nearly all white paper products in Europe today, such as office paper, coated paper, and coated carton board. 2,3 It is common that an OBA- containing paper of which the total spectral radiance fac- tor in blue is 30 or 40% higher than the ideal white (reflectance 5 100%), when measured under UV-rich light illuminations, such as CIE D 65 . However, unlike the other three ways, the whiteness improvement by using OBAs depends on the UV content or more precisely the spectral power distribution (SPD) of the illumination source. Standard illuminants, D 65 and C, are the standard illu- minations in paper industry, mimicking outdoor and indoor daylight conditions. Correspondingly, in graphic
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