Fig. 1. Schematic illustration of the conventional UV-filtering and -adjustment techniques. The UV amount is regulated by moving the GG395 UV filter in and out of the optical path. The light beam that passes through the UV filter has wave- lengths longer than 400 nm.
mination, which strongly impacts color measurement, there is no well-documented method for the adjustment of the UV content. The objective of this work is to examine how effective or reasonable the one-point-matching UV adjustment is in colorimetric measurements standardized by ISO 5631-1, 2 2, and 2 3. 15–17
industry, the standard illuminants are D 50 and A. These illuminations have different UV contents; D 65 is the strongest followed by D 50 . When the SPDs are normal- ized at k 5 370 nm around which the excitation efficiency peaks, the C illuminant has the lowest UV content, 4 lower than the A illuminant. It is obvious and natural that the same print is perceived very differently when viewed under different illuminations, provided there are OBAs involved. According to the ISO standards applicable for paper- making industries, 5–8 adjustments of the UV contents of the standard illuminants, C and D 65 , are achieved by matching to single-assigned values, namely, ISO bright- ness for C and CIE whiteness for D 65 , respectively. The reasons CIE whiteness was chosen as the UV-adjustment criteria were given by Bristow, 9 as it is simple and rea- sonably accurate in reproducing the spectral radiance fac- tor corresponding to the assigned CIE whiteness value. However, these methods have intrinsic weaknesses, for instance, fluorescent metamerism. Jordan et al . 4 pointed out that a true D 65 colorimetry of fluorescent paper requires matching the SPD of the illumination because the detailed shape of the emission spectrum depends on the joint features of the UV spectrum of the illumination weighted by the corresponding excitation spectrum. Hence, the one-point-matching technique can hardly ensure that the adjusted illumination is equal (or close) to the standard illuminant, D 65 or C, in papermaking indus- tries. In other words, two illuminations of different UV spectroscopic characteristics may still result in identical CIE whiteness or ISO brightness, which is known as fluo- rescent metamerism. Optical brightening agents contribute not only to paper properties, for example, whiteness and brightness, but also affect printed colors. Fluorescent light emitted by OBAs enhances optical dot gain and leads to more satu- rated color print. 10,11 In graphic industries, D 50 and A are the standard illuminants according to ISO standards. 12–14 However, despite significant UV content of the D 50 illu-
UV-FILTERING AND -ADJUSTMENT TECHNIQUES
According to the ISO standards, the standard illuminants used in papermaking and graphic industries are CIE D 65 and C and D 50 and A, respectively. From the colorimetric point of view, illuminant A gives more fluorescence con- tribution than illuminant C. However, it is still reasonable to focus on illuminants D 65 , D 50 , and C, because illumi- nant A is realizable by an incandescent lamp and needs no UV adjustment. To generate different illuminations having different UV contents within a spectrophotometer, UV-filtering technique is used, often with only one or a few UV filters. Spectrophotometers used in paper industries apply basi- cally two major types of UV-adjustment techniques 18 : the traditional filtering method introduced by G € artner and Griesser 19 in the 1970s, and the numerical UV-filtering technique introduced by Imura et al . 20 in 1990s. These two techniques are further explained in detail.
Conventional UV-Adjustment Technique with Adjustable Filters
Typically, a spectrophotometer based on the conven- tional UV-adjustment technique (used in papermaking industry) is equipped only with one xenon lamp as the light source. The standard illuminations are, in practice, obtained by applying the adjustable UV filter, as shown in Fig. 1. The most commonly used is GG395 UV filter with the cutoff wavelength at 400 nm. This filter removes (absorbs) UV radiations below 400 nm.
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COLOR research and application
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