TABLE I. The weighting factors for standard illumi- nation conditions.
TABLE II. Differences between the chromatic coordinators.
Instruments Illuminations L Ass 2 L Cal a Ass 2 a Cal b Ass 2 b Cal
1 2 a for UV-full
a for UV-reduced
D E *
Instruments
Illuminations
2 0.195 0.365 2 0.170 0.700
2 0.140 0.437 2 0.637 0.959
CM3630d
D65
2 0.0051
1.0051 0.3618 0.6649 0.6281 0.1891 0.4069
Konica-Minolta
D 65
C
2 0.344 0.063 0.359 2 0.490 0.248 0.603
C
0.6382 0.3351 0.3719 0.8109 0.5931
D50 D65
0.079 0.248
D 50 D 65
Color-Touch 2
Color-Touch 2
C
0.080 0.046 0.133 0.162
2 0.125 0.129 0.290
C
D50
0.228
D 50
100% of UV-full (1 2 a > 1) or to have negative ( a < 0) contributions when composing (matching) the standard illuminations. Second, it eliminates the need for lengthy and repeated filter-position adjustments to ensure both correct reflectance scale and the UV content. Third, this also makes it possible that the UV content be adjusted not only to have the best agreement for the whiteness but also for the tint value (which, in case of the adjustable fil- ter is not possible). Figures 5 and 6 depict the spectra of the IR2 reference standards, measured under the three standard illumina- tions, C, D 50 , and D 65 and with the two spectrophotome- ters, CT2 and CM3630d, respectively. In the figures on the left, the solid, dashed, and dotted lines represent the calculated spectra of the respective illumination condi- tions, using Eq. (8) or (11) and with the weighting factors listed in Table I. The corresponding values assigned by NRC are denoted as “ 1 ,” “*” and “o,” respectively. The figures on the right hand side show the differences between the assigned spectra and the calculated ones. As shown, the calculated spectra and the assigned ones are largely in good agreement, except for the wavelengths around 430 nm. Moreover, the differences between the assigned values and the calculated ones decrease quickly toward longer wavelengths. Consider the fact that only single assigned values were used in the UV content adjustment, such an agreement is reasonably satisfactory. When the spectra are known, one can calculate all kinds of optical and chromatic quantities, for instance, CIE whiteness and ISO brightness values. The maximal differ- ence between the CIE whiteness (D 65 /10 8 ) value calcu- lated with the assigned spectra and the calculated ones is 0.005. For ISO brightness (C), the corresponding value is 0.062. These differences are (much) smaller than toleran- ces required by the ISO standards based on the one-value UV content adjustment techniques. Figure 7 depicts the differences of the color coordina- tors calculated using the assigned spectra and the calcu- lated ones corresponding to the three standard illuminations. As the major discrepancies between the assigned and the obtained spectra occur in the blue spec- tral band where the fluorescence contributes most, it is natural that the biggest color differences are for the b* and a* coordinators. The differences between the corre- sponding chromatic coordinators calculated from the assigned spectra and the simulated ones are listed in
The numerical UV-filtering and -adjustment technique offers a number of advantageous flexibilities when com- pared with the conventional UV-adjustment technique with a moveable UV filter (GG395). First, it allows the device to “act” beyond its upper or lower physical limits. For instance, it enables the device to have more than Fig. 7. Differences of the color coordinators, CIE XYZ and CIELAB, calculated from the assigned total spectral radiance factors and the ones obtained by matching the assigned whiteness and brightness values of the standard illuminants, D 65 , D 50 , and C. In the figures, the gray bars represent the CT2, and the black bars represent the CM3630d.
Volume 42, Number 1, February 2017
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