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The wide-gamut system colorimetry has been standardized for ultra-high definition television (UHDTV). The chromaticities of the primaries are designed to lie on the spectral locus to cover major standard system colorimetries and real object colors. Although monochromatic light sources are required for a display to perfectly fulfill the system colorimetry, highly saturated emission colors using recent quantum dot technology may effectively achieve the wide gamut. This paper presents simulation results on the chromaticities of highly saturated non-monochromatic light sources and gamut coverage of real object colors to be considered in designing wide-gamut displays with color filters for the UHDTV.
© 2014 Optical Society of America
1. Introduction
With technological advancements regarding the television and associated fields, the resolution of the television system has come to play a most important role in delivering an “immersive” experience to the viewer. In this light, the ultra-high definition television (UHDTV) is a next-generation television system that provides a better viewing experience than the currently popular high definition television (HDTV). As regards the technicalities of UHDTV, in August 2012, the ITU-R issued Recommendation BT.2020 (Rec. 2020) [1] specifying the video parameter values for UHDTV production and international program exchange. Important features of Rec. 2020 include high pixel counts of 4K (3840 2160 pixels) and 8K (7680 4320 pixels), a higher frame frequency of 120 Hz, and a wider color gamut than that of HDTV specified in Recommendation ITU-R BT.709 (Rec. 709) [2]. Table 1 lists the chromaticity coordinates of the red (R), green (G), and blue (B) primaries specified in Rec. 2020 and the corresponding wavelengths of monochromatic light. Rec. 2020 covers real object colors as well as the major standard system colorimetries and reproduces images more realistically [3,4]. Figure 1 shows the chromaticities of the RGB primary sets of standard system colorimetries: Rec. 709 (for HDTV), Adobe RGB [5] (as a de facto standard in professional color processing), and SMPTE RP 431-2 [6] (for the reference digital cinema projector, informally known as DCI-P3), and Pointer’s colors [7] (representing the maximum gamut of real object colors under Illuminant C). Pointer’s colors shown in Fig. 1 are transformed to the D65 white point with the CAT02 chromatic adaptation transform [8].
Table 1. Chromaticity Coordinates of the RGB Primaries Specified in Rec. 2020 and Corresponding Wavelengths of Monochromatic Light
Fig. 1 Chromaticities of the RGB primary sets of the standard system colorimetries for Rec. 709, Adobe RGB, DCI-P3, and those of Pointer’s colors transformed to the D65 white point with the CAT02 chromatic adaptation transform: (a) the CIE 1931 (x, y) chromaticity diagram, (b) the CIE 1976 (u′, v′) chromaticity diagram.
Monochromatic light sources such as lasers are required for a display to perfectly fulfill the Rec. 2020 system colorimetry. While liquid crystal displays (LCDs) are considered to be promising as UHDTV displays and laser-backlit LCDs are expected to be available in the near future, non-monochromatic light sources may well be used from the viewpoints of cost and performance. The key aspects to consider while using non-monochromatic light sources are the spectral bandwidths and peak emission wavelengths.
The quantum dot LED (QD-LED) is emerging as a next-generation LCD backlight source for wide-gamut LCDs [9]. Currently available red and green QDs approximately have Gaussian emission spectra at 30 nm full-width at half-maximum (FWHM) when pumped by blue LEDs at 20 nm FWHM [10]. Their central wavelengths are precisely tunable. However, even if sharp emission spectra of light sources are realized, crosstalk due to overlap in the color filters used for LCDs can reduce the purity of the RGB primaries. Current LCD color filters have been designed without considering the Rec. 2020 wide-gamut system colorimetry. Therefore, it is an urgent necessity to optimize color filters for the new colorimetry.
In this study, we explore the chromaticities of RGB primaries for displays suitable for the Rec. 2020 system colorimetry using currently available light sources. We simulate the chromaticities of non-monochromatic light sources and gamut coverage of real object colors by assuming QD-LED and laser backlit displays with color filters for realizing wide-gamut displays.
2. Chromaticities of Gaussian emission spectra
We calculated the chromaticities of non-monochromatic light sources with Gaussian emission spectra. If ideal color filters having no crosstalk are used, the chromaticities of the RGB primaries are determined by their central wavelengths and FWHMs. For the Rec. 2020 system colorimetry, it would be preferable if the chromaticities of the primaries lie on the constant hue loci of the Rec. 2020 primaries from the viewpoints of gamut mapping and balance of the coverage of the Rec. 2020 color gamut.
Figure 2(a) shows the chromaticities of the constant FWHM loci at 10, 20, …, 110 nm FWHM meshed with the constant central wavelength loci at intervals of 5 nm and the CIELAB constant hue loci of the Rec. 2020 primaries. Figures 2(b)–2(d) show enlarged sections of Fig. 2(a) around the green, blue, and red primaries, respectively. In Fig. 2(b), it can be observed that the green primaries of Adobe RGB and DCI-P3 are close to the locus at 50 nm FWHM and that the green primary of Adobe RGB is close to the constant hue locus of the Rec. 2020 green primary. In Fig. 2(c), it can be observed that the blue primary of Rec. 709 (same as those of Adobe RGB and DCI-P3) lies close to the locus at 110 nm FWHM. In Fig. 2(d), we note that the loci at different FWHMs merge to the spectral locus around red.
Fig. 2 (a) Constant FWHM loci at 10, 20, …, 110 nm FWHM meshed with constant central wavelength loci at intervals of 5 nm of Gaussian emission spectra and the CIELAB constant hue loci of the Rec. 2020 primaries: (b)–(d) enlarged sections around the green, blue, and red primaries, respectively, with the Adobe RGB, DCI-P3, and Rec. 709 primaries.
Table 2 lists the central wavelengths of Gaussian emission spectra with the CIELAB constant hue of the Rec. 2020 primaries at FWHMs ranging from 0 to 50 nm. Assuming feasible QD-LEDs with red and green QD emissions at 30 nm FWHM and blue LED emission at 20 nm FWHM, their central wavelengths are 638.9 nm, 531.6 nm, and 466.1 nm, respectively. The set of the light sources for display primaries can cover the gamuts of Adobe RGB, DCI-P3, and Rec. 709.
Table 2. Central Wavelengths and FWHMs of Gaussian Emission Spectra on the CIELAB Constant Hue of the Rec. 2020 Primaries
3. Computation of coverage of Pointer’s gamut
A popular method to evaluate display gamuts is to compare the triangle area connecting the chromaticity points of the RGB primaries to those of some standard RGBs on the CIE xy or u′v′ chromaticity diagram. It is easy to calculate the area, and most display manufacturers define color gamut coverage in areal dimensions. However, it is not reasonable that the whole area is taken into account even if the triangle protrudes outside the standard RGB ones to be compared. Further, some manufacturers exploit their advertisement of the larger ratio calculated in the two diagrams to market their product.
In this study, the CIELAB color space was used to compute gamut coverage. It is reasonable to evaluate the color gamut in terms of volume rather than area because perceptible color is represented in a color solid on the basis of the trichromatic nature of human vision, wherein a color solid is a three-dimensional representation of a color model. Although there are a few latest color appearance models that predict perceptual attributes, there is no evidence of the advantages of using them over CIELAB in gamut volume simulation [11]. To compute the coverage of Pointer’s gamut by a display, the boundaries of the object color gamut and display gamut are sampled at regular lightness-value and hue-angle intervals under Illuminant D65. The details of the computation of the gamut coverage are provided in the Appendix.
3.1 Coverage by display with ideal color filters
We computed the display gamut with RGB light sources having Gaussian emission spectra for primaries on the constant hue loci of the Rec. 2020 primaries and ideal color filters having no crosstalk. The relative intensity of each light source was optimized to have a white point corresponding to the Rec. 2020 reference white (D65). In the simulation, the chromaticity of the red primary was set to that of the Rec. 2020 red primary because the red primary with ideal color filters can remain on the spectral locus regardless of its FWHM, as seen in Fig. 2(d). Further, the red primary exclusively affects the gamut in red and magenta while both green and blue primaries relate to the coverage of cyan. Figure 3 shows the coverage ratio of Pointer’s gamut by a display with the blue light source at 0–100 nm FWHM and the green light source at 0–50 nm FWHM, with both lying on the constant hue loci of the Rec. 2020 primaries. The coverage ratio is more than 99.9% with monochromatic RGB light sources. The coverage is significantly reduced when the FWHM of the green light source is over 30 nm. Assuming feasible QD-LEDs with red and green at 30 nm FWHM and blue at 20 nm FWHM, the coverage ratio is 99.6%.
Fig. 3 Contour plot of the coverage ratio of Pointer’s gamut (%) by a display with the blue light source at 0–100 nm FWHM and the green light source at 0–50 nm FWHM, with both lying on the constant hue loci of the Rec. 2020 primaries, and the Rec. 2020 red primary.
Figure 4 shows the coverage ratio of Pointer’s gamut by a display with the blue light source for a central-wavelength range of 457–477 nm at 20 nm FWHM, the green light source for a central-wavelength range of 522–542 nm at 30 nm FWHM, and the Rec. 2020 red primary. The coverage by the blue light source at the central wavelength of 466.1 nm and the green light source at the central wavelength of 531.6 nm, which lie on the constant hue loci of the Rec. 2020 primaries, is close to the peak.
Fig. 4 Contour plot of the coverage ratio of Pointer’s gamut (%) by a display with the blue light source with a central-wavelength range of 457–477 nm at 20 nm FWHM, the green light source with a central-wavelength range of 522–542 nm central wavelength at 30 nm FWHM, and the Rec. 2020 red primary.
3.2 Coverage by display with real color filters
Crosstalk between currently available color filters reduces the purity of light sources for primaries. Figure 5(a) shows the transmission spectra of the color filters as provided in [10]. First, we select a set of monochromatic RGB light sources whose wavelengths are 630 nm, 532 nm, and 467.1 nm, respectively, corresponding to those of Rec. 2020. The relative intensity of each light source after being transmitted through the color filters was optimized to have a white point corresponding to the Rec. 2020 reference white (D65). Figures 5(b)–5(d) show enlarged sections around the green, blue, and red primaries, respectively. The primaries shift inward due to the crosstalk between the color filters. The gamut coverage ratio of Pointer’s gamut is decreased from more than 99.9% to 95.3%. To mitigate the effect of the crosstalk between the color filters, the wavelengths of the green and blue light sources need to be separated from the corresponding wavelengths of Rec. 2020. When the wavelength of the green light source is set at 535 nm, which corresponds to that of the peak transmittance of the green filter, and that of the blue light source is set at 461 nm, the chromaticities of the green and blue primaries lie close to the constant hue loci of the Rec. 2020 primaries as shown in Figs. 5(b) and 5(c) while that of the red primary remains almost in the same position (not shown in Fig. 5(d)). The gamut coverage ratio is 98.4%, which is an improvement of 3.1% from 95.3%.
Fig. 5 RGB primaries with color filters: (a) Transmission spectra of color filters and normalized spectra of the optimized laser light sources (461 nm, 535 nm, and 630 nm) and QD-LED (459 nm, 535 nm, and 638.9 nm) light sources, (b–d) the chromaticities of the selected green, blue, and red light sources, respectively, with and without color filters, constant FWHM loci at 10, 20, …, 110 nm FWHM of Gaussian emission spectra, and CIELAB constant hue loci of the Rec. 2020 RGB primaries.
When the XYZ tristimulus values of the peak white after passing through the color filters are (95.0, 100.0, 108.9), the tristimulus values of the optimized monochromatic RGB light source (630, 535, and 461 nm) before passing through the color filters are (63.3, 26.1, 0.0), (19.7, 79.8, 2.6), and (23.0, 5.1, 133.2), respectively. Regarding the blue color filter, the transmittance at these wavelengths are 0.35%, 4.19%, and 78.0%, respectively. The xy chromaticity of the red and blue light after passing through the blue color filter is (0.144, 0.032), which is close to that of monochromatic light at 461 nm (see Fig. 2(c)). However, when the crosstalk from the green light is added, the xy chromaticity shifts to (0.145, 0.057), thereby resulting in a significant decrease in purity. Regarding the green color filter, the transmittance at these wavelengths are 0.31%, 85.5%, and 1.65%, respectively. The xy chromaticity of the red and green light after passing through the green color filter is (0.195, 0.780), which is close to that of monochromatic light at 535 nm. However, when the crosstalk from the blue light is added, the xy chromaticity shifts to (0.193, 0758). The small crosstalk of 1.65% of the blue light significantly decreases the purity of the green primary.
Next, we select a set of non-monochromatic light sources with 30 nm FWHM for the green and red light sources and 20 nm FWHM for the blue light source, assuming feasible QD-LEDs. When the central wavelengths of the RGB light sources are 638.9 nm, 531.6 nm, and 466.1 nm, respectively, corresponding to the values listed in Table 2, it is observed in Figs. 5(b)–5(d) that the primaries shift inward due to the crosstalk. The gamut coverage ratio of Pointer’s gamut is decreased from 99.6% to 92.0%. When the central wavelength of the green light source is set at 535 nm again (for the purpose of comparison with the previously selected monochromatic light source) and the central wavelengths of the blue light sources is set to 459 nm, the chromaticities of the green and blue primaries lie close to the constant hue loci of the Rec. 2020 primaries as shown in Figs. 5(b) and 5(c), respectively. The chromaticity of the red primary remains almost in the same position (not shown in Fig. 5(d)). The gamut coverage ratio is 96.3%, which is an improvement of 4.3% from 92.0%.
3.3 Comparison of coverage ratios
Table 3 lists the estimated coverage ratios of Pointer’s gamut in the three-dimensional CIELAB color space and Rec. 2020 area ratios calculated in the two-dimensional xy and u′v′ chromaticity diagrams by displays with the selected sets of RGB light sources. The color filters decrease the gamut coverage ratio from more than 99.9% to 98.4% in the case of monochromatic RGB light sources and from 99.6% to 96.3% in the case of non-monochromatic RGB light sources. The ranking between the coverage ratios of Pointer’s gamut and area ratios in the u′v′ chromaticity diagram is consistent, but it is not so for the ratios in the xy chromaticity diagram.
Table 3. Estimated Coverage Ratios of Pointer’s Gamut in the CIELAB Color Space and Rec. 2020 Area Ratios Calculated in the xy and u′v′ Chromaticity Diagrams by Displays with the Selected Sets of RGB Light Sources
While the choice of the central wavelengths of RGB light sources may further be optimized for the given color filters, the crosstalk between the color filters should be reduced sufficiently to highlight the larger gamut-coverage capacity of the Rec. 2020 wide-gamut system colorimetry.
4. Conclusion
We presented our simulation results on the chromaticities of non-monochromatic RGB light sources and gamut coverage of real object colors for designing Rec. 2020 wide-gamut displays. Since current LCD color filters significantly reduce the purity of the RGB primaries even when using monochromatic light sources, it is an immediate requirement to optimize color filters for the new colorimetry. We believe that our findings will contribute to the practical realization of the UHDTV system colorimetry on displays.
Appendix
Method for computing coverage of Pointer’s gamut by display
The coverage ratio of Pointer’s gamut by a display is calculated as the volume ratio of Pointer’s gamut covered by the display gamut to the whole Pointer’s gamut in the CIELAB color space. Each gamut is represented by 100 loci computed at regular lightness-value intervals of 1 unit. Each locus consists of 360 maximum chroma values sampled at regular hue-angle intervals of 1°. The sum of the areas of the loci approximates the volume. The following subsections provide the detail of the gamut computation.
Pointer’s gamut computation
Pointer [7] published the maximum chroma values at 36 hue angles of 0°, 10°, …, 350° and 16 lightness values of 15, 20, …, 90 in the CIELAB color space under Illuminant C. The color data were transformed into those under Illuminant D65 using the CAT02 chromatic adaptation transform [8]. Fig. 1 shows the chromaticities of the transformed colors. Pointer’s colors have traditionally been used as a target gamut of real object colors for wide-gamut displays. Further, the coverage of Pointer’s gamut was analyzed in the design process of the Rec. 2020 system colorimetry [3].
We resampled Pointer’s colors for the gamut computation. The colors transformed by CAT02 slightly deviate from the sampling points of the original 36 hue angles and 16 lightness values (which complicates the computation) to obtain 100 loci sampled at regular lightness- and hue-value intervals. To solve this problem, we performed brute-force computation. We sampled colors at regular lightness-value intervals of 0.02 ( = 0.02, 0.04, …, 99.98) and hue-angle intervals of 0.02 ( = 0°, 0.02°, …, 359.98°) by linearly interpolating the original Pointer’s color data with black ( = 0) and white (= 100) points. After applying the CAT02 chromatic adaptation transform to the 89,982,000 colors (4999 lightness values 18,000 hue angles), we extracted 36,000 colors close to the sampling points at regular intervals of = 0.5, 1.5, …, 99.5 and = 0°, 1°, …, 359°, which resulted in a precision of about 0.01% for each lightness value and hue angle. The maximum chroma values were used for computing the volume , which is approximated as .
Display gamut computation
The chroma values on the boundary of display gamut were computed at regular intervals of = 0.5, 1.5, …, 99.5 and = 0°, 1°, …, 359° by locating the chroma values on the gamut boundary at each lightness value and hue angle. When there were multiple gamut boundaries at one lightness value and one hue angle, we chose the smallest one. This is the case of yellow highlight, and it does not significantly affect gamut coverage computation.
Coverage of real object color gamut
The maximum chroma values of boundary of Pointer’s gamut covered by the display gamut consists of the smaller chroma values sampled at regular intervals of = 0.5, 1.5, …, 99.5 and = 0°, 1°, …, 359° between the two gamuts: . The volume is approximated as . Subsequently, the coverage ratio of Pointer’s gamut by the display is calculated as .
Acknowledgments
The authors are indebted to Zhenyue Luo of CREOL, the College of Optics and Photonics, University of Central Florida, Orlando, for providing the transmission spectra of the color filters used in the study.
References and links
1. ITU-R Recommendation BT.2020, “Parameter values for ultra-high definition television systems for production and international programme exchange,” 2012.
2. ITU-R Recommendation BT.709-5, “Parameter values for the HDTV standards for production and international programme exchange,” 2002.
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4. ITU-R Report BT.2246–2, “The present state of ultra-high definition television,” 2012.
5. Adobe Systems Inc., “Adobe RGB (1998) Color Image Encoding,” 2005.
6. SMPTE RP 431-2, “D-Cinema Quality — Reference Projector and Environment,” 2011.
7. M. R. Pointer, “The gamut of real surface colour,” Color Res. Appl. 5(3), 145–155 (1980). [CrossRef]
8. “A colour appearance model for colour management systems: CIECAM02,” CIE 159:2004 (CIE, 2004).
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