Preferred balance between luminance and color gamut in mobile displays

Abstract— To improve the image quality of a mobile display, the balance between color‐gamut size and luminance was studied in two subjective experiments. The first experiment was performed during the Asian Society for Information Display (ASID) conference in Nanjing, February 2004. Nearly 600 participants ranked the quality of images displayed for fixed combinations of color‐gamut size and display luminance on small color supertwisted nematic (CSTN) and thin‐film transistor (TFT) twistednematic (TN) displays. In the second experiment, a broader range of color‐gamut sizes and luminance levels were simulated on a cathode‐ray tube (CRT) display, and 20 participants were asked to score perceived image quality. The results of these experiments were used to model image quality as a function of color‐gamut size and display luminance for images differing in the level of chromaticity of their content. This model can be used to estimate the increase in luminance required to compensate for a reduction in color‐gamut size.     

Single display gamut size metric

To measure the relative gamut sizes of wide‐gamut displays, it is herein proposed that the CIE 1931 xy chromaticity diagram be used rather than the nominally perceptually uniform CIE 1976 u′v′ chromaticity diagram. High correlations were found between the area‐coverage ratios in the xy diagram and the volume‐coverage ratios in the CIE 1976 L^*a^*b^* color space for major standard wide‐gamut color spaces. It is also demonstrated herein that performing planimetry in the uniform u′v′ diagram does not yield accurate relative display gamut sizes, even though the large sizes obtained using the u′v′ diagram are often reported regardless of the fact that its uniformity is valid only when the luminance factor is constant. The single display gamut size metric using the xy diagram will facilitate the unbiased development of wide‐gamut displays.     

Spatially uniform colors for projectors and tiled displays

Abstract— A major issue when setting up multi‐projector tiled displays is the spatial non‐uniformity of the color throughout the display's area. Indeed, the chromatic properties do not only vary between two different projectors, but also between different spatial locations inside the displaying area of one single projector. A new method for calibrating the colors of a tiled display is presented. An iterative algorithm to construct a correction table which makes the luminance uniform over the projected area of one single projector is presented first. This so‐called intra‐projector calibration uses a standard camera as a luminance measuring device and can be processed in parallel for all projectors. Once the color inside each projector is spatially uniform, the set of displayable colors — the color gamut — of each projector is measured. On the basis of these measurements, the goal of the inter‐projector calibration is to find an optimal gamut shared by all the projectors. Finding the optimal color gamut displayable by n projectors in time O(n) is shown, and the color conversion from one specific color gamut to the common global gamut is derived. The method of testing it on a tiled display consisting of 48 projectors with large chrominance shifts was experimentally validated.     

New display concept for realistic reproduction of high-luminance colors

A new display concept for reproduction of high-luminance colors based on a liquid crystal display has been developed using a brighter backlight unit and color mapping algorithms. The new concept is able to display brighter colors close to a peak luminance of a display white than conventional displays so that realistic scene of brighter colors is better reproduced. It may also be one of the future display solutions needed to extend the color gamut in the direction of brighter colors, which is a principal limitation in conventional displays even in high-dynamic range display systems. With the new concept, an xvYCC- (extended-video YCbCr) compatible display can be easily realized.     

Investigation into the stability condition of correlated color temperature of white illumination sources based on trichromatic light-emitting diodes

We present a simple method to obtain a stable correlated color temperature (CCT) for the variation in light output power (LOP) of a light-emitting diode (LED) comprising a trichromatic LED-based white light source. A mathematical model was developed to determine the condition for the stable CCT operation using colorimetric analyses, and the stable CCT condition can be obtained by taking the derivative of the CCT with respect to the power ratio between the LEDs. Specifically, the stable CCT point that is invariant to the change in the LOP of a green LED can be uniquely determined at a certain value of the power ratio between red and blue LEDs. It was found that the chromaticity coordinate of the mixed white light moves along the isoCCT line of the Planckian locus as the LOP of a green LED varied under the stable CCT condition. The existence of the stable CCT point was also verified experimentally. The proposed method is expected to be advantageously employed in optimizing trichromatic LED-based white illumination sources for improved color stability.     

Inverse display characterization: A two‐step parametric model for digital displays

Abstract— A simple additivity model is often used as a basic model for digital‐display characterization. However, such a simple model cannot satisfy the needs of demanding color‐management applications all the time. On the other hand, systematic sampling of the color space and 3‐D interpolation is an expensive method in terms of measurement and computation time when precision is needed. This paper presents an enhanced method to characterize the XYZ‐to‐RGB transform of a digital display. This parametric method exploits the independence between the luminance variation of the electro‐optic response and the colorimetric responses for certain display types. The model is generally applicable to digital displays, including 3‐DMD projectors, single DMDs, CRTs, LCDs, etc., if the independence condition is satisfied. While the problem to solve is a 3‐D‐to‐3‐D transformation (from XYZ to RGB), the proposed parametric model is the composition of a 2‐D transform followed by a 1‐D transform. The 2‐D transform manages the chromatic aspects and, in succession, the 1‐D transform manages the luminance variations. This parametric digital model is applicable in the field of color management, with the objective of characterizing digital displays and applying a reference look such as a film look.     

Adapting softcopy color reproduction to ambient illumination

Abstract— The perceived colors of an image seen on a self‐luminous display are affected by ambient illumination. The ambient light reflected from the display faceplate is mixed with the image‐forming light emitted by the display. In addition to this direct physical effect of viewing flare, ambient illumination causes perceptual changes by affecting the adaptation state of the viewer's visual system. This paper first discusses these effects and how they can be compensated, outlining a display system able to adjust its output based on prevailing lighting conditions. The emphasis is on compensating for the perceptual effects of viewing conditions by means of color‐appearance modeling. The effects of varying the degree of chromatic adaptation parameter D and the surround compensation parameters c and N_c of the CIECAM97s color‐appearance model were studied in psychophysical experiments. In these memory‐based paired comparison experiments, the observers judged the appearance of images shown on an LCD under three different ambient‐illumination conditions. The dependence of the optimal parameter values on the level of ambient illumination was evident. The results of the final experiment, using a category scaling technique, showed the benefit of using the color‐appearance model with the optimized parameters in compensating for the perceptual changes caused by varying ambient illumination.     

The luminance resolution characteristics of multi‐primary‐color display

Abstract— In this paper, the resolution characteristics of multi‐primary‐color (MPC) display systems are analyzed. That four‐primary‐color (4PC) displays can increase the effective resolution for achromatic images in the luminance domain by a factor of two as compared to conventional RGB‐based displays with MPC‐specialized subpixel rendering, which is proposed in this paper, is demonstrated. Five‐ and six‐primary‐color (5PC and 6PC) display systems can reproduce denser luminance data than conventional RGB‐based display systems and solve a problem of MPC displays, viz. an increase of production costs and a decrease in the aperture ratio caused by increasing the number of subpixels in one pixel. This is an essential advantage of MPC display systems, which is related to the combination of the proposed color‐filter architecture and image processing. Thus, a completely new advantage of MPC display systems, in addition to their well‐known capabilities of color reproduction and power saving, is proposed.     

All solution‐processed white quantum‐dot light‐emitting diodes with three‐unit tandem structure

Quantum‐dot light‐emitting diodes (QLEDs) are promising candidates for next generation displays. White QLEDs which can emit red, green and blue colors are particularly important; this is because the combination of white QLEDs and color filters offers a practical solution for high‐resolution full‐color displays. In this work, we demonstrate all‐solution processed three‐unit (red/green/blue) white tandem QLEDs for the first time. The white tandem devices are achieved by serially connecting the red bottom sub‐QLED, the green middle sub‐QLED and the blue top sub‐QLED using the inter‐connecting layer (ICL) based on ZnMgO/PEDOT:PSS heterojunction. With the proposed ICL, the two‐unit tandem QLEDs exhibit a high current efficiency of 22.22 cd/A, while the three‐unit white QLEDs exhibit evenly separated red, green and blue emission with a CIE coordinate of (0.30, 0.44), a peak current efficiency of 4.75 cd/A and a high luminance of 4206 cd/m². Displays based on the developed white QLEDs exhibit a wide color gamut of 114% NTSC. This work confirms the effectiveness of the proposed ZnMgO/PEDOT:PSS ICL and the feasibility of making all‐solution processed tandem white QLEDs by using the proposed ICL.     

Color management of reflective‐type LCDs in terms of adaptation of the human visual system to light‐source variation

We have investigated the color management, in terms of the color adoption property of the human visual system, of a reflective‐type TFT‐LCD (R‐LCD). Since the R‐LCD depends on ambient light as the light source, it is expected that the colorimetric color on the R‐LCD must be changed if the source of the ambient light is changed. However, due to the adaptation property of the human visual system, the eye does not perceive colorimetrically corrected colors as the same color even for an R‐LCD. In this research, first, we conducted a subjective experiment to obtain the RGB code value that is required in order to display a corresponding color on the R‐LCD under varying ambient‐light conditions. The result of the experiment shows that the corresponding color of the experimental results was reasonably approximated by the color obtained by using the von Kries model. Secondly, we proposed a color‐compensating mechanism that is described as a cascaded simple 3 × 3 linear matrix. Actual colors displayed are adjusted according to the ambient light. The evaluation of the picture quality of the R‐LCD showed that the proposed model is effective.     

Legibility and subjective preference for color combinations in text

This study examined legibility performance and subjective preference for text/background color combinations displayed on a video monitor. Luminance contrast was fixed at two preoptimized levels, either with text brighter than the background (10:1) or vice versa (1:6.5). In Experiment 1, 32 subjects rated about 800 color combinations. No evidence suggested differential effects of luminance polarity or hue, with the only exception that cool background colors (blue and bluish cyan) tended to be preferred for the light-on-dark polarity. Saturation had the most important influence on ratings. Any desaturated color combination appears to be satisfactory for text presentation. In Experiment 2 a reduced set of 18 color combinations was investigated with a new sample of 18 subjects. Reading and search times as well as multidimensional ratings were evaluated. There was no evidence for an influence of luminance polarity or chromaticity on performance. Subjective ratings corresponded well with the results of Experiment 1.     

Hue rotation (HR) and hue blending (HB): Real‐time image enhancement methods for digital component video signals to support red‐green color‐defective observers

The mutual understanding of color‐normal observers (CNOs) and color‐defective observers (CDOs) is now essential because personal color information display environments have been widely adopted. However, existing tools for CDOs offer only color discrimination; they fail to support color impression (ie, saturation and contrast). Therefore, we need a novel tool that offers help in distinguishing opponent colors, while preserving color saturation. We introduce two key techniques for realizing this difficult goal. The former is the repeated sequential display of the original and processed images to support the formation of unified correct percepts that provide discrimination of both red‐green and yellow‐blue opponent colors. One image, ie, original, exhibits correct yellow‐blue but distorted red‐green information for CDOs while the other, ie, processed, provides synthesized distinguishable red‐green but confusable yellow‐blue information for CDOs; here, hue rotation (HR) is useful for advanced users whereas hue blending (HB) is suitable for general. The latter is realized by the real‐time video processing available on smartphones; our algorithms support direct processing of the digital component video signal formats (eg, Y, C_R, and C_B). Subjective tests suggest that the two above‐mentioned algorithms will, along with embedding a lightweight real‐time dichromatic simulation facility for CNOs, greatly help the mutual understanding of CNOs and CDOs.     

A perceptually based metric to characterize the viewing‐angle range of matrix displays

Abstract— The current specification of a display's viewing angle as the angle within which the contrast ratio is larger than 10:1 appears not to be predictive for the acceptable viewing‐angle range obtained from perception experiments. In our search towards a perceptually relevant specification for the viewing angle, the physical characteristics of the display that are most related to the viewing‐angle‐dependent image quality were analyzed. This was done for two types of liquid‐crystal displays and one plasma TV. The results indicate that a combination of the luminance and chromaticity coordinates of the higher gray levels predicts the degradation in image quality as a function of viewing angle. As a consequence, a new definition of a display's viewing‐angle range is proposed based on these characteristics.     

Efficient white organic light‐emitting diodes based on a balanced split of the exciton‐recombination zone using a graded mixed layer as an electron‐blocking layer

Abstract— Efficient white organic light‐emitting diodes with both a graded mixed layer as the blue‐emitting layer and an electron‐blocking layer, and a DPVBi:Rubrene layer as a yellow‐emitting layer have been demonstrated. The mixing of the two colors occurs due to a balanced split of the exciton‐recombination zone by the graded mixed layer serving as the electron‐blocking layer. The white organic light‐emitting diode with an ITO/2‐TNATA 30 nm/NPB 30 nm/DPVBi:Rubrene (1.0 wt.%) 5 nm/NPB:DPVBi (9:1) 150 nm/NPB:DPVBi (5:5) 75 nm/NPB:DPVBi (3:7) 75 nm/NPB:DPVBi (2:8) 75 nm/NPB:DPVBi (0.5:9.5) 75 nm/BCP 5 nm/Alq³ 30 nm/LiF 0.5 nm/Al 100 nm structure is chosen as a device with an optimal configuration among devices investigated in this study. The employment of the graded mixed layer in the device is effective in suppressing the color shift at different voltages. The white light, with a Commission Internationale d'Eclairage chromaticity coordinates of (0.33, 0.34), is obtained with an applied voltage of 10.5 V for the device. At the applied voltage, the luminance is 4882 cd/m² and the current efficiency is 5.03 cd/A.     

A method for selecting display primaries to match a target color gamut

Abstract— A method for selecting primaries of a wide‐gamut display is proposed, in which display color gamut is designed to match a target color gamut in CIELAB color space. A standard deviation of the relative maximum chroma of display and target color gamuts is defined. The selection method optimizes display primaries for the minimum standard deviation so that display and target color gamuts are similar in shape. It is shown that the color gamut of a laser display designed by this method is similar in shape to the theoretical maximum, or optimal, color gamut of objects. It is also shown that the color gamut of an LED display can be designed to include 99.7% of the gamut of Pointer's real‐world surface colors. LED primaries are selected to minimize the standard deviation of the relative maximum chroma of effective display color gamut and a target color gamut which is defined to include Pointer's real‐world surface colors. For both the laser and LED displays, it is necessary to constrain the red‐primary wavelength to avoid excessive optical power for the red primary.     

基于NSST的改进双通道PCNN多聚焦RGB图像融合

针对光学成像设备景深有限、图像部分失焦的问题,提出一种基于非下剪切波变换(NSST)的改进双通道脉冲神经耦合网络(PCNN)融合算法。首先,该算法采用Lab颜色空间分割RGB图像的亮度分量和色度分量间的关联性得到亮度和色度通道子图;然后,亮度通道子图使用NSST重构,色度通道子图使用能量匹配融合;针对融合时阈值设置和点火量化产生的误差,提出改进双通道PCNN模型融合,并结合对比敏感度函数(CSF)自适应设定PCNN参数;最后,亮度和色度重构图通过逆Lab得到最终融合图。实验结果证实,该算法可有效减小失真,抑制伪影并保留边缘轮廓信息,提升全场景清晰度。     

Optically adjustable display color gamut in time-sequential displays using LED/Laser light sources

Recently, various wide color gamut displays have been developed. Because of color gamut differences between wide gamut displays and the standard definitions, the color reproduction on such display system is not equal to original images. To overcome this, a color transformation such as gamut mapping algorithm is applied in conventional systems. But this manipulates only the video signal that would reduce the signal dynamic range and produces quantization error of video signal.In this paper, we propose a new optical Color Gamut Processing instead of an image data mapping. In the method, an individual color gamut of the standard definitions can be composed of an optical mixture of original primary colors. As a result, a perfect color match for the various standards can be realized within a display gamut without any signal artifacts.Moreover the same concept is applied to the video stream in order to elevate scene luminance. A scene-adaptive Dynamic Color Gamut Processing measures a color gamut for every video frame and adjusts a matched display color gamut so that it is able to increase display contrast in comparison to conventional display systems.     

Analysis of standard chromaticity gamut area metrics

It is herein proposed to measure display gamut sizes by employing the International Telecommunication Union—Radiocommunication Sector Recommendation BT.2020 (Rec. 2020) area‐coverage ratios in the xy chromaticity diagram rather than the standard gamut area metrics that use the horseshoe‐shaped spectrum chromaticity area as the target in the u′v′ chromaticity diagram. It is more reasonable to use the Rec. 2020 gamut than the spectrum gamut as the target because the Rec. 2020 area‐coverage ratios in the xy diagram are better correlated than the spectrum area‐coverage ratios with the volume‐coverage ratios of object color gamuts that are visually significant in displaying natural scenes.     

Diffractive backlight grating array for mobile displays

Abstract— The display backlight unit (BLU) is the most power‐consuming subunit in mobile liquid‐crystal displays. The state‐of‐the‐art BLUs utilize scattering, refractive, and reflective microstructures to generate a uniform distribution of white light through the display. More effective means of transmitting light through the display color filters could be obtained by using diffraction, but previously proposed diffractive backlights do not fully utilize all the possibilities to design gratings effectively for optimal color separation and outcoupling. This paper presents a new pixelated diffractive backlight grating array as an approach for overcoming these obstacles in BLU design. A model array was fabricated to couple out red, green, and blue primary colors from the respective subpixel locations. The results show that it is possible to manufacture such an array and that the light couples out as intended, giving a starting point to design mobile‐display modules with low light‐transmission losses.     

Masking model for accurate colorimetric characterization of LCD

Abstract— A color management system (CMS) such as ICC profile or sRGB space have been proposed for color transformation and reproduction of cross media. In such a CMS, accurate colorimetric characterization of a display device plays a critical role in achieving device‐independent color reproduction. In the case of a CRT, colorimetric characterization based on a GOG model is accurate enough for this purpose. However, there is no effective counterpart in liquid‐crystal displays (LCDs) since the characterization of an LCD has many difficulties, such as channel interaction and non‐constancy of channel chromaticity. In this paper, a new method of display characterization is proposed which is applicable to the assessment of color reproduction of LCDs. The proposed method characterizes an electro‐optical transfer function considering both channel interaction and non‐constancy of channel chromaticity. Experimental results show that the proposed method is very effective in the colorimetry of LCDs.