Luminance Range Research Articles

Double Emitting Layer Based Solution Processed WOLEDs Simultaneously with High Power Efficiency and Good Color Stability

AbstractA double emitting layer (EML) device configuration is successfully demonstrated for power‐efficient and color‐stable white organic light‐emitting diodes (WOLEDs) that are fabricated via orthogonal solvent processing. In this case, a self‐host blue dendrimer B‐G2 is independently used as the bottom EML due to its excellent nondoped device performance and perfect alcohol resistance, and an alcohol‐soluble yellow phosphor Ir(Flpy‐CF3‐EG)3 functionalized with 2‐(2‐methoxyethoxy)ethyl group is newly developed as the upper EML combined with the arylphosphine oxide‐containing electron transporting materials. Unlike single EML based white devices, the unwanted charge trap from long‐wavelength phosphors can be easily and effectively prevented by tuning the energy level alignment in the upper EML, leading to a nearly unchanged white electroluminance with a negligible Commission Internationalede l'Eclairage (CIE) variation of ΔCIE (0.008, 0.005) in a luminance range from 2.2 to 1015 cd m−2. Meanwhile, a promising power efficiency of 51.5 lm W−1 is realized, which can be further optimized to 104.5 lm W−1 at maximum and 62.0 lm W−1 at 1000 cd m−2 after the addition of an out‐coupling half‐sphere. The simultaneous achievement of high power efficiency and good color stability clearly indicates the great potential of double EML in solution processed WOLEDs.

Humans Trust Central Vision More Than Peripheral Vision Even in the Dark

SummaryTwo types of photoreceptors in the human retina support vision across a wide range of luminances: cones are active under bright daylight illumination (photopic viewing) and rods under dim illumination at night (scotopic viewing). These photoreceptors are distributed inhomogeneously across the retina [1]: cone-receptor density peaks at the center of the visual field (i.e., the fovea) and declines toward the periphery, allowing for high-acuity vision at the fovea in daylight. Rod receptors are absent from the fovea, leading to a functional foveal scotoma in night vision. In order to make optimal perceptual decisions, the visual system requires knowledge about its own properties and the relative reliability of signals arriving from different parts of the visual field [2]. Since cone and rod signals converge on the same pathways [3], and their cortical processing is similar except for the foveal scotoma [4], it is unclear if humans can take into account the differences between scotopic and photopic vision when making perceptual decisions. Here, we show that the scotopic foveal scotoma is filled in with information from the immediate surround and that humans trust this inferred information more than veridical information from the periphery of the visual field. We observed a similar preference under daylight illumination, indicating that humans have a default preference for information from the fovea even if this information is not veridical, like in night vision. This suggests that filling-in precedes the estimation of confidence, thereby shielding awareness from the foveal scotoma with respect to its contents and its properties.

Stimulus range bias leads to different settings when using luminance adjustment to evaluate discomfort due to glare

Luminance adjustment is widely used to evaluate discomfort due to glare. This paper reports an experiment conducted to investigate two factors of the luminance adjustment procedure, stimulus range bias and direct vs indirect control. Stimulus range bias describes the influence on subjective evaluations of the range of stimuli available to the test observer, with range being the minimum and maximum available glare source luminance in the current context. For the glare source, an artificial window, there were three ranges, having maximum luminances of 5 106, 7288 and 9469 cd/m2. The results suggest that luminance range had a significant effect on settings made, sufficient to change settings by an amount equivalent to one step of a Hopkinson-like discomfort sensation scale. The mean luminance associated with just intolerable discomfort with the low range was less than that associated with just uncomfortable with the high range. Past experiments have used direct control, where the observer makes the adjustment directly, and indirect control, where the observer instructs the experimenter to make the adjustment actions. Both methods were used in the current experiment. It was found that range bias was larger when using direct control than with indirect control. These findings contribute to an understanding of why different studies of discomfort glare have reported different results and hence proposed different discomfort models.

Understanding RGB Color Spaces for Monitors, Projectors, and Televisions

AbstractColor imaging has many exciting new advances related to gamut, luminance, color spaces, quantum dots, and high dynamic range (HDR). In this short primer—the first of a two‐part series—we examine three color‐space specifications: Recommendation (Rec.) 709, Digital Cinema Initiatives (DCI)‐P3, and Rec. 2020.

Spatial contrast sensitivity from star- to sunlight in healthy subjects and patients with glaucoma

Glaucoma is traditionally considered an asymptomatic disease until later stages. However, questionnaire studies revealed visual complaints related to various tasks, especially under extreme luminance conditions (such as outdoor at night on an unlit road or outside in the sun). We measured contrast sensitivity (CS) over a luminance range of 6 log units spanning the scotopic to photopic range and we aimed (1) to determine whether Weber’s law also holds under extremely high luminance conditions and (2) to compare CS as a function of spatial frequency and luminance between glaucoma patients and healthy subjects. We included 22 glaucoma patients and 51 controls, all with normal visual acuity. For the second aim, we used a subgroup of 22 age-similar controls. Vertically oriented sine-wave gratings were generated with a projector-based setup (stimulus size 8x5 degrees). CS was measured monocularly at 1, 3, and 10 cycles per degree (cpd); mean luminance ranged from 0.0085 to 8500 cd/m2. ANOVA was used to analyze the effect of glaucoma, luminance, and spatial frequency on logCS. In controls, Weber’s law held for 3 and 10 cpd; for 1 cpd, CS dropped above 1000 cd/m2 (P = 0.003). The logCS versus log luminance curves did not differ grossly between patients and controls (P = 0.14; typically 0–0.2 log units); the difference became larger with decreasing luminance (P = 0.003) but did not depend clearly on spatial frequency (P = 0.27). We conclude that differences between glaucoma and healthy were relatively modest for the spatially redundant, static stimulus as used in the current study.

Further investigation on the modified hyperbolic function in the CAM16 color appearance model

AbstractIn the International Commission on Illumination (CIE) color appearance model CIECAM02, a modified hyperbolic function is used to represent luminance adaptation. The same nonlinear function is also used in the new color appearance model CAM16 [Color Res Appl., 2017;42:703‐718]. Although the modified hyperbolic function seems reasonable based on physiological evidence, it has an infinite slope at the origin, which causes instability for both the forward and inverse modes of the CIECAM02/CAM16 models. In this article, various possible extensions to the nonlinear luminance adaptation function in CIECAM02/CAM16 are reviewed and evaluated. Based on these investigations, the Gill extension to the hyperbolic function that is used to represent luminance adaptation [Proceedings of 16th Color and Imaging Conference, pp. 327‐331, 2008], is recommended at both the lower end (q < qL) and the upper end (q > qU), where q is the appropriate Rc, Gc, Bc (or Rwc, Gwc, Bwc) response. In addition, the new recommended function can be readily inverted for use in the appropriate inverse appearance model. From an extensive analysis using available experimental data sets, we also propose that, for the lower and upper limits of the luminance range in the extended model, the values qL = 0.26 and qU = 150 be used, respectively.

High Dynamic Range Image Watermarking Based on Tucker Decomposition

High dynamic range (HDR) imaging technique has received much attentions in the recent years for its abundant details and wide dynamic range of luminance. In order to display HDR images on low dynamic range display devices, tone mapping (TM) process is required. However, the TM can be deemed as an inevitable attack in the field of HDR image copyright protection. In this paper, a highly robust HDR image watermarking algorithm based on Tucker decomposition is proposed. Firstly, Tucker decomposition is carried out on HDR color image to obtain the first feature map of the core tensor, which contains most of the energies in the HDR host image. Then, a Auto-Regressive prediction method is used to establish a local correlation model of the first feature map, so that watermark can be embedded in the first feature map according to the prediction result and its true value. Furthermore, a low-complexity luminance masking approach is designed based on modified specular free map to exclude the areas not suitable for watermark embedding, and an embedding intensity selection strategy is used to balance the imperceptibility and robustness of the watermark. The experimental results show that the proposed algorithm is robust against existing TM attacks while keeping better image imperceptibility and embedding capacity.

Synaptic inhibition tunes contrast computation in the retina.

Inhibition shapes activity and signal processing in neural networks through numerous mechanisms mediated by many different cell types. Here, we examined how one type of GABAergic interneuron in the retina, the A17 amacrine cell, influences visual information processing. Our results suggest that A17s, which make reciprocal feedback inhibitory synapses onto rod bipolar cell (RBC) synaptic terminals, extend the luminance range over which RBC synapses compute temporal contrast and enhance the reliability of contrast signals over this range. Inhibition from other amacrine cells does not influence these computational features. Although A17-mediated feedback is mediated by both GABAA and GABAC receptors, the latter plays the primary role in extending the range of contrast computation. These results identify specific functions for an inhibitory interneuron subtype, as well as specific synaptic receptors, in a behaviorally relevant neural computation.

Modeling perceptual color confusion of helmet-mounted display symbology as a function of see-through contrast

In military aviation helmet-mounted displays (HMDs) or head-up displays, light from the ambient scene is added to the symbology to create a complex mixture of colors, textures, and luminances. In the case of color mixing, the color of the transparent symbology symbols shifts toward the colors of the ambient background that the symbology overlays. The magnitude of the shift depends on the contrast of the symbology against the background. Against a darkened background, there is negligible shifting of symbology color. However, during daylight conditions, symbology colors shift toward the background hue. Using CIELAB distances between symbology colors as a measure of color discrimination, confusion contrast thresholds are calculated for each of seven symbology colors mixed with fourteen different background colors over a wide range of luminance contrasts. Confusion contrast thresholds are calculated for color normal and color vision deficient (CVD) observers. For CVD observers, colors are filtered using the RGB coefficients developed by Machado. Using the color discrimination data presented here as well as previous assessments of HMD luminance requirements based on observer ratings of the quality of symbology, luminance guidelines for see-through displays are presented, which correct for a calculation error made previously.

Effect of the Decrease in Luminance Noise Range on Color Discrimination of Dichromats and Trichromats.

Color vision assessment can be done using pseudoisochromatic stimuli, which has a luminance noise to eliminate brightness differences between the target and background of the stimulus. It is not clear the influence of the luminance noise on color discrimination. We investigated the effect of change in the luminance noise limits on color discrimination. Eighteen trichromats and ten congenital dichromats (eight protans, two deutans) had their color vision evaluated by the Cambridge Colour Test, and were genetically tested for diagnostic confirmation. The stimuli were composed of a mosaic of circles in a 5° circular field. A subset of the circles differed in chromaticity from the remaining field, forming a letter C. Color discrimination was estimated in stimulus conditions differing in luminance noise range: (i) 6–20 cd/m2; (ii) 8–18 cd/m2; (iii) 10–16 cd/m2; and (iv) 12–14 cd/m2. Six equidistant luminance values were used within the luminance noise limits with the mean stimulus luminance maintained constant under all conditions. A four-alternative, forced-choice method was applied to feed a staircase procedure to estimate color discrimination thresholds along eight chromatic axes. An ellipse model was adjusted to the eight color discrimination thresholds. The parameters of performance were threshold vector lengths and the ellipse area. Results were compared using the Kruskal-Wallis test with a significance level of 5%. The linear function model was applied to analyze the dependence of the discrimination parameters on the noise luminance limits. The first derivative of linear function was used as an indicator of the rate of change in color discrimination as a function of luminance noise changes. The rate of change of the ellipse area as a function of the luminance range in dichromats was higher than in trichromats (p < 0.05). Significant difference was also found for individual thresholds in half of the axes we tested. Luminance noise had a greater effect on color discrimination ability of dichromats than the trichromats, especially when the chromaticities were close to their protan and deutan color confusion lines.

Design and validation of a compact embedded photometric device for real-time daylighting computing in office buildings

Studies have shown that the exposure to daylight can have substantial visual/non-visual benefits for building occupants. To optimise daylighting provision while maintaining a comfortable visual environment, daylighting control systems have been investigated by architectural design and academic research for a number of years. However, real-time regulation of daylighting in buildings requires transient daylighting simulation with high accuracy and, daylighting simulation, as performed to date, is significantly impacted by the employed sky luminance distribution models which only crudely reproduce the real sky characteristics due to oversimplification and limited luminance sampling inputs.In this paper, an embedded photometric device is proposed to combine high dynamic range (HDR) imaging based high-resolution sky luminance monitoring with quasi real-time on-board daylighting computing, composed of a low-cost image sensor and a field programmable gate array (FPGA) micro-processor. A deliberate calibration procedure of the whole imaging system, regarding its spectral response (spectral correction error f’1 = 8.89%), vignetting effect and signal response, was formulated and validated experimentally. The device was made to measure a wide luminance range (150 dB) including that of the direct solar disk, sky vault, and landscape simultaneously. Finally, experiments during predominant clear and overcast sky conditions were conducted respectively to assess its performance in daylighting simulation, both qualitatively and quantitatively. The experimental results demonstrated its quality in solar tracking as well as its capability to reduce daylighting simulation error to 17∼13 of that of a common practice using the conventional Perez all-weather sky model for workplane illuminance calculation in office buildings.

Adaptive perceptual quantizer for high dynamic range video compression

Although high dynamic range (HDR) videos are a very fascinating way to represent real-world scenes, they need a huge amount of memory to store and transmit due to the high bit-depth. Thus, it is a major challenge for HDR video coding to efficiently compress them without sacrificing perceptual quality. Perceptual Quantizer (PQ) transfer function provides a solution to this problem, which is adopted as the HEVC Main 10 Profile-based Anchor. However, PQ is not adaptive to HDR contents, thus reducing the coding efficiency. In this paper, we propose adaptive transfer function based on PQ for HDR video compression, called adaptive PQ. Different from PQ which uses a fixed mapping curve from luminance to luma, the proposed transfer function adaptively maps luminance to luma according to HDR contents. Thus, adaptive PQ is able to efficiently utilize possible luma values. Moreover, adaptive PQ achieves better perceptual uniformity in the luminance range than PQ. Experimental results demonstrate that adaptive PQ achieves a significant performance improvement in HDR video coding over PQ in terms of visual quality and bitrate.

A 4K/60p HEVC Real-Time Encoding System With High Quality HDR Color Representations

This paper describes a 4K/60p high efficiency video coding (HEVC, the latest coding standard) real-time encoding system with high quality high dynamic range (HDR) color representations. The HDR technology provides attractive video contents with a wider dynamic range of luminance, which can make shadows and brighter details appear clearly without any blown out highlights or blocked up shadows. We propose a new rate control method for HDR video coding. The method consists of two algorithms for improving perceptual quality by reducing degradation of HDR-specific color artifacts. The first one, adaptive block size and prediction mode decision, is conducted to suppress perceptual degradation due to prediction error in high chroma areas. The second, local quantization parameter control, is carried out to improve the visual quality with appropriate bit allocation in low chroma areas. The method was implemented in an HEVC real-time encoding system and its performance was assessed by measuring the color difference metrics by 3.86% at the most degraded frame, while peak signal-to-noise ratio remained almost the same. The subjective quality of the decoded HDR images was improved by reducing color artifacts.

Fast Intra Coding of High Dynamic Range Videos in SHVC

Compared with the conventional standard dynamic range (SDR) content, high dynamic range (HDR) content supplies viewers with more immersive experience by offering a much higher range of luminance. Most of current consumer devices cannot afford to this emerging technology, and content providers decide to create both an HDR version and an SDR version of the same video. In this letter, scalable high efficiency video coding (HEVC) scalable extension of HEVC (SHVC) serves as the coding framework where the base layer (BL) is an 8-b SDR version and the enhancement layer (EL) is a 12-b HDR version. Recently, many fast coding algorithms for SDR videos are proposed, and there is an urgent demand for fast coding algorithms for EL HDR videos. With the coding information of the BL SDR videos, this letter proposes a fast algorithm to reduce the complexity of intra coding for EL HDR videos. First, depth information of neighboring coding tree units (CTUs) in the HDR version and the colocated CTU in the SDR version is used for early coding unit (CU) depth determination. Moreover, four classifiers are trained to predict the CTU depth range. Two classifiers are trained for CTUs in frames with a high average luma, and another two classifiers are used for CTUs in frames with a low average luma. Experimental results show that the proposed algorithm achieves 43% encoding time saving on average, with only a 0.54% Bjøntegaard delta bit rate (BDBR) increase compared to the original SHVC test model.

Optimal Linear Precodings for Multi-Color, Multi-User Visible Light Communication System with Fairness Considerations

With fairness consideration, optimal linear precoding designs for multi-color, multi-user visible light communication (VLC) systems are presented in this paper. Utilizing both the spatial and multi-color resources, the precoding designs are proposed to mitigate the impact of the multi-user interference (MUI) and the multi-color crosstalk. With the constraints of chromaticity, luminance, and signal range, the precoding designs are formulated to achieve the max-min fairness and the maximum sum-rate. Since the closed-form expression for the capacities is not available, the lower and upper bounds are chosen as the performance criterions. To make the optimization problems be easy to be solved by using standard optimization packages, the non-convex problems are cast into convex ones. Subsequently, algorithms are developed to find optimal solutions. Extensive simulation results indicate that the proposed precoding schemes outperform the conventional pseudo inverse method.

Novel Robust High Dynamic Range Image Watermarking Algorithm Against Tone Mapping

High dynamic range (HDR) images are becoming pervasive due to capturing or rendering of a wider range of luminance, but their special display equipment is difficult to be popularized because of high cost and technological problem. Thus, HDR images must be adapted to the conventional display devices by applying tone mapping (TM) operation, which puts forward higher requirements for intellectual property protection of HDR images. As the robustness presents regional diversity in the low dynamic range (LDR) watermarked image after TM, which is different from the traditional watermarking technologies, a concept of watermarking activity is defined and used to distinguish the essential distinction of watermarking between LDR image and HDR image in this paper. Then, a novel robust HDR image watermarking algorithm is proposed against TM operations. Firstly, based on the hybrid processing of redundant discrete wavelet transform and singular value decomposition, the watermark is embedded by modifying the structure information of the HDR image. Distinguished from LDR image watermarking, the high embedding strength can cause more obvious distortion in the high brightness regions of HDR image than the low brightness regions. Thus, a perceptual brightness mask with low complexity is designed to improve the imperceptibility further. Experimental results show that the proposed algorithm is robust to the existing TM operations, with taking into account the imperceptibility and embedded capacity, which is superior to the current state-of-art HDR image watermarking algorithms.

Visual glare limits of HDR displays in medical imaging

Emerging display technologies are proposing monitors for medical imaging with an extended dynamic range of luminance. Those devices offer the opportunity to extend the range of visual information displayed, but the limits of the human visual system (HVS) in perceiving such information can cancel the advantages. To investigate this problem, we present a set of experiments, to assess the visual response of the HVS to controlled high dynamic range (HDR) content. They analyse the effects of glare. Using a typical HDR display, tailored for medical imaging applications, they first study the dependence of the visible dynamic range from the inter‐ocular glare, induced by different backgrounds, then the effect of glare on the detection of test elements on medical radiographic images. Finally, they assess the influence of luminance‐equivalent backgrounds with different structure in the detection of test patches. The results of the experiments confirm the glare as a major player in influencing visual information detection. Glare has a significant impact in limiting the amount of visual information actually perceived, consequently limiting analysis capabilities of such images. This confirms the importance of investigating and considering the characteristics of human vision in the design and test of HDR imaging systems.

P024] The enlightening of radiologists

Purpose The working environment for radiologists typically consists of dark rooms in order to increase image contrast. From an ergonomic viewpoint, dark rooms are far from ideal. Darkness is likely to cause fatigue, loss of concentration and thus a potential degradation of diagnostic accuracy. Also, a low screen luminance requires a rigorously controlled ambient illuminance in order to avoid contrast loss due to fluctuating reflections. The purpose of this talk is to examine the scientific reasons for using dark reading rooms. Are they indisputable or do old habits simply die hard? By using state-of-the-art displays – is it possible that an increase in light could also be beneficial to image quality as well as ergonomic conditions? Methods Display contrast was determined by calculating the number of just noticeable differences (jnd) for several display luminance settings. Since the jnd is based on a limited number of biological processes, effects caused by absolute luminance level, eye adaptation, luminance range and visual acuity were also examined. Results For a constant luminance ratio, the contrast increased with luminance. An increase in black level allowed for an increase in ambient light, i.e. less dark reading rooms. By using a high maximum luminance and a reduced luminance range, bright reading rooms could be used without compromising image quality. Unfortunately, most standards and guidelines require a minimum luminance range, forcing the maximum luminance to be higher than technically achievable when increasing the minimum luminance. A reduced luminance range lowers the contrast but can also have positive effects since the human visual system performs better for small differences in luminance. An increased black level causes the eye to always work in photopic mode where visual acuity is higher and small details are more easily detected. Conclusions With bright medical displays available, it is theoretically possible to use higher luminance settings in brighter rooms while maintaining – or increasing – image quality. In brighter rooms, radiologists will be able to work in normal office light and relatively large fluctuations in ambient lighting can also be allowed without any significant reductions in image contrast.

Visual Performance as a Function of Luminance in Glaucoma: The De Vries-Rose, Weber's, and Ferry-Porter's Law.

To determine whether the De Vries-Rose, Weber's, and Ferry-Porter's law, which describe visual performance as a function of luminance, also hold in patients with glaucoma. A case-control study with 19 glaucoma patients and 45 controls, all with normal visual acuity. We measured foveal and peripheral contrast sensitivity (CS) using static perimetry and foveal and peripheral critical fusion frequency (CFF; stimulus diameter 1°) as a function of luminance (0.02 to 200 cd/m2). ANOVA was used to analyze the effect of glaucoma and luminance on CS and CFF; analyses were adjusted for age and sex. Foveally, logCS was proportional to log luminance at lower luminances (de Vries-Rose) and saturated at higher luminances (Weber); glaucoma patients had a 0.4 log unit lower logCS than controls (P < 0.001), independent of luminance. Peripherally, the difference was more pronounced at lower luminances (P = 0.007). CFF was linearly related to log luminance (Ferry-Porter). Glaucoma patients had a lower CFF compared with controls (P < 0.001), with a smaller slope of the CFF versus log luminance curve, for both the fovea (6.8 vs. 8.7 Hz/log unit; P < 0.001) and the periphery (2.5 vs. 3.4 Hz/log unit; P = 0.012). Even in apparently intact areas of the visual field, visual performance is worse in glaucoma patients than in healthy subjects for a wide range of luminances, without a clear luminance dependency that is consistent across the various experiments. This indicates impaired signal processing downstream in the retina and beyond, rather than an impaired light adaptation in the strictest sense.

Illumination-Adapted Transceiver Design for Quadrichromatic Light-Emitting Diode Based Visible Light Communication

With the development of lighting industry and visible light communication (VLC) technology, multicolor light-emitting diode (LED) has been advised to replace phosphor-converted LED because of its higher modulation bandwidth and better lighting quality. In this paper, we developed an illumination-adapted transceiver design for a quadrichromatic LED based multicolor multiple-input multiple-output VLC system. The multicolor crosstalk caused by spectral overlapping is considered to make the channel model more realistic. The optimization problem of transceiver design is formulated to minimize total mean-square error between transmitted and received signals with the constraints of chromaticity, luminance, and signal range. Since the optimization problem is nonconvex, we develop an iterative algorithm that updates optimization variables to get the optimal transceiver and color mix ratio in the specified color temperature (CT). Simulation results indicate that the proposed method outperforms the existing design methods with the consideration of the crosstalk and has a better system performance with larger CT value.