2400 Nits Edge-Lit LCD and Adaptive EOTF for HDR and Brilliant Images Michael Grüning, Julian Ritter, Maxim Schmidt, Chihao Xu Institute of Microelectronics, Saarland University, Saarbrücken, Germany Abstract For displaying HDR images, an Edge-Lit LCD was amended and achieves 2400 nits brightness. A new adaptive EOTF combining the strength of Gamma function and Barten CSF is developed. Based on brilliance analysis, dull images are displayed at lower power without glaring viewers, while brilliant images appear vivid and naturalistic. Author Keywords HDR, Edge-Lit LCD, High Brightness, Brilliance, EOTF, Local Dimming 1. Introduction Up to now, HDR is just implemented on Direct-Lit LCDs or AMOLED displays. Both display types are expensive. OLED TVs reach a luminance of 700 nits. Newest direct-Lit LCDs TVs reach 2000 nits, but are thick. The most important feature of HDR displays is the very high luminance (significantly >1000 nits), so that a naturalistic impression can be produced. Edge-Lit LCDs are wide-spread and used in most TVs. The maximum luminance is in the range of 700 nits or below. The challenge on Edge-Lit LCD is the generation of high luminance at reasonable power consumption, since the heat is concentrated on one or two bars where the backlight LEDs are mounted. In this paper we describe an Edge-Lit LCD prototype capable to produce 2400 nits or even more, its impact on visual perception and power consumption. The power consumption will be reduced by local dimming as well as a new adaptive EOTF (electro-optical transfer function). In dependence of the brilliance analysis [1], low contrast images are displayed at moderate power consumption without glaring the viewer and brilliance of high contrast images are displayed and shown off. 2. Amendment of an Edge-Lit LCD The AUO P460HVN04.3 panel was chosen as the starting point. It is a 46inch FHD panel with LEDs placed at the top and bottom edges. The achievable luminance is higher than 1200 nits, which is a benchmark for commercially available Edge-Lit panel. The power consumption of the backlight is 155 Watt. For further increasing the panel luminance, the FHD panel is resized by the company Annax to a resolution of 1920x716. The shorter distance between the top and bottom edges should allow a higher brightness. The two original LED bars are replaced by more efficient LEDs. Each bar contains 8 LED strings, so that 16 individually controllable LED units are available and local dimming is enabled. The LED strings are driven at high current pulse mode by an LED driver we developed. In order to avoid thermal overheating, temperature sensors are placed at each LED string. This way, we can explore the limit of this prototype. For DC operation and displaying of a white image, 160 Watt electrical power is provided by the LED driver. The temperature of LEDs, measured on the outside of the casing, is 48 °C which is similar as with the original panel, while the ambient temperature is about 20°C. A luminance of 2400 nits is achieved. This high luminance value shall allow displaying HDR images on an Edge-Lit panel. For videos, an even higher peak luminance is feasible, as local spot may temporally and spatially moving, so that the operation temperatures of LEDs are still in the specified range. As impressive as 2400 nits may appear, the power consumption is high. For a panel of 16/9 aspect ratio, even higher power will be needed. LEDs may be overstressed. In addition, such a high luminance is not always favorable. It may glare the viewer, if the image contains a large bright area. On the other hand, such a high luminance on a brilliant image may impress the viewer. Thus, it will make sense, to display an image with an adaptive EOTF. 3. Adaptive EOTF The standard EOTF for SDR (standard dynamic range) image data is the well-known Gamma function. Bright details may get lost on the display, although they are included in image data. There are EOTFs based on the Barten CSF (contrast sensitivity function) [2] for HDR like the PQ-function in the standard SMPTE ST 2084[3]. Such a function is valid for ideal display with high gray scale resolution e.g. 16 bits under optimal environment like in a dark room. The peak luminance is specified at 10.000 nits. For an LC display with a limited contrast, a substantial part of lower gray values will be effectively equalized by the PQ function, so that dark details are lost. In order to combine the strength of the SDR and the HDR EOTF, both functions are summed into one single EOTF, as the equation below shows. The full scale of the EOTF is normalized to the maximum luminance of 2400 nits, with our panel. The weight w can be chosen between 0 and 100%. For this prototype, it is set to 30%, so that the maximum luminance of SDR data generates 700 nits like the most quality TV panels. The factor b is between 0 and 100%. If b is zero, the EOTF is the well-known Gamma function. If b is 100%, the maximum luminance reaches 2400 nits. The PQ function has a share of 70%. For most lower gray values, a perceivable luminance will be generated and can be differed from another lower gray value. As described in the next chapter, the factor b can be varied in dependence of the image content. The combined EOTF function is plotted in figure 1. The gamma value is set at 2.4. The equation of the adaptive EOTF for a pixel gray value gv can be defined as: () =  ∗   + ( − ) ∗  ∗ () For lower gray value, the output is dominated by the first term of the equation above. Without this term, these gray values would appear at the black level the LCD panel can produce. For higher gray value, the output is dominated by the PQ function. The different luminance for different gray values can still be percept which is a crucial feature of HDR. Such an EOTF should combine the advantages of SDR and HDR EOTF. One issue remains that low contrast image with large bright part may glare the viewers. In addition, the power consumption is unnecessary high. On the other side, a high peak luminance for a small region like a light source may produce a naturalistic image and make the image vivid. The factor b in the equation above is introduced to bridge the two opposite 22-3 / M. Grüning SID 2019 DIGEST • 307 ISSN 0097-996X/19/4801-0307-$1.00 © 2019 SID