The technical features of HDR contents described in the previous column can be summarized as follows. First, the average brightness required for HDR contents is similar to that for SDR contents. Second, HDR contents require not only high brightness for small highlight regions but also deep black levels for dark regions. In fact, these features strongly align with the merit of OLED displays: pixel dimming. Unlike most consumer LCD or LED displays, each pixel on OLED display is individually controllable. Note that LED displays are technically a type of LCD display because they also have a liquid crystal layer to control the amount of light emitting from the backlight. Likewise, QLEDs are a type of LCD display as well. Most consumer LCDs usually have one, dozens, or hundreds light sources while OLED displays have the same number of light sources as their resolution. For example, it has about 8 million light sources for a 4K (3840 x 2160) OLED display (See Figure 7). In general, an LCD which has more than one light source independently controllable is called a local dimming LCD. What does the number of dimming blocks have to do with the image quality of HDR contents? Basically, the less light sources a display has, the more pixels a single light source needs to cover. Let’s assume that there is a 4K LCD with 40 dimming blocks or light sources. A single dimming block of this LCD should cover approximately 200,000 pixels. This means if a single pixel represents white while 199,999 pixels render black, the single light source for 200,000 pixels should still be turned on. Although liquid crystals corresponding to the black pixels block the light, it is impossible to do this without any leakage due to intrinsic limitations. Therefore, algorithms are crucial for local dimming LCDs to determine the luminance of each dimming block based on the registered images. However, such approaches are not perfect for all cases.
Figure 7. Comparison of dimming technology. The peak highlight and black levels are strongly dependent on dimming technology.
Here is a somewhat extreme example to demonstrate how difficult algorithmic approaches are. Let’s say that there is an HDR image where one-pixel sized star on the black background which should deliver up to 500 cd/m2. Also, a single dimming block of the local dimming LCDs mentioned above can deliver up to 1,000 cd/m2. In order to represent the star pixel as intended, the dimming block corresponding to the star pixel needs to be turned on at the half of its maximum power. In this case, how does the black regions look like? The black regions cannot be seen as real blacks. However, if the dimming block is completely turned off, there is no star to be seen on the display. For this reason, such algorithmic approach cannot be an impeccable solution for delicate image rendering.
Figure 8. Simultaneous contrast. The intensity of each small gray square in the different backgrounds is physically identical but the gray square in the black background appear lighter than that in the white background.
In contrast, OLED display can technically represent the highlighted regions in its entirety, regardless of its size, while showing deep black levels at the same time. Also, due to simultaneous contrast effect, a psychophysical phenomenon of the human visual system, deep black levels of an OLED can even make us to better detect highlights than on an LCD with the same luminance. If we consider these facts, it may be possible to call OLED as a native HDR display.
Looking at the features of HDR contents and display technologies introduced so far, in