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Understanding OLED Pixel Dimming

  • Date Posted2019.04.18
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OLED’s various picture quality strengths, such as “perfect black”, “presentation of vivid colors”, and “elaborate details” have been widely covered through different media outlets. Today, I will discuss pixel dimming, the technology that makes the advantages of OLED possible. 

A pixel is the smallest unit used to express images in most of today’s displays, including OLED, LCD, and PDP. Subpixels in red, green, and blue form one pixel. The more pixels a display has, the higher the resolution becomes to enable more detailed no_no_no_expression of images. For instance, a basic FHD (1920 x 1080) has about 2 million pixels, and UHD or 4K (3820 x 2160) has over 8 million pixels. In other words, UHD/4K is nearly four times more elaborate in terms of expressing images.


Figure 1. The difference in black levels between LCD TV (left) and OLED TV (right) 

Coming back to the subject of pixel dimming, the technology allows each pixel to control its brightness independently from other pixels. The important thing that we should keep in mind here is that each pixel of OLED is an independent light source, which means it’s self-illuminating. In contrast, each pixel in other TVs such as LCD or LED (including QLED) is not independently controlled. All the pixels in LCD/LED TVs share one backlight as the light source. Each pixel’s brightness depends on the amount of light permeated through a substance called liquid crystal. For example, if there’s an image that is half white and half black, an OLED TV will emit light with half of the pixels to express white, while turning the other half of the pixels off completely. However, in case of LCD/LED TVs, the backlight stays on, while half of the liquid crystals permeate the light and the other half block out light. In principle, both OLED TV and LCD/LED TV must show an identical image, but LCD/LED TVs cannot express perfect black that OLED TVs are capable of. This is because liquid crystal cannot completely block out the light. In Figure 1, you can see the clear difference between the levels of black. If you google “OLED LCD black”, it’s easy to find more examples comparing the black levels between OLED and LCD/LED.

(http://www.displaymate.com/TV_OLED_LCD_ShootOut_1.htm, https://www.cultofmac.com/400910/none-more-black-lgs-oled-displays-show-that-nothing-is-really-something/ )

To improve this shortfall, the recent LCD/LED TVs have added a new light source called “local dimming”, which controls the light source by block or regional unit. In Figure 2, you can see two different methods of local dimming implemented in LCD/LED TVs, Direct-lit and Edge-lit. Shown at the center, direct-lit employs a total of 150 (15x10) blocks. At the far right, the Edge-lit method uses twelve blocks or light sources (12x1). While OLED TVs can independently control over 8 million light sources (UHD), LCD/LED TVs using Direct-lit can only control 150 light sources and 12 for Edge-lit. With a fewer number of light sources that can be controlled independently, it’s more difficult to express detailed images. For instance, if some of the pixels within one block have to express white, while others express black, the block has to shine at the brightest level. In turn, this undermines the quality of black. On the other hand, if the block is darkened or turned off to emphasize black, the white will come out darker. 


Figure 2. OLED Pixel Dimming (Left), LCD/LED TV Local Dimming (Middle and Right)

The picture quality difference between OLED TVs and LCD/LED TVs becomes more obvious when you watch a video of a starfield, which shows stars in the night sky. (Figure 3 is an actual example of an LG OLED TV and a Direct-lit local dimming LCD/LED TV) As opposed to the video from the first column that expresses each star clearly, they are hardly seen on the 150-block LCD/LED, though the darkness is somewhat better. From this, we can guess this LCD/LED TV was designed to prioritize expressing black levels. When a comet passes by (the second column), OLED can express all the stars, the comet, and clear black. But the LCD TV has losses in terms of picture quality, such a halo around the comet and only shows stars around the comet that were not seen before. The stars at the edges are still difficult to see.


Figure 3. The difference in picture quality between OLED pixel dimming and LCD/LED local dimming 

As such, pixel dimming in OLED and local dimming in LCD/LED are vastly different when it comes to picture quality. High-end LCD/LED TVs that are currently available in the market have 100-200 blocks (some have 600, though rarely) with the Direct-lit method, while Edge-lit TVs have 12-24 blocks. On the other hand, the super high-end LCD/LED monitors used by video production experts have 2000 blocks because they are required to express much more elaborated details. These experts’ interest in OLED monitors is also increasing over time, particularly with the advent of HDR (High Dynamic Range). In the next column, we will discuss what this new HDR standard is, and the strengths of OLED in terms of this standard.

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