OLED Pixels Switch Off, LCD Black Is Just Blocked Light

An LCD pixel showing black still has a backlight shining behind it. The liquid crystals twist to block as much of that light as they can, but they cannot block all of it. A small amount leaks through. An OLED pixel showing black has no backlight behind it at all. The pixel is the light source, and when it needs to be black, the pixel simply stops emitting. There is no light to block because there is no light being made. That is what “absolute black” means: the pixel is off, and the only light reaching your eye from that part of the screen is reflected ambient light bouncing off the panel surface. The difference sounds small. It changes how the display is designed, how it is measured, and what it can show in a dark room.

The backlight was the problem

Liquid crystal displays work by modulating a backlight. The backlight is always on. The liquid crystals in each pixel rotate to control how much of that light passes through the color filters and out to your eye. To show white, the crystals let light through. To show black, they try to block it.

They cannot block it completely. The liquid crystal layer has imperfections, the polarizers are not perfect, and the panel assembly has tiny gaps. The result is that some light always escapes. A typical IPS panel has a contrast ratio around 1000:1, meaning the brightest white is about a thousand times brighter than the darkest black. A VA panel does better, reaching 3000:1 or more because its crystals can align more completely. But even the best LCD cannot reach zero. There is always a floor, and that floor is set by the backlight leakage.

OLED removed the backlight. Each pixel is an organic compound that emits light when current flows through it. No backlight, no leakage. When the pixel is off, it emits nothing. The contrast ratio becomes the ratio of the brightest white to zero, which is mathematically undefined. Manufacturers call it infinite. A 2020 paper in Light: Science & Applications lists the contrast ratio for OLED as ∞ in a comparison table against LCD’s 5000:1 and micro-LED’s ∞.

What the measurement standard says

The International Electrotechnical Commission publishes IEC 62341-6-5, the technical specification for measuring OLED dynamic range properties. It defines test patterns for peak luminance, black level and constancy, and local contrast. Black level is measured with a specific pattern that includes a black area and a surrounding background. The standard exists because “pixel off” is not enough to describe what a viewer sees. The panel still reflects room light, the measurement instrument has a floor, and the black level depends on what else is on the screen.

A 2025 study in the Chinese Journal of Liquid Crystals and Displays measured an LG CX OLED TV and a Mini LED TV under ambient light levels of 0, 130, and 450 lux. At 0 lux, the OLED’s black level was 0.000 cd/m². The Mini LED’s black level was 0.003 cd/m². At 130 lux, the OLED’s black level rose to 0.078 cd/m², and the Mini LED’s rose to 0.116 cd/m². The gap narrowed because both panels were reflecting the same room light. Absolute black in a dark room is not the same as absolute black in a living room with windows.

The 2025 Perfect Black certification

In October 2022, UL Solutions, the safety science company, tested LG Display’s OLED panels against a new criterion. The test compared black levels in an absolute darkroom against black levels in a 500-lux illuminated environment, roughly the brightness of a living room in daylight. To be certified as “Perfect Black,” a display had to maintain the same black level in both conditions. The threshold was 0.24 nit. LG’s OLED panels measured 0.15 nit, about 40 percent lower than the criterion. They were the first panels to receive the certification.

The certification matters because it shifts the question from “does the pixel turn off” to “does the black look black when the room is bright.” A pixel that is off still reflects ambient light. LG’s claim is that the panel surface itself reflects less, so the black stays black even under illumination. UL measured the reflection, not the pixel state.

What absolute black changes in practice

A dark scene on an OLED shows stars against black space without a grey haze. Subtitles stay crisp because the pixels behind the text are actually off. The bloom that appears around bright objects on a local-dimming LCD is absent because there are no dimming zones to leak. A 2007 study by Satoru Kubota at Seikei University measured OLED and LCD panels under a range of ambient illuminance and found that OLED’s black level was lower than LCD’s across the entire range, with the gap largest below 100 lux. The subjective image quality of fireworks images was especially high on OLED, the study found, because the bright points of light appeared against a true black background.

What is still not measured

The 2025 UL certification and the IEC standard both measure black level under specific conditions. Neither measures what happens after hours of use. OLED materials degrade over time, and a pixel that has been driven hard will emit less light than a pixel that has been driven less. The black level of a degraded pixel is not the same as the black level of a new one, because the pixel is still off but the surrounding pixels may be dimmer. Burn-in is the visible result. The compensation cycles that run when the display is off rebalance the pixels, but they cannot restore the material. How much absolute black degrades over the life of a panel, and how much that matters to a viewer, is still an open question. The standards measure a new panel in a lab. Your panel is not new, and your room is not a lab.