Black Crush

What black crush is

Black crush is the loss of near-black detail. Several dark steps above reference black are merged into pure black, so shadow textures such as a dark suit, a night sky or a cave wall turn into flat patches. TFTCentral defines shadow detail as the detail visible in near-black content and calls losing it "crushed" or "clipped" to black, a common OLED issue. It differs from low contrast: the display can have excellent black and still crush the first steps above it.

How it happens: levels, gamma and the panel

Four layers can cause it. The first is video levels. In 8-bit video, reference black is 16 and white is 235, and in 10-bit video the range is 64 to 940, per Wikipedia's Rec. 709 article. Spears and Munsil note that values 1 to 15 are below black and that some players or displays "don't bother transmitting or displaying the values in the digital signal below 16." If a chain clips at 16 when it should pass 17 and up, detail disappears.

The second is the Brightness control, which is really a black-level control. Spears and Munsil describe it as "the input level that the display will consider absolute black." Set it too low and every level near black is pushed under the display's black, which crushes shadows. Set it too high and black turns grey. The third is the EOTF. ITU-R BT.1886 (approved 2011-03-08) is the reference EOTF for flat-panel studio displays. Light Illusion explains that BT.1886 lifts near-black brightness based on the display's real black level, and that if a colourist graded with that lift and the footage is viewed on a plain 2.4 power-law display, "the shadows will appear crushed." Its example is a display with 0.01 nit black and 100 nit white.

The fourth is the panel, and it is where OLED differs. A true 0.00 nit black makes a steep cliff at the bottom of the gamma curve. TFTCentral calculates that on an accurate 2.2 gamma at 200 nits, RGB 1 targets 0.0010 nits, RGB 2 targets 0.0047 nits, RGB 3 targets 0.0114 nits, RGB 4 targets 0.0214 nits and RGB 5 targets 0.0350 nits. The visibility threshold in a typical dim room is about 0.01 nits (0.01 to 0.1 range), so RGB 1 and 2 fall below it and RGB 3 is the first visible shade. A 2.6 gamma pushes the first visible shade to RGB 6. An LCD hides the problem: a 1000:1 IPS panel at 200 nits has a 0.2 nit black, so RGB 1 targets 0.201 nits, already visible. TFTCentral's tests were on monitors, and it found WOLED gamma shifting with refresh rate (blacks crush more at 280 Hz than at 60 Hz) while QD-OLED stayed consistent. TV behavior differs, so treat this as an explanation of the mechanism.

Black crush vs low contrast vs banding

Crush is a loss of steps at the dark end; color banding is a lack of steps that shows as visible contours, and dithering is the usual remedy for banding. Raised blacks (grey) is the opposite error from crush. A TV can show all three on different content. Dolby Vision 2's Precision Black feature, which DisplayDaily describes as lifting near-black detail while keeping blacks deeper than HDR10 on the same hardware, addresses the creative side: Dolby Vision 2 content can tell the TV how to handle dark scenes, though no measurement of the effect has been published in the sources I read.

How to test with a PLUGE pattern

PLUGE (picture line-up generation equipment) was developed by the BBC to calibrate black level. Wikipedia lists the classic dark patches at 0, 3.5, 7.5, 11.5 and 0 IRE, designed so monitors would not be calibrated too dark. The Spears and Munsil version uses bars at -4%, -2%, +2% and +4%. In a dark room, if the display passes below-black, raise Brightness until all four bars show, then lower it until the two left (below-black) bars vanish while both right bars stay visible. If the display cannot pass below-black, raise brightness until both right bars show, lower it until the faintest disappears, then raise it a notch or two. The whole procedure takes about 30 seconds.

TestUFO offers a browser version: adjust until you "barely can see moving black squares on a dim-grey background," using RGB 0 against RGB 1, in 8-bit for SDR and 10-bit for HDR. Use a source that outputs the range your display expects (limited-range 8-bit video runs 16 to 235), because a mismatch moves the black point. Gamma needs its own pass; the TV gamma and EOTF tracking entries cover it, and TV calibration settings gives starting values.

Fixes and common confusions

Adjust Brightness a notch at a time against the pattern. Compare BT.1886 and a gamma of 2.2 in your actual room, since Light Illusion notes the BT.1886 lift can look washed out in less than ideal viewing conditions. TFTCentral lists a slight near-black offset as a fix on monitors, provided black stays at 0.00 nits, and a gamma that follows roughly 1.6 to 1.8 near black before rejoining 2.2. Test with a pattern before blaming a streaming service for crushed dark scenes. Finally, "black crush" is sometimes confused with native contrast, which measures the gap between black and white, not the steps in between.