Black Level Test: Check Near-Black Detail

What this black level test checks

A black level test here is a visual check of the darkest part of the picture. It asks which near-black patches you can still tell apart from black, and whether a screen filled with black looks evenly dark. The patch values are 8-bit code numbers, not a light meter.

• Near-black steps: nine patches labeled by code value, from 0 through 32, on a black background. Code 0 matches the background.
• Black field: one solid black screen for glow or uneven darkness across the center, edges, and corners.

It does not report candelas per square meter, contrast ratio, or a pass against a studio standard. Uneven gray in the midtones belongs on the Gray Uniformity Test. A halo around a bright object belongs on the Blooming Test.

How to use this black level test

Start with Near-black steps. Nine patches sit on a black background. From left to right, their 8-bit code values are 0, 1, 2, 4, 8, 12, 16, 24, and 32. Code 0 is the same black as the background. Each later patch is a slightly lighter neutral gray. Sit at your normal viewing distance, set the browser zoom to 100 percent, and dim the room lights enough that you can see the screen without a lamp reflecting in it. Note the first code you can distinguish from the background. Then switch to Black field and scan the center, edges, and corners.

Use Full screen so the white page around the tool is out of view. Hide controls when you want the black field judged without bright buttons sitting on top of it. Click the pattern or press Escape to bring the controls back. If you change a monitor setting, change only one control, look again, and then return it. A useful note is specific: “code 8 became visible after brightness was lowered one step,” rather than a pass or fail for the whole display.

What the near-black code steps show

These patches are CSS colors, written as equal red, green, and blue values. Code 16 is 16/255 of the full 0–255 range, about 6.3 percent of that code scale. It is not 6.3 percent of the screen’s light. Video and computer gray are encoded so that small code changes near black represent a much smaller fraction of white. ITU-R BT.1886 specifies a reference electro-optical transfer function for flat-panel displays used in HDTV production, with a power of 2.404 and separate parameters for white luminance and black luminance. As an illustration of that exponent only, raising the normalized code 16/255 to the power 2.404 gives about 0.13 percent of full-scale white. That arithmetic describes the code. It does not measure the candelas per square meter coming off your panel.

The recommendation’s own note says that confirming a display against the reference function calls for a dark-room measurement. Its appendix also discusses a moderate black-luminance example of 0.1 cd/m² when someone is matching older CRT behavior. This page does not measure that quantity and does not claim the patches follow BT.1886. The steps are here so you can see, on this browser and this display, which of those code values still look different from black.

Black level, brightness, and crushed shadows

On many displays the control labeled brightness moves the black level rather than the peak white. Raising it can make code 0 look gray and can reveal more of the lowest patches. Lowering it can make codes 1, 2, and 4 disappear into the background. That disappearance is often called crushed shadow detail. It can be a setting, a picture mode, a dynamic-contrast feature, or the way the panel behaves at the bottom of its range. The right setting is the one that keeps a black field looking black while preserving the shadow separation you need. Making every patch obvious by lifting black until the background looks gray is not a better result.

Contrast, gamma, HDR or SDR mode, and “dynamic contrast” or local dimming can change the same patches without a change to the brightness number. Compare modes by name. A vivid or game mode and a cinema mode are different pictures, not two readings of one fixed panel. Laptop content-adaptive brightness can also move the image while you watch, so disable that feature if you want two viewings to be comparable.

How the room and the browser change a black field

A black field that looks gray in daylight can look black at night. Reflections from a window or a lamp add light on top of the panel, which hides the difference between code 0 and code 4. View the screen from straight ahead first. Some panels wash out as you move off axis, and that shift belongs to viewing angle rather than to the code values themselves. Browser night-mode filters, operating-system night light, and blue-light tools also tint or lift the image. Turn those off for the comparison, then turn them back on if you want to see their effect as a separate check.

The page around the pattern is bright. On a local-dimming screen, that white area can brighten zones that extend into the black stage, especially near the edges of the tool. Full screen plus hidden controls reduces that influence. Even then, a browser is not a video-signal generator. Color management, HDR tone mapping, and the graphics driver can reshape near-black CSS colors before they reach the panel. Repeat the observation in a second browser if the first result is hard to believe.

Black level versus uniformity, banding, and bright detail

A black level check asks whether tones just above black remain separate, and whether black itself stays dark. The Gray Uniformity Test asks a different question: whether one flat gray looks even from edge to edge. The Gray Banding Test asks whether a smooth ramp breaks into visible stripes. A display can crush code 2 and still show a smooth ramp through the mid grays, or it can keep a smooth ramp and still show a bright corner on a flat field. Keep the notes separate.

The bright end of the scale has its own page. The White Saturation Test uses near-white steps so you can see whether highlight tones merge. Near-black and near-white problems often come from different controls. If a bright object on a black page grows a halo, use the Blooming Test instead of judging that halo from these small patches. Point defects, a single pixel that stays lit or dark, belong on the Dead Pixel Test. A broad glow in a corner is not a dead pixel.

What a browser black level test cannot measure

This tool cannot report black luminance, contrast ratio, gamma, or compliance with BT.1886. It cannot tell an LCD, an OLED, and a projector apart, and it cannot see the room light that is reflecting from the screen. Two people looking at the same panel can disagree about code 1 and code 2 because those steps are near the limit of visibility. Write down the first code you can see and the viewing conditions, then use that note when you change one setting. A single glance in a bright room is a weak reason to return a display or to keep raising brightness until the blacks turn gray.