Brightness Test

How to test your monitor brightness

1. Test in the room light you normally use, and turn off automatic brightness so the screen holds steady while you look.
2. Press Start test. Move through the patterns with Next and Previous, the arrow keys or a tap, and press Esc or Exit to leave.
3. On Brightness steps, check that all 11 bars look different, with roughly even steps from black on the left to white on the right.
4. On Shadow detail, find the first bar you can tell apart from the black 0% bar on the left.
5. On Highlight detail, find the highest number you can still tell apart from the white 100% bar on the right.
6. Use Black, Gray 50% and White as flat references when you compare settings or screens.

Can a website measure screen brightness?

No website can measure screen brightness: brightness is the light a screen gives off, measured in nits (candelas per square meter), and only a light meter or colorimeter can read it. A web page can only send known signal levels, such as the labeled gray bars here, and let you judge by eye how your display shows them.

What brightness should you set your monitor to?

No single number is right. The setting depends on the light around the screen, and one that looks right in daylight often looks glaring at night. Set it by eye in the light you usually work in:

• Open White and hold a sheet of white paper next to the screen. If the screen looks much brighter than the paper, turn brightness down; if it looks gray beside it, turn brightness up.
• Open Shadow detail. The lowest bars should just separate from black. If they vanish when you lower brightness, raise it until they return. If they stay hidden at every setting, your screen is crushing the shades just above black; the Contrast Test helps find out why.
• Check Highlight detail last. If the brightest bars merge into white, lower contrast rather than brightness.
• For photo editing or matching prints, white is set to a target brightness in nits with a colorimeter. The guide to adjusting a monitor by eye covers what you can do without one.

What do the percentages on the bars mean?

They are signal levels, not amounts of light. Each bar sends that share of the full signal, and the display turns the signal into light along its gamma curve. On a display that follows the sRGB standard, the 10% bar gives off only about 1% of white’s light. Your eyes respond to light along a similar curve, so the bars still look roughly evenly spaced. The Gamma Test checks the display’s curve.

One percent of the 0 to 255 range is 2.55 levels, so after rounding the 1% steps are 2 or 3 levels apart rather than exactly equal.

What this test can detect

• Near-black bars that look the same as black at your current brightness setting.
• Near-white bars that merge into white, often a sign that contrast is set too high.
• Neighboring bars in Brightness steps that look identical, or one jump that looks much bigger than the others.
• White that looks much brighter or grayer than a sheet of paper beside the screen.
• A screen that dims or brightens by itself as you switch between Black and White.

How it works

The page draws plain blocks of gray with CSS, not images, so nothing is scaled or compressed. Brightness steps has 11 bars in 10% steps from black to white. Shadow detail has 11 bars from 0% to 10%, and Highlight detail 11 bars from 90% up to white, each 1% apart. Black, Gray 50% and White are flat fields. Each percentage becomes one of the 256 levels of 8-bit gray: 50% is level 128.

Nothing is measured. Start test fills the screen with the Fullscreen API, or fills the browser window where that is not available.