What a black screen is good for
A pure black image (RGB 0, 0, 0) is the quickest way to see problems that hide behind normal content. Any pixel that stays lit stands out immediately, and on LCD monitors the backlight shows through wherever it leaks.
Check for stuck and bright pixels
Look closely at the black screen from about an arm's length. A pixel that glows red, green, blue or white is stuck on. For a complete check, the dead pixel test cycles through red, green, blue, white and black so you also catch pixels that are stuck off.
Check backlight bleed and IPS glow
Darken the room, set the monitor to full brightness and look straight at the black screen. Bright patches near the edges that stay put when you move your head are backlight bleed. A silvery glow in the corners that shifts as you move is IPS glow, a normal trait of IPS panels. Most makers accept light bleed only if it is severe, so take a photo with your phone set to a low exposure if you plan a return.
OLED and power
On OLED screens black pixels are fully off, so a black screen uses very little power and looks perfectly dark. It is a good way to see how deep your OLED's blacks are next to an LCD.
Need a different colour?
Use the white screen to check for dark pixels, dust under a laptop screen or uneven brightness.
Questions
How do I make my screen completely black?
Press "Go full screen" above. The whole display turns black. Press Esc, or double click, to leave full screen.
Does a black screen save power?
On OLED and AMOLED screens, yes: black pixels are switched off, so a black screen uses very little power. On LCD screens the backlight stays on, so the saving is small.
Can I use a black screen to find dead or bright pixels?
Yes. A stuck pixel shows up as a bright dot on black. For a full check through red, green, blue and white, use the dead pixel test.
How do I check backlight bleed?
Turn the room lights off, show the black screen at full brightness and look at the edges and corners. Light leaking in as cloudy patches is backlight bleed. A faint glow that changes with your viewing angle is IPS glow, which is normal.
Will the black screen turn off my monitor?
No. The monitor stays on and shows black. Your computer may still dim or sleep after its usual idle time.
Does it work on a phone or TV?
Yes, in any modern browser. iPhones do not allow full screen for web pages, so the page fills the browser window instead; turn the phone sideways and hide the address bar by scrolling.
More tests
Dead pixel testFull screen colours to spot dead and stuck pixelsWhite screenA full white screen, one clickRefresh rate testYour monitor Hz, measured in the browser
Mouse testCheck every button, side buttons and the scroll wheel
Keyboard testPress every key, with ISO, AZERTY and QWERTZ layouts
Controller testButtons, sticks, triggers and vibration for any gamepad