What a red screen is used for
- Checking that every red subpixel fires, one channel at a time
- Preserving dark adaptation for astronomy, darkroom work and night navigation
- Testing red channel uniformity and colour banding
- A warning or stop signal on a spare display
- Photography and video work needing a flat red backdrop
Red screen online: #FF0000 at full screen
A pure red field drives the red subpixel to maximum and the other two to zero, which is why it is a single-channel test. Any dot that is not red on this page is a subpixel that is not doing its job. Run red, green and blue in turn and you have checked all three channels of every pixel on the panel.
Red also has a use that has nothing to do with testing. Human night vision is far less sensitive to long-wavelength red light, so a red screen lets you read something at night without destroying the dark adaptation that took twenty minutes to build. Astronomers have used red torches for this reason for a century, and a red screen at low brightness does the same job.
Red is the channel most likely to show banding on cheap panels, because 8-bit red gradients on a poorly calibrated display step visibly. If this screen looks blotchy rather than flat, that is worth knowing about the display.
How to get a full screen red background
Press the fullscreen button on the player above, or the F key. Escape returns you to the page. On a desktop browser F11 does the same job and hides the browser's own toolbars as well.
The colour box is a control, not a picture: use the hex field to enter any value, pick another shade from the palette, or download the current colour as a PNG at your screen's resolution. Whatever you set is written into the address bar, so the link you copy reproduces exactly what you are looking at.