Why a red screen finds faults white misses
Every pixel on your display is three subpixels: one red, one green, one blue. White lights all three, so a pixel with a failed green subpixel still looks nearly white and you will walk straight past it. A full screen red lights only the red subpixels, which means any pixel whose red element has died appears as a black dot on an otherwise perfect red field. It is the only way to isolate that channel.
Red is also the hardest primary for many panels to render evenly, so a red field will expose banding, blotching and gradient steps that a grey ramp hides.
Red light and night vision
Red is the one colour that leaves dark adapted eyes mostly alone. Astronomers, darkroom printers, night shift crews and pilots have used red lighting for a century for exactly this reason: the rod cells responsible for low light vision are far less sensitive to long wavelengths. A dim red screen is a usable reading light at 3am that will not cost you twenty minutes of readjustment when you look back into the dark.
Drop the brightness slider to 15 or 20 percent for this. Full brightness red is still a lot of light, and the point of using red at all is to keep the total low.
Other uses
- Checking a phone or camera screen for a failing red channel, which shows as a magenta or yellow cast on photos.
- A solid backdrop for photographing small objects, when you want a coloured bounce rather than a neutral one.
- Testing whether a projector or television is clipping saturated reds, which appear flat and detail free when it is.