A small glossary of screen warmth

Most of the words around warm screens get used loosely, and the loose use is where the confusion starts. This glossary covers the terms you meet when you set up a screen warming app on a Mac, with Sundown as the worked example because it is the one we make.

Kelvin (K). The unit used to describe the colour of white light. Lower numbers are warmer, so 2700 K looks like an ordinary warm household bulb and 1000 K is a deep orange. It describes colour, not brightness, which is why a dim screen can still be very blue.

6500 K. The usual daytime white on a Mac. When a warming app is set to neutral, this is where it sits, and a good app should leave the display untouched at that point.

Warmth ladder. The range of Kelvin values an app lets you choose from. Sundown goes from 6500 K down to 500 K in 100 K steps. Night Shift, which comes with macOS, is usually described as reaching about 2700 K at its warmest, and f.lux goes down to 1200 K.

Gamma table. The lookup the Mac uses to map the numbers an app draws into the light the panel shows. Screen warming apps on the Mac work by adjusting it, turning down blue and some green so white comes out warmer. It is also why two warming apps running at once can fight each other.

Preset. A named warmth you can pick in one tap. In Sundown 1.5 the presets run from Energy at 6000 K through Reading at 4000 K and Evening at 2000 K down to Midnight at 500 K.

Schedule. The plan that changes warmth across the day. In Sundown it is four cards, Daytime, Afternoon, Evening and Night, each holding steps with a time and a warmth, and since 1.5.1 each step lands when you set it to.

Menu bar app. An app whose main control is an icon at the top right of the screen. On macOS 26 the system can hide menu bar icons, and the switch to show one again is under Allow in the Menu Bar in System Settings.

HDR and EDR. Ways the Mac shows brighter than normal highlights in some videos and photos. They can interact badly with a warmed display. Before Sundown 1.5.1, an old safety guard could switch the warmth off and leave the display white when HDR content was on screen, and 1.5.1 fixed that.

Blue light. The short wavelength part of what a screen gives off. It is the part a warmer setting reduces most. For a rough estimate of your own evenings, there is the Blue Light Calculator, a free web tool that estimates the blue light from your screen after sunset from four inputs: your device, your screen hours after sunset, any filter you use, and your bedtime.

Flicker. Some displays dim by switching the backlight on and off very quickly. Whether that matters for eye strain is less settled than people assume, and Screen flicker and eye strain research collects the studies with a plain note on where the evidence is thin.

If you want to try any of this on your own Mac, Sundown runs on macOS 14 or later, with a Classic build for macOS 12 and 13.