Colour temperature is measured in kelvin, and on a screen it runs backwards from what the word "temperature" suggests: the higher the number, the bluer and cooler the white looks, and the lower the number, the warmer and more orange it gets. A Mac display normally sits near 6500K, which is the standard daylight white that most screens are tuned to. Screen-warming tools shift the display towards a warmer white.
This page is a plain reference for what the different ranges look like, so the numbers in a settings panel mean something before you drag a slider. These descriptions are approximate: appearance varies with the display, software, room lighting and your visual adaptation. Kelvin settings are not measurements of blue light reduction.
| Range | What the screen looks like |
|---|---|
| 6500K | A common neutral white point for general display use and web content. |
| 5000K to 4000K | A mild to noticeable warm cast compared with 6500K. Whites look warmer. |
| 3500K to 2700K | Clearly warm. Night Shift on a Mac works around this area, at about 2700K at its warmest. |
| 2500K to 1200K | Strongly warm, shifting towards orange-red at the lower end. Blue and green content can appear dimmer. 1200K is the lowest setting f.lux offers. |
| 1000K to 500K | A strong red cast. Text readability depends on brightness and contrast. Unsuitable for judging colours accurately. |
There is no single right number, because rooms, displays and eyes differ. A practical way to find yours is to start in the 3000K area after sunset, then step lower when the room lights go down, and keep the lowest settings for reading in a dark room. If you want to compare settings by the numbers, the blue light calculator is a quick way to do it.
Brightness also affects how comfortable a screen feels. A warm screen at full brightness in a dark bedroom can still feel harsh, so it usually helps to lower both together as the night goes on.
The built-in Night Shift stops at about 2700K, and f.lux stops at 1200K. Sundown is a Mac menu bar app that goes from 6500K all the way down to 500K, so the red end of the table above is reachable, and it keeps a flicker-free display while it dims. You can set a schedule so it warms at sunset on its own, or move it by hand. For a closer comparison of the two apps, see this f.lux alternative for Mac page.