Update 12/21/15 (how appropriate — the solstice!)

The folks at f.lux are hearing from people who want to take all the blue out of the light from their computer screens.

The honcho’s response, about a month ago, seems to bring to an end the question of whether f.lux can actually do that. Here is his (guessing at gender) thoughtful response. 


Every mode from 1900K and lower filters all “blue pixels” from your display.

If you’re trying to filter all alerting light you also need to filter all the green too. 1200K lets just a little bit of this through, which massively improves your ability to use your computer at night, at a very tiny cost to extra light you see.

However–there are two more things that mean (in reality) f.lux doesn’t filter all blue:

  1. LCD screens are imperfect and let through some of the backlight color. For most displays this is <0.1% though.
  2. Many panels are trying to “match” sRGB and allow through some blue and green on purpose, even when displaying saturated red. This is because the red primary they would achieve otherwise is too saturated. (You’d prefer they use a “wide red” primary but instead they mix in some blue and green.)

But in the end, the circadian effects of low levels of light like this seem to be extremely small. Blocking 90% is probably good enough to keep screens from messing up sleep.