December 9, 2018 Update - sct

I found source code for program sct which allows user to set color temperature. It has "cribbed the code" from redshift and provides mapping for red, green and blue values:

/* cribbed from redshift, but truncated with 500K steps */
static const struct { float r; float g; float b; } whitepoints[] = {
    { 1.00000000,  0.18172716,  0.00000000, }, /* 1000K */
    { 1.00000000,  0.42322816,  0.00000000, },
    { 1.00000000,  0.54360078,  0.08679949, },
    { 1.00000000,  0.64373109,  0.28819679, },
    { 1.00000000,  0.71976951,  0.42860152, },
    { 1.00000000,  0.77987699,  0.54642268, },
    { 1.00000000,  0.82854786,  0.64816570, },
    { 1.00000000,  0.86860704,  0.73688797, },
    { 1.00000000,  0.90198230,  0.81465502, },
    { 1.00000000,  0.93853986,  0.88130458, },
    { 1.00000000,  0.97107439,  0.94305985, },
    { 1.00000000,  1.00000000,  1.00000000, }, /* 6500K */
    { 0.95160805,  0.96983355,  1.00000000, },
    { 0.91194747,  0.94470005,  1.00000000, },
    { 0.87906581,  0.92357340,  1.00000000, },
    { 0.85139976,  0.90559011,  1.00000000, },
    { 0.82782969,  0.89011714,  1.00000000, },
    { 0.80753191,  0.87667891,  1.00000000, },
    { 0.78988728,  0.86491137,  1.00000000, }, /* 10000K */
    { 0.77442176,  0.85453121,  1.00000000, },
};

The three columns above are values for Red, Green and Blue.

November 4, 2018 Update - Eyesome

I wrote a collection of bash scripts called Eyesome that adjust brightness and gamma gradually at dawn and dusk over a 90 minute to 120 minute period. Although user can change the settings, a good starting point for full brightness setting is:

xrandr --output DP-1-1 --gamma 1:1:1 --brightness 1.0

The nighttime setting (for one of the three monitors) is:

xrandr --output DP-1-1 --gamma 1.0:0.88:0.76 --brightness 0.55

Note: Up to three monitors can be defined with separate settings.

Original Answer

Finally found one answer: MORE OF XRANDR OR BLUE LIGHT EXPOSURE This script is presented:

#!/bin/sh 

night_mode() { 
  for disp in $(xrandr | sed -n 's/^\([^ ]*\).*\<connected>.*/\1/p'); do 
    xrandr --output $disp --gamma $1 --brightness $2 
  done } 
case $1 in 
  off) night_mode 1:1:1 1.0 ;; 
  *) night_mode 1:1:0.5 0.7 ;;
esac

The script reduces blue light component and brightness of all Xorg displays attached. Simply save it in your bin directory, chmod +x it and, if you want to, add a cron job or keyboard shortcut to call the function.

How it works? Simply type in the script name without any parameters to reduce blue light. To return to normal setting, add off parameter after the command.

Those values in the thousands you referred to are temperatures in Kelvin. Each has a corresponding black-body (a star or anything that emits all wavelengths of light) emission spectrum.

So, if you pick a band of visible light (red, green or blue, for example) then find where the curves intersect those bands and this gives you their proportions. (Note: the scale on the graph is logarithmic; the differences are much greater than they appear)

I think 5777K is our Sun (it is kind of white). Remember, this is just the raw emitted light, all kinds of atmospheric effects can bend or scatter or absorb light along the way at different times of day depending on the angle of the Sun, thickness of atmosphere, what chemicals are in the atmosphere, etc. Read this paper for more info on these phenomena: Variation of outdoor illumination as a function of solar elevation and light pollution .

You definitely don't need all this to make a nice gamma changing script but it's good to have some idea of what you are actually trying emulate then you can pick nice values.

For me, best results were obtained by installing redshift (apt install redshift) and using it either as:

redshift -oP -O 4500 -b 0.5

for a temperature of 4500 and brightness of 0.5, or in combination with xrandr (and, optionally, xrandr-invert-colors to invert colours):

xrandr --output eDP-1 --brightness 0.5
xrandr-invert-colors
redshift -o -O 4500

Note that calling xrandr on its own will cancel any redshift effect. The -P option of redshift also resets the screen brightness and color instead of incrementally superimposing adjustments. Incremental adjustments (no -P option) are needed if using in conjunction with xrandr like in the second use case.

Finally, instead of using xrandr-invert-colors, one can also use a negative brightness value in xrandr (but not in redshift), but the result is not as nice.