Automatic brightness control including external monitor

Solution 1:

Have a look at eyesome Multi-monitor brightness and color temperature based on sun times. Below is a snipped from the Ask Ubuntu answer.

eyesome

Eyesome is a bash script running as a daemon and sleeping most of the time 24/7. It automatically adjusts screen brightness (and optionally gamma too) for your laptop display via hardware interface and up to two other monitors using xrandr software control.

At sunrise (the time is automatically obtained from the internet each day), your screen brightness and gamma (aka color temperature, hue or tint) is adjusted gradually. Before sunset gradual adjustments are made for nighttime settings. In between these periods they eyesome daemon sleeps many hours.

What distinguishes eyesome from Redshift, Night Light and similar products is the ability to simultaneously control three monitors with unique settings. Also notable is eyesome controls brightness (both hardware and software) in addition to color temperature.

Edit Configuration - Monitor 1 tab

Clicking on Monitor 1 tab reveals this panel in my configuration (yours may be different):

Don't be daunted by these settings because most are automatically obtained by eyesome. You will need to set the daytime and night brightness/levels though.

If the gamma values for Red, Green and Blue are intimidating, you can fine tune them later with the Override button from the Main Menu. There you have access to a slider control for color temperature where 3500 is typical nighttime setting and 6500 is typical daytime setting. Typical daytime setting is 1.0 for Red, Green and Blue channels which is every systems default.

Edit Configuration - Monitor 3 Tab

Clicking on Monitor 3 Tab reveals this panel in my configuration (yours may be different):

Monitor 2 Tab is not shown because it is a new TV with adaptive brightness and Smart OS. It requires no overrides by Eyesome.

When eyesome is installed configuration is automatically created for the most part. If later on you attach a different monitor you may have to enter the xrandr monitor name.

Where to find Eyesome

The full program can be found on github.

A new website called Pippim is under construction and Eyesome will be moved there soon.