Change brightness of secondary screen in ubuntu

Its very easy to do via command line. Type the following command in terminal.

xrandr -q | grep " connected"

You will get something like this

eDP-1 connected primary 1366x768+0+0 (normal left inverted right x axis y axis) 345mm x 194mm
DP-1 connected 1366x768+1366+0 (normal left inverted right x axis y axis) 410mm x 230mm

I am using an external monitor. If you want to reduce the brightness of external screen just type

xrandr --output DP-1 --brightness 0.5