Turn off iMac main screen while working on external I have an external display on my iMac and I would like to work only on the external display and turn off/disable the iMac one, just like the "clamshell mode" on a Macbook for example. It works with Windows in Bootcamp, but not in the Mac's partition. The iMac's display is too shiny and it gives me headaches when I work while the sun is shining. 

How can I turn off the iMac's screen so it won't stress my graphic card, display etc.?
I succeeded by using switchresx.
Terminal command

brew cask install switchresx


then, open System Preferences, SwitchResX

Click icon, setup a display set with a disabled primary display, setup hotkey (ctrl-opt-f1 for me), save&amp;close SwitchResX

Then, if you want to use single external display mode, press the hotkey et voilà