I have an external monitor connected to my laptop. When for example watching video on my external screen, I would like to have the option of easily turning off or dimming the laptop screen. But I would like to have the possibility to quickly pop on the laptop monitor to check something and then turn it back off. So far the best I've come up with is maximizing a black image on the laptop screen. Is it possible to "sleep" individual monitors?
If you are running in X, you might be able to use
xrandr. Something like
xrandr --output VGA1 --off
can turn the display off.
Select the display preferences . If you have multiple monitors connected it will show you both of them there. From there you can select and turn off a particular monitor of your choice.
Here the on/off options are disabled since there is only one monitor I have. When you have multiple monitors you can turn any one of them off. On a centos linux you can select display options from System->Preferences->Display. If you are using Ubuntu just press the Windows button and search for Display.