Thoughts on leaving display on for extended lengths of time

There's absolutely no harm in doing that. I do that on a regular basis. My iMac's display gets left on, sometimes for many hours on end, doing absolutely nothing.

I did this with my 2009 27" iMac for 3 years, and am now doing it with my 2012 iMac.

*EDIT*

Let me add to that. There have been a few times I've forgotten to turn my display off/set my system to sleep, and have come to check on my iMac the next day and the screen is still on. So at least 12-16 hours with nothing moving on the screen. No issues.

 

I've been looking for a way to turn the iMac display on and off while still leaving the external one on. I can't find anything either.

 

I don't think you should have an issue. My 2011 21.5" iMac was left on many, many times for days/weeks with no issues.

With that said, I have a new 2021 27" iMac that I just had to exchange due to Target Display mode not working. The replacement came in last week and I immediately hooked it up to the first 2012 iMac and successfully tested Target Display mode. I left it like this for the first day (using the older 2012 iMac as the computer and the new iMac as a second display). Within a few hours I saw my first taste of image retention on the newest iMac. Since then, I've tried to make it happen again with no luck (including leaving up the checkered test patterns). Until today. I connected my TV to the iMac as a second screen and within an hour or so I have full screen image retention on the new iMac (and of course it had to be my RDP session into my windows box, leaving PC images all over my damn Apple screen...)

I need to jump over to the image retention forums and see if added stress on the GPU is causing this burn in or not, but since it seems to happen (to me) only when I have a second monitor running, I'd be very cautious about this. (I did talk to Apple who refunded me $150 so I could purchase AppleCare, on top of the other $100 they refunded to me out due to the issues.)

 

These are not CRT or Plasma screens, leaving it on won't hurt it. You used to be able to completely dim the screen, so as to save the backlight. Mine won't go all the way black anymore. Maybe a terminal tweak?

 

you could also just turn brightness way down and display a full screen black image that's practically turning it off although not quite, but should work as you want it and not cause any retention or anything.

 

Would the old Exposé Hot Corner - Sleep Display work for you? I haven't tested it, for use with Home Sharing in iTunes, but I'll do it a bit later and get back to you.

Also, apparently I'm so used to my MacBook, which will let you dim the screen all the way, but I did check on the new iMac and it doesn't allow that.

Edit: I double checked that sleeping the display doesn't sleep the computer, iTunes Home Sharing still works just fine. Try it with an external monitor, and see if it works the way you wanted.