I think the only way to do that on an iMac would be to crack the case open and unplug the video cable . . . . I don't think there is a way to do it with software. I know on the Apple laptops you can turn the brightness to the point the backlight is off, but I am not sure you can do that with an LCD display in an iMac.
OH you could setup your iMac as a Dual Monitor system, making the external monitor the main screen. Once you have done that, just change the background on your iMac screen to a black background and put the brightness on the lowest setting. That would work too!!!!
You could also put it lengthwise on the floor like a slim tower unit and put a towel over the screen. You could still access all the ports and you wouldn't have a huge blank screen box on your desk. Just be careful not to block the vent ports.
Seriously?! You can't turn off the iMac screen? It's annoying to have the brightest monitor I've seen on while watching the larger tv screen. Anyway (besides a towel, my current solution) to do this?
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My solution is still a towel. Is there not a better solution?
This works with 10.5.x, not sure if works with 10.4.x, but if you hit the shift+control+eject on an apple keyboard it will turn off the built in monitor.