MacBook – Turn off macbook screen when connected to a monitor

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When connecting my 2017 15' macbook pro to a 4k external monitor it gets a little bit warmer (65° celsius, while it's only at 35 without the monitor) than I want it to be, even if it's just idling.
Since I don't ever use the macbook screen as a secondary screen when connected, I thought I might turn it off completely and hopefully cool the machine down a little bit. Is there any way of doing this? I figured out I could just set the brightness to 0 but will this prevent the machine from actually rendering images?

Best Answer

If the idea was to just hide doubled picture on laptop display, you can just lower brightness level to 0 using fn + F1