The best alternatives to Luna Display are Duet Display, Lunar, and DUO by Mobile Pixels. If these 3 options don't work for you, we've listed a few more alternatives below.
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Duet turns your iPad or iPhone into the most advanced extra display for your Mac & PC. Developed by a team of ex-Apple engineers, Duet offers unparalleled performance and display quality - all with zero lag.
The built-in display brightness of your Macbook or iMac is synchronized to the external monitor.
Custom brightness and contrast values are computed for each external monitor by passing the built-in brightness through a curve algorithm that is completely configurable by the user.
BetterDummy creates a virtual display which you can utilize as a mirror main for your display on the Mac. This technique allows all Macs (including Apple Silicon) to have any HiDPI "Retina" resolution on any display you use.