Did you know that you can use certain Apple iMacs as monitors when they become surplus to requirements using a special feature called Target Display Mode. It works automatically on iMacs that are not switched on, they automatically become a monitor in this special mode.
Target Display Mode
Target Display Mode is a special mode that certain iMacs can use. It enables you to re-use old hardware in a different way.
With 2009 and 2010 27″ iMacs, these have a mini Display Port connector and have to use a special cable to work with.
On 2011 – mid 2014 iMacs, they used Thunderbolt and Thunderbolt 2 connectors respectively. You can get an adapter that enables a more modern Mac to this with a Thunderbolt 3 to Thunderbolt 2 adapter.
There are some stipulations, these iMacs must be running High Sierra or below. Additionally, the Mac you are using to display from must be of 2019 or earlier and must be running Catalina or below. This mode does not work with the new M1 Macs.
Unfortunately, late 2014 Apple changed switched to Retina displays and decided the increased density of the pixel display was unsuitable for Thunderbolt and dropped the Target Display Mode from future iMacs.
So if you have an iMac older than the Retina displays you can use it as an additional monitor. Depending on what Apple Mac you are using as a source will define how many iMacs you can use as external displays, with the Mac Pro supporting 6 displays.
Alternatively, you can use something called LunaDisplay, which is a dongle that you connect to the machine that will be displaying the content and then the software runs on the display as a driver. It means you can use an iMac or even an iPad as a display and input device.
There is also a version for the PC so you can use an iMac for a display and an iPad as a display and input device.
The cost is $129 for the USB-C version suitable for both PC and Mac, Mini DisplayPort for older Macs and HDMI for older PCs.
One downside of LunaDisplay is that the iMac display cannot work in Retina mode.