Brightness for screens, particularly for external displays aren’t the easiest to manage. For one, the Windows 10 brightness control will not work for external displays. It only works with internal displays by default. If you have a multiple display set up, you likely have to set the brightness for your displays separately. An easy way to sync brightness across multiple displays is to use an app called ClickMonitorDDC.
Sync brightness across multiple displays
ClickMonitorDDC has both a portable and an installable version. If you don’t have admin rights on your system, the portable version will work just as well. Pick which one you want to use and download it.
Run the app. It will run in the system tray by default where it displays the RGB values, contrast, and brightness values for a monitor. A window will open listing the monitors connected to your system. At the top left, there’s a button that will open the app’s settings.
Before you click it though, you should copy the name of your monitors as they appear. To do that, right-click a monitor and select the Change option from the context menu. In the window that opens, copy the name of the monitor. Repeat this for both(all) your monitors. Now you’re ready to go to the app’s settings.
On the settings window, enable the ‘Automatically use brightness from source monitor’ option. Enter the name of the monitor that you want to copy the brightness level from in the first field. In the second field, enter the name of the monitor you want to sync the brightness level to, click Apply, and the brightness for both will be synced.
Keep the app running and whenever you change the brightness for the source display, the brightness for the second display will change accordingly. More importantly, if you have adaptive brightness enabled, this app will sync the change in brightness from your internal display to the external one. It will basically extend the feature to a display that would otherwise not have it.
This can work both ways i.e., you can sync the brightness from the external monitor to the internal one as well. All you have to do is change which monitor you’ve entered as the source (first field) and which one you’ve entered in the ‘For’ field (second one). The app is set to run automatically when you log on to your desktop even if you’re running the portable version.