Apple Silicon users: Download the beta.
Download: here (macOS Catalina required, app is signed and notarized)
Alternatively, you can use Homebrew Cask: $ brew install --cask brightness-sync
The app doesn't automatically check for updates. If you want to stay up-to-date, I recommend selecting "Watch->Custom->Releases" at the top of this page if you have a GitHub account, or using the Homebrew Cask installation method.
This app is only compatible with the LG UltraFine monitors developed for Mac in cooperation with Apple, specifically:
22MD4KA, 24MD4KL, 27MD5KA and 27MD5KL
These monitors work differently than other monitors and this app was developed because existing solutions didn't support them. If you have a different (UltraFine) monitor, please check out another solution like Lunar.
This is a small menu bar app to mitigate the problem of the first-generation LG UltraFine not being able to automatically adjust brightness. It will poll the brightness of your built-in display and synchronize it to your LG monitors. This way you can use the ambient light sensor of your MBP or iMac for all your (UltraFine) displays! This also means manually adjusting the brightness of all your displays at once is easier, a single swipe on your Touch Bar or press on your keyboard will do.
The difference between this app and some existing apps is that this app uses a private framework of macOS to control the backlight of the LG UltraFine the same way the secondary slider of your Touch Bar will do. Other apps might virtually darken the display with the backlight staying the same, this will greatly reduce contrast.
Because this app relies on your Mac’s ambient light sensor, unfortunately it won't help with automatic brightness if your MacBook is in clamshell mode or you have connected your UltraFine to for example a Mac Mini.
You can use the offset slider to control your UltraFine's brightness in relation to your built-in display (e.g. the difference between them). You might want to do that if your monitors are at different angles, you perceive a difference between them that you want to correct, or for various other reasons. If you are interested in controlling and finetuning this offset throughout the day, disable the offset lock in the menu. Now you can use various other ways to easily control the offset. For more info see the release notes for v2.2.0. Otherwise, just keep the lock enabled.
I only have one 27-inch LG UltraFine display, so I can do only limited testing. Let me know if you have issues.
Requires macOS 10.15. If you're on 10.14, you can download v1 from the releases page.
The app polls the brightness pretty aggressively (every 0.1 seconds), which results in a small energy impact of around 0.3-0.5 according to Activity Monitor. However it will automatically stop the polling when no UltraFine displays are connected and because those monitors are also a power source, this effectively means it will never run when on battery power.
If you want to change the default update interval, it is exposed as an internal user preference BSUpdateInterval. It can be set via the terminal:
$ defaults write dev.vandijk.Brightness-Sync BSUpdateInterval 0.5 where 0.5 is the desired delay in seconds. The app must be restarted for the change to take effect.
This app was designed with 1st generation UltraFines in mind that don't support auto brightness. I started work on supporting mixed setups with both 1st gen and 2nd gen displays in v2.3.0. If no built-in display is detected, it will use a 2nd gen display as the "source" and sync its brightness to all other connected displays. The app will currently override the auto brightness of all 2nd gen displays that aren't used as the source, because some people have reported the auto brightness of the 2nd gen to not be so reliable and are using this app to sync the brightness of the built-in display to the 2nd gen. You should probably turn off the built-in auto brightness of the 2nd gen in the Settings if you do this. Other people might want to only sync to 1st gen and let the 2nd gen handle its own auto brightness. I'm considering making this an option for those people.