Brightness Sync2.3.2

Brightness Sync copies the brightness of a Mac’s built-in display to connected LG UltraFine monitors, allowing one brightness setting to govern every screen. It is a small menu bar utility that reads the ambient-light sensor in a MacBook Pro or iMac and applies the resulting brightness to the external displays. The app targets the first-generation LG UltraFine models that could not adjust their brightness automatically, including the 22MD4KA, 24MD4KL, 27MD5KA, and 27MD5KL.

It controls the monitors’ actual backlight through a private macOS framework, in the same manner as the secondary brightness slider on a Touch Bar. That approach avoids merely darkening the image in software, which can leave the backlight unchanged and significantly reduce contrast. An offset slider lets you deliberately keep the UltraFine brighter or dimmer than the built-in screen, useful when screens sit at different angles or appear unequal.

Brightness Sync depends on the Mac’s ambient-light sensor, so automatic adjustment does not work when a MacBook is closed in clamshell mode or when the displays are connected to a Mac mini. It polls brightness every 0.1 seconds, typically adding about 0.3–0.5 energy impact in Activity Monitor, and stops polling when no UltraFine is connected; because these monitors also supply power, it effectively does not run on battery power. Other UltraFine models are outside its compatibility target, with Lunar suggested as an alternative.

The MacOS System Tools release 2.3.2 supports Intel and Apple Silicon and is freeware. It is intended for Mac users combining a built-in display with the specified LG UltraFine screens who want ambient-light changes and manual brightness adjustments synchronized without sacrificing display contrast.