The True Tone Subsystem
macOS laptops (MacBook Pro, MacBook Air) and advanced Apple external displays feature True Tone and automatic brightness adjustment. These features rely on hardware ambient light sensors embedded near the webcam. The software layer responsible for taking raw sensor data and mathematically translating it into smooth backlight adjustments on your screen is managed by the CoreBrightness framework.
To ensure smooth transitions (so your screen doesn’t strobe violently when a cloud passes over the sun), CoreBrightness caches temporal data and user-defined offset curves. If you manually adjust the brightness in a dark room, macOS remembers that preference and adjusts the algorithm’s baseline.
If the CoreBrightness preference cache becomes corrupted, the symptoms are immediately visible and highly disruptive. Your screen might suddenly dim to 0% and refuse to brighten, True Tone might get stuck permanently applying a harsh yellow tint regardless of the ambient lighting, or the screen brightness might rapidly flicker when waking from sleep. Resetting the NVRAM/PRAM sometimes fixes hardware sensor glitches, but it often fails to resolve software-level cache corruption. You must purge the CoreBrightness cache via the Terminal.
Locating the CoreBrightness Caches
Because display profiles are tied both to the physical hardware and the specific user’s visual preferences, the cache files are split between the root Library and the user’s local Library.
Purging the Cache via Terminal
You must use the Terminal with elevated (sudo) administrator privileges to safely delete these deeply integrated system preference files.
- Open the Terminal application.
- First, delete the global system-level CoreBrightness state file:
sudo rm -f /Library/Preferences/com.apple.CoreBrightness.plistPress Enter, type your Mac’s administrator password (the characters will remain invisible), and press Enter again.
- Next, delete the user-specific brightness preference overrides (if they exist):
sudo rm -f ~/Library/Preferences/com.apple.CoreBrightness.plistRestarting the WindowServer
Deleting the .plist files does not instantly fix the screen flickering because the corrupted algorithm is still actively running in the Mac’s RAM, managed by the WindowServer daemon.
You must forcefully terminate the WindowServer process to flush the RAM and force macOS to reload the clean default settings.
WARNING: Executing the next command will immediately log you out of your Mac and forcefully close all open applications without saving. Save your work before pressing Enter.
sudo killall WindowServerThe Recalibration
Your screen will instantly go black, followed by the Apple logo, returning you to the login screen.
During this reboot sequence, the CoreBrightness framework initialized, looked for the cache files, and realized they were missing. It immediately polled the ambient light sensor for a fresh hardware reading and generated a clean, uncorrupted algorithm baseline.
When you log back in, True Tone and automatic brightness adjustments will be fully restored to smooth, factory-calibrated functionality.