Indicate ambient brightness.
The higher brightness value, the faster a fan to rotate.
To calibrate, tap on the right top arrows. Right after enabling Auto-Brightness control, the value shall be approx.400 at a family living room.
Accuracy of a light sensor may vary between different devices. The brightness value is indicative.
Note: Respecting iOS API specification, this app refer a brightness of a display backlight to determine the ambient brightness. Thus, Auto-Brightness control needs to be enabled from iOS Settings.