The ambient light-sensing (ALS) system automatically adjusts the brightness of your display and keyboard illumination according to the light available in your current environment.
Display and keyboard can be independently disconnected from ALS, so it will not automatically adjust their brightness. For more information, see "PowerBook G4 (17-inch), (15-inch FW 800): How to Turn Off Ambient Light Sensor (ALS) Automatic Brightness Adjustment".
Multiple sensors are used to approximate the ambient light. The brightness of both the display and keyboard backlights are adjusted, based on this approximation.
The display and keyboard brightness can be adjusted to match the ambient light level. This setting is not static like previous display backlight settings. The brightness level indicators will vary based on the measured ambient light. For more information, see "PowerBook G4 (17-inch), (15-inch FW 800): About Adjusting Brightness on Ambient Light Sensor (ALS)".
When the Ambient Light Sensor (ALS) feature is on and the display or keyboard backlight brightness is adjusted manually, the relative brightness over the entire ambient light response curve is similarly adjusted.
Note: Not all PowerBook G4 (15-inch FW 800) models include the backlit keyboard and ambient light sensor. To determine if your model includes these features see if the F8, F9, and F10 keys have the ambient light sensor icons printed on them.
F8, F9 and F10 keys on keyboard
If these icons are not printed on the keys, then these features are not present in your model.