If you select the option "Illuminate keyboard in low light conditions" in the Keyboard pane of System Preferences, the computer will turn on the keyboard's illumination in low light conditions.
If the computer is idle for a certain number of minutes, the keyboard's illumination turns off. You can set the amount of time before the illumination is turned off in the Keyboard pane of System Preferences. Drag the slider under "Turn off when keyboard is not used for:" to the amount of time you wish to use.
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.
If the computer is idle for a certain number of minutes, the keyboard's illumination turns off. You can set the amount of time before the illumination is turned off in the Keyboard pane of System Preferences. Drag the slider under "Turn off when keyboard is not used for:" to the amount of time you wish to use.
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.
Figure 1 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.