About Adjusting Brightness on Ambient Light Sensor (ALS)

This document describes the brightness adjustments for the PowerBook G4 (17-inch), PowerBook G4 (17-inch 1.33GHz), PowerBook G4 (17-inch 1.5GHz), PowerBook G4 (15-inch FW 800), or PowerBook G4 (15-inch 1.5/1.33GHz) computer's Ambient Light Sensor (ALS).

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On these PowerBook G4 computers, several function keys serve as shortcuts to adjust the brightness of the display and the keyboard illumination. F1 dims the display and F2 brightens the display. F9 dims the keyboard illumination and F10 brightens the keyboard illumination.

When the Ambient Light Sensor (ALS) is on, the function keys associate the current brightness setting to the current sensor reading.

If you use Mac OS X 10.3, the default settings for Exposé also use the F8, F9 and F10 keys. If your computer has an Ambient Light Sensor and is also running Mac OS X 10.3, hold down the "fn" key with the function key in order to activate the Exposé functions.
You can also remap the keys for the Exposé features by using the Exposé pane of System Preferences.

Since the system is automatic, as the ambient light changes, the display and keyboard illumination levels vary from the base level you established. As ambient light levels grow brighter or dimmer, the keyboard and display change from the level you selected to compensate for the brighter or dimmer setting. They return to the originally selected brightness when the ambient light level is the same as when you originally set the controls.

When the ALS is not turned on for displays, the brightness control keys (F1 or F2) work as on previous PowerBook computers, setting a fixed brightness of the display backlight.

Unlike the display brightness keys, the keyboard brightness can only be adjusted when the ALS is engaged with the keyboard illuminated (as in a dimly lit room). If you press the keyboard illumination control keys (F8, F9 or F10) at any other time, you will see a dimmed graphic with the prohibitory sign (see Figure 1) to remind you that the ambient light is not low enough for the keyboard illumination to have activated.

Alert that ALS cannot be configured


  1. Artificially activating the keyboard backlight by covering the sensors may not produce the desired results. The keyboard illumination is not normally visible in ambient lighting conditions brighter than a dimly lit room.
  2. 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.


Last Modified: Feb 20, 2012

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