Chrome OS Keyboard Backlight Behavior

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On devices that possess backlit keyboards, powerd is responsible for adjusting the backlight brightness.

Powerd reads raw percentages from keyboard_backlight_user_steps preference, scales the first step as 0%, second step as 10% and last step as 100%, and calculates the rest of the scaled percentages linearly.

If an ambient light sensor is present, powerd uses its readings to determine the keyboard backlight brightness level. In a well-lit environment, the backlight is turned off. In a dark environment, the backlight is turned on at a moderate level (pursuant to user activity, as described below). The ambient light ranges and corresponding backlight brightness percentages are read from the keyboard_backlight_als_steps preference. The percentages in this preference should be scaled percentages.

If no ambient light sensor is present, powerd reads a single brightness percentage from the keyboard_backlight_no_als_brightness preference and uses that instead when the backlight is turned on. The percentage in this preference should be scaled percentage.

When fullscreen video is detected, powerd forces the keyboard backlight off so as to not distract the user. The keyboard backlight is also dimmed or turned off in lockstep with the display backlight in response to user inactivity or the system suspending or shutting down.

If the device‘s touchpad is capable of detecting when the user’s hands are hovering over it, powerd turns the backlight on when hovering is detected and turns the backlight off soon after hovering ceases (the duration is supplied by the keyboard_backlight_keep_on_ms preference, which defaults to 30 seconds). Additionally, hovering forces the keyboard backlight on even while fullscreen video is detected.

If the device is incapable of detecting hovering, the backlight is turned on only in response to user activity (e.g. keyboard or touchpad events) instead. This is controlled by the keyboard_backlight_turn_on_for_user_activity preference, enabled by default.

The user is able to adjust the keyboard backlight brightness by holding Alt while pressing the Brightness Up or Brightness Down keys. The brightness moves between the raw percentage steps in the keyboard_backlight_user_steps preference. On the UI, the keyboard backlight brightness controller bar moves between the scaled percentage steps. Once the user has manually adjusted the brightness, powerd refrains from making any automated adjustments until the system reboots.