The backlight brightness for external displays is controlled using DDC/CI. Powerd only accepts requests to increase/decrease the external display's brightness, not to set to an absolute brightness. If there is more than one external display, the brightness is changed on all of them (assuming they all implement DDC/CI).
Some Chromeboxes work with external displays that also contain an ambient light sensor in the display bezel. Powerd can separately control the brightness of these displays based on the ambient light level detected by the sensor in the display. This feature is disabled by default but can be enabled with the external-ambient-light-sensor preference. It can also be dynamically enabled/disabled via DBus with SetExternalDisplayALSBrightness.
For external displays with ambient light sensors, the brightness levels used are controlled by the external-backlight-als-steps preference. The default levels used set the brightness very low in low light conditions, and to full brightness otherwise.
For boards that support external displays with ambient light sensors, if automatic brightness control has been disabled, an exception is made to allow setting an absolute brightness to external displays with ambient light sensors. This does not apply to external displays without an ambient light sensor.
