How actual display brightness is determined

Windows 8 cleanly separates brightness policies from the actual display brightness level.

The following formula determines how Windows determines the Actual Display Brightness Level. This value is subsequently provided to the monitor driver (Monitor.sys), which in turn notifies the Windows Display Driver Model (WDDM) display driver to change the display brightness accordingly:

Actual Display Brightness Formula

where Normal Brightness Policy:, Dim Brightness Policy, and ALS Offset are defined as follows:

  • Normal Brightness Policy: Depending on whether the Windows PC is plugged-in (AC) or runs on battery (DC), this value would be equal to the AC or DC value in the display brightness setting. As a result, this value may change due to AC/DC transitions, or any action that would change the value of the display brightness setting. These actions include brightness changes through the UI or hot keys (see Settings charm and Hardware buttons (hot keys)), as well as direct changes to the setting’s value in the power policy store (see Brightness control settings and actions). For example, based on the Windows 8 defaults, display brightness is 100% in AC and 40% in DC. If a system is in DC and the user changes the brightness from 40% to 55% through the Settings Charm brightness slider, they have effectively changed the display brightness setting (and thus Normal Brightness Policy) from 40% to 55%.
  • Dim Brightness Policy: This is a value between 0 and 100. When Windows dims the display, this would be equal to the dimmed display brightness setting (see Brightness control settings and actions), defaulted to 50. When the display is not dimmed, it will be equal to 100.
  • ALS Offset: This is a positive integer representing an offset to be applied to the display brightness due to outside lighting conditions. On systems without an Ambient Light Sensor (ALS), or during normal lighting conditions, the ALS Offset is 100. On systems with an integrated ALS sensor, the ALS Offset will be set to a value other than 100 after a sustained change in the lighting conditions. In general, dark environments result in ALS Offset values below 100, and bright environments result in ALS Offset values above 100. For further details, see Integrating Ambient Light Sensors with Computers Running Windows 8.



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