Advanced Configuration and Power Interface Specification

ACPI-Defined Devices and Device-Specific Objects
_ALC
_ALT
_ALR
_ALP
The current ambient light color chromaticity reading, specified using x and y coordinates per the
CIE Yxy color model. [Optional]
The current ambient light color temperature reading in degrees Kelvin. [Optional]
Returns a set of ambient light illuminance to display brightness mappings that can be used by an
OS to calibrate its ambient light policy. [Required]
Ambient light sensor polling frequency in tenths of seconds. [Optional]
9.2.1 Overview
This definition provides a standard interface by which the OS may query properties of the ambient
light environment the system is currently operating in, as well as the ability to detect meaningful
changes in these values when the environment changes. Two ambient light properties are currently
supported by this interface: illuminance and color.
Ambient light illuminance readings are obtained via the _ALI method. Illuminance readings indicate
the amount of light incident upon (falling on) a specified surface area. Values are specified in lux
(lumen per square meter) and give an indication of how “bright” the environment is. For example, an
overcast day is roughly 1000 lux, a typical office environment 300-400 lux, and a dimly-lit
conference room around 10 lux.
A possible use of ambient light illuminance data by the OS is to automatically adjust the brightness
(or luminance) of the display device – e.g. increase display luminance in brightly-lit environments
and decrease display luminance in dimly-lit environments. Note that Luminance is a measure of
light radiated (reflected, transmitted, or emitted) by a surface, and is typically measured in nits. The
_ALR method provides a set of ambient light illuminance to display luminance mappings that can be
used by an OS to calibrate its policy for a given platform configuration.
Ambient light color readings are obtained via the _ALT and/or _ALC methods. Two methods are
defined to allow varying types/complexities of ambient light sensor hardware to be used. _ALT
returns color temperature readings in degrees Kelvin. Color temperature values correlate a light
source to a standard black body radiator and give an indication of the type of light source present in
a given environment (e.g. daylight, fluorescent, incandescent). ALC returns color chromaticity
readings per the CIE Yxy color model. Chromaticity x and y coordinates provide a more
straightforward indication of ambient light color characteristics. Note that the CIE Yxy color model
is defined by the International Commission on Illumination (abbreviated as CIE from its French title
Commission Internationale de l'Eclairage) and is based on human perception instead of absolute
color.
A possible use of ambient light color data by the OS is to automatically adjust the color of displayed
images depending on the environment the images are being viewed in. This may be especially
important for reflective/transflective displays where the type of ambient light may have a large
impact on the colors perceived by the user.
9.2.2 _ALI (Ambient Light Illuminance)
This control method returns the current ambient light illuminance reading in lux (lumen per square
meter). Expected values range from ~1 lux for a dark room, ~300 lux for a typical office
environment, and 10,000+ lux for daytime outdoor environments – although readings may vary
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