Dell Studio 1558 (Early 2010) - Manual (Page 54)

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Monitors and Displays
You can enable or disable the ambient light sensor feature by pressing the
<Fn> and left-arrow key combination.
NOTE:
Do not cover the ambient light sensor with any adhesive labels. If covered,
the sensor—when enabled—automatically sets the display brightness to the
minimum level.
NOTE:
The ambient light sensor is disabled when your computer is connected to
an electrical outlet.
Using the display brightness key combinations disables the ambient light
sensor and the display brightness increases or decreases accordingly.
NOTE:
Rebooting the computer returns the ambient light sensor to it’s prior setting
(either enabled or disabled).