N900 light meter
The light meter lets the software on the N900 sense ambient light levels.
Hardware
The device is a TAOS TSL2563.
Software
It is handled by the kernel module tsl2563.
The kernel driver exposes the lux value in the sysfs file
cat /sys/class/i2c-adapter/i2c-2/2-0029/lux 21
Under MeeGo it's available as
cat /sys/class/i2c-adapter/i2c-2/2-0029/device0/illuminance0_input
Note: please check! This might only be one of the two different spectral channels and thus yield vastly incorrect LUX values when massive IR or UV light has impact on the sensor. The lux value of N900 is generic and derived from an algorithm calculating visible lux from roughly a difference between the two sensor values the ALS provides
Third party applications that use this sensor include Luxus Light Meter and photographic light meter.
The sensor is also used to modulate the backlight in response to ambient light, although this behaviour can be disabled.