Ambient Light Sensor
Showing up as an input is something identified as a temt6200_light.
That's probably something doing an a2d conversion of Vishay Semiconductors TEMT 6200 ambient light sensor
You can read the ambient light value from /sys/class/input/input4/result
Putting the pre in complete darkness:
root@castle:/sys/class/input/input4# cat result 4
Exposing it to low light levels
root@castle:/sys/class/input/input4# cat result 11