The Light Sensor emits a redstone signal if it detects a minimum threshold light level. The threshold can be altered by shift-right-clicking with a Screwdriver or Sonic Screwdriver.
The levels require light thresholds of 1, 5, 9, or 13, with the default being 1. For reference, daylight is 15, a torch is 14, a redstone torch is 7, moonlight is 4, and complete darkness is 0. Light decreases by 1 for each block's distance from skylight or a light-emitting block: thus, the block immediately beside a torch block will have light level 13; the ground underneath the torch (or wall behind it) will be illuminated at 14. Almost all uses for Light Sensors will require raising the threshold from the default. As the light threshold is increased, a gray "shutter" closes over the blue surface of the sensor. To move the shutter, use the screwdriver by Shift+Right Click