The Light Sensor has only one output side for red alloy cables, redstone, an adjacent circuit, machine, lamp or anything that can be controlled with redstone signals.
It transmits a redstone signal with different intensities, which can be simply measured with a Wire Debugger. The signal intensity is dependent on the light intensity incident on the sensor, both natural and artificial light sources (without the Lamps, the Inverted Lamps and the Illumar Feedback Buttons) affect the sensor alike.
Note that ONLY the strongest light source, which reaches the sensor, is used for the signal (there is no addition of various light sources).
This fact is important combined with the three different modes of the Light Sensor (open, half-closed, closed) which are shown in the picture. These modes act as filters for different light sources or as barriers:
Open: The sensor detects all light sources.
Half-closed: The sensor detects only natural lights (sun and moon).
Closed: The sensor detects only artificial light sources (torches, lamps etc.).
Using the screwdriver from Project Red by holding the crouch key, you can switch between modes. Without the crouch key, the orientation of the gate can be changed freely in all four directions.
The following intensities are a few examples which are measured directly on the output with the Wire Debugger (artificial light sources were standing right next to the sensor):