How does optical sound-on-film work?

Well, there are two major optical sound-on-film systems: variable area and variable density. Both system use the same principle: a narrow beam of flicker free light from an exciter lamp falls through the soundtrack (which is located on an opaque area next to the actual picture) on a solar cell. As the film moves the changing amount of light causes a changing signal to be produced by the solar cell. These signals are amplified and sent to the speakers.
Now for the difference between variable area and variable density: with variable density the sound is represented by shades of grey, as opposed to variable area where the changing width of the clear portion of the opaque area encodes the sound. For an illustration on variable area, click on the picture.
There were several subsystems of the variable area system such as unilateral, bilateral, dual unilateral and dual bilateral. The figure shown is an example of a bilateral soundtrack.