http://www.youtube.com/watch?v=HIzuOeyu-pI

In this video, a Cyndustries Photon module is used to control a modular synthesizer with light:

The Photon Interface module comes with a black metal Littlelite brand high-intensity gooseneck lamp, a rheostat dimmer, heavy duty right-angle 4-pin panel connectors, and an independent 12 Volt DC power transformer for system lighting right where you need it.

The lower half of the module features a sensitive photocell with its own dedicated voltage processor which can be used as a performance controller when your hand passes between photocell and lamp.

The voltage processor has an adjustable attenuator and comparator and offers both positive and inverted outputs. Rushing the photocell suddenly with your hand will also generate a Gate Output for triggering different things, and you can switch between an external signal input or an adjustable internal voltage supply simply with the pull of a knob.

via hexenduction:

This is the Photon Interface module by Cynthia. The positive CV output from the Photon is controlling the filter frequency of the Serge Creature and later the tempo of the Cynthia VC Tabla. The lower half of the module features a highly sensitive photocell with its own dedicated voltage processor which makes for a great performance controller when your hand passes between photocell and lamp. The gate out from the VC Tabla is triggering the Pulse Divider on the Serge Gator. These divisions are doing various tone changes in the Creature. 2 of the Malekko Echo Bright 600 pedals are being used for delay.