The optical transducer was a facility of Geordi La Forge's VISOR.
In 2369, Doctor Beverly Crusher adjusted the optical transducer to block some of the pain receptors in La Forge's brain after he had been shot by a terrorist at Arkaria Base, although the effect was only partial. The optical transducer was later used to generate a hypersonic pulse that rendered all but Data unconscious. (TNG: "Starship Mine")
The Star Trek Encyclopedia (4th ed., vol. 2, p. 108) described the function of this component. This component "received electromagnetic radiation from the environment and translated it into bioelectric impulses that could be interpreted by the brain."