Optical transducer system for monitoring movable parts, optical modulator and musical instrument using the same

An optical modulator (30) is hung from a key (2a, 2b) moved on a key trajectory so as to be moved together with the key (2a, 2b), and a light beam (B, B1, B2), which is stationary with respect to a key bed (6a), is radiated through the optical modulator (30); a large number of miniature refractors (32c) are formed on the light output surface (32b) of the optical modulator (30), and the density of miniature refractors (32c) per unit area is varied in a direction in which the optical modulator (30) is moved together with the key (2a, 2b) so that the current key position is converted to the amount of light passing through the optical modulator (30); since the miniature refractors (32c) and other portions of optical modulator (30) have a unitary structure, a molding process is used for the optical modulator (30), and the molding process make the production cost low.