A rotation information record surface of a rotary disk is disposed at an intermediate position between a fixing point of the rotary disk and a rotary shaft and a fixing position of a detection head, so that a stable gap can be obtained with a variation due to thermal expansion being reduced. In order to attain this, the fitting portion of the rotary disk is shaped to have a first fitting portion and a second fitting portion. The first fitting portion is provided for attaining satisfactory coaxiality, and the second fitting portion is adapted to form a clearance into which bonding means such as an adhesive is to be infused. With the above structure, an optimum detection position (i.e. an optimum gap position) would not be varied by a change in the ambient temperature.
A reflection encoder is disclosed having a light emitting element in a light receiving element that are separated by a light shielding body that prevents unwanted light from entering the light receiving element. In variations of the invention, the light shielding body may be integrally formed with a mold resin portion that holds the light emitting element and light receiving element, formed by a sheet or plate, or formed using an opaque liquid resin that is poured between transparent resin bodies that encapsulate the light emitter and receiver. In operation, light radiated from the light emitting element is reflected by a code wheel and then received by a light receiving element. Other variations include varying the height at which the light receiving and detecting elements are disposed relative to the code wheel and tilting these elements towards each other so as to increase light efficiency.
