Photoelectric encoder having a plane mirror disposed parallel to the optical axis of a concave mirror

A photoelectric encoder assembly comprising:

• a light emitting device that emits concentrated parallel rays of light, including a light source, a first reflecting member with a curved surface having an optical axis and being spaced from the light source that reflects light rays emitted from the light source as parallel light rays, and a second reflecting member with a planar surface positioned parallel to the optical axis that reflects light rays emitted from the light source onto the first reflecting member;
• a main scale including a light transmissive plate having a longitudinal axis and a plurality of opaque gratings spaced along the longitudinal axis, the main scale being disposed adjacent the light emitting device to receive the reflected light rays from the light emitting device and being movable with respect to the light emitting device;
• and
• an index scale including a light transmissive plate having a longitudinal axis and a plurality of opaque gratings spaced along the longitudinal axis, the index scale disposed adjacent to and aligned with the main scale to receive the reflected light rays from the light emitting device that have passed through the main scale, wherein the index scale is fixed with respect to the light emitting device such that displacement of an object is detected based on relative movement between the main scale and the index scale.