An optical encoder for measuring the rotary angle of a rotating shaft comprising an electronic board with light emitter and light sensor means disposed between a pair of overlying light guides having a common symmetry axis that coincides with the shaft rotation axis, the light guides receiving light from a light emitter means on the electronic board and returning light to a light sensor means the electronic board; an optical pattern of two concentric annular sections attached to the rotating shaft and an optical pattern of two concentric annular sections on a stationary device. A method for measuring the rotary angle of the rotating shaft by emitting light from the electronic board into the light guides and processing the signals received on the electronic board from light rays returning through the light guides after interacting with the said optical patterns.