US Patent 7821626 Angle measuring device and method

By diffracting light emitted from a collimator lens into lights of different orders by a transmission diffraction grating, applying the diffracted lights to a flat surface of an object, forming the light that has been reflected on the flat surface of the object and thereafter retransmitted through the transmission diffraction grating into an image in a position located apart from the optical axis of the outgoing light and measuring the angle of the object with respect to the optical axis of the collimator lens using the image location and the order of the light formed as the image, the angle in a wide range can be measured with a high resolving power.