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Abstract

An optical axis monitoring apparatus for monitoring a variation in the optical axis of light beams in an optical system. The apparatus includes a light source for generating a light beam to propagate along a first optical path. A diffraction optical element is disposed along the first optical path for diffracting the light beam into at least two orders of diffraction. At least one of the orders of diffraction propagates along the first optical path and another of the orders of diffraction propagates along a second optical path. A first monitoring element id disposed along the first optical path between the light source and the diffraction optical element for monitoring the first light beam and generating an incoming beam signal in response to light incident thereon. The incoming beam signal being indicative of the condition of the first light beam. A second monitoring element is disposed opposite the diffraction optical element from the first monitoring element and along said the second optical path. The second monitoring element is for receiving light diffracted into the other of the orders of diffraction by the diffraction means and generating an outgoing beam signal in response to light incident thereon. The outgoing beam signal is indicative of the condition of the light beam emanating from the diffraction optical element.

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