Patent ID: 7295324

Claim:
A speckle-image-correlation optical displacement sensing readhead, the readhead providing speckle pattern images used to determine a relative displacement measurement along at least a first measurement axis relative to a member having a surface that gives rise to speckles when illuminated with coherent radiation, the readhead comprising: a coherent light source that emits a light beam that illuminates a portion of the surface to produce a speckle pattern that corresponds to the illuminated portion, the illuminated portion depending on the relative displacement; a readhead optical system having a nominal focal plane, the readhead optical system including an image detector positioned along an optical path of the readhead optical system to receive an image of the speckle pattern; and at least one stabilizing element, wherein the stabilizing element stabilizes the speckle pattern images used to determine the relative displacement measurement with respect to potential changes in the speckle pattern that might otherwise occur due to causes other than the relative displacement along at least a first measurement axis, the at least one stabilizing element comprising a warm-up period monitoring system that provides a signal when the speckle pattern images used to determine a relative displacement measurement are nominally stable.