Patent ID: 6624403
Filing Date: 2003-09-23
Classification: G02B

Abstract:
An apparatus for evaluating whether a sample is in the focal plane of a primary focusing element of a measurement device comprising:a light source for generating a monitor beam, said beam being directed to reflect off the sample via the primary focusing element; a secondary focusing element for focusing the reflected monitor beam; means for splitting the reflected monitor beam into first and second beam portions, with the beam path of the first portion being longer than the second portion so that the focal planes of the two beam portions are spatially separated; an aperture in the path of the beam portions and positioned such that when the sample is in the focal plane of the primary focusing element, said aperture is located in between the focal planes of the two beam portions; and a detector for monitoring the intensity of the respective first and second beam portions after passing through the aperture and generating first and second output signals, said first and second output signals providing information about the location of the sample with respect to the primary focusing element.