Patent ID: 6717124
Filing Date: 2004-04-06
Classification: G02B

Abstract:
A focusing system for an optical system for imaging light from a sample, the system comprising:a) a photodetector for receiving and detecting a light beam from the sample along an optical path, the photodetector having at least four parts positioned at at least two different radial distances from a center of the photodetector, each part independently detecting the intensity of the light falling upon it and generating a light intensity signal indicative thereof; b) at least one lens in the optical path between the sample and the photodetector for focusing the light beam coming from the sample; c) a knife edge positioned in the optical path between the sample and the photodetector for blocking at least one, but not all, of the parts of the photodetector from receiving the light beam from the sample; d) a stage for holding the sample; e) a focus mechanism for controllably moving at least one lens or the stage along the optical path in response to a focus signal to adjust the focus of the system; and f) a controller for generating the focus signal in response to the light intensity signals, by: i) subtracting light intensity signals from substantially opposite parts of the photodetector to generate a focal error signal indicative of a direction in which the image of the sample is out of focus; ii) comparing light intensity signals from parts of the photodetector at different radial positions to determine an amount by which the image of the sample is out of focus; iii) generating a focus signal in response to which the focus mechanism will adjust the focus of the system in the direction and by an amount sufficient to bring the focal error signal closer to zero; and iv) repeating steps (i) through (iii) until the focal error signal is within a predetermined acceptable range.