Patent ID: 6548795
Filing Date: 2003-04-15
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 two parts, each of which independently generates a light intensity signal indicative of the intensity of the light beam falling upon it; 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 light from the sample; d) a light adjuster positioned in the optical path between the sample and the photodetector for controllably adjusting the area of the light beam in response to a light adjustment signal; e) a stage for holding the sample; f) a focus mechanism for controllably moving the 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 g) a controller for generating the light adjustment signal and 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 and amount by which the image of the sample is out of focus; ii) if the absolute value of the focal error signal is larger than a first predetermined acceptable value, generating a light adjustment signal to cause the light adjuster to reduce the area of the light beam; iii) generating a focus signal to cause the focus mechanism to adjust the focus of the system in the direction and by an amount sufficient to bring the focal error signal closer to zero.