Method and apparatus for automatic focusing of a confocal laser

Abstract

A microscope system moves a target in a first direction relative to a low power objective lens and, during the relative motion, generates and records values of an electronic focus signal that depends on the magnitude of light reflected by the target. Then, a host workstation calculates a first estimate of position ('focus position') of the target at which the microscope system is focused, by a median point method. In the median point method, the host workstation calculates the sum of the recorded values and determines the position along the range of motion at which half of this sum was exceeded, to be a first estimate of the focus position. From the intensity values of the first pass, optimal sensor gain is set for subsequent passes. Second and third estimates of the focus position can be calculated in a similar manner if necessary and the target is moved to the most recent estimate of the focus position. In one embodiment, the microscope system uses an area scan in which the largest value of an electronic focus signal at a set of points within an area of the target is recorded at a given elevation of the target. The largest of the recorded values is then used to estimate the focus position of the brightest layer of the target. In one embodiment, the microscope system focuses on a predetermined layer using an offset from the brightest layer.