Patent ID: 8481904

Claim:
An autofocus system for use in an optical system that is capable of examining a target surface and that includes an objective lens defining a system in-focus region, comprising: a light source emitting an outgoing reference beam of light, wherein a portion of the outgoing reference beam is reflected from the target surface as an incoming reference beam; a first photodetector disposed in the path of the incoming reference beam and adapted to generate a first electrical signal according to the position of the incoming reference beam on a sensing surface of the photodetector; a second photodetector disposed to receive a portion of the light emitted by the light source and adapted to generate a second electrical signal proportional to the light incident on a sensing surface of the second photodetector; wherein the first photodetector comprises a pair of photodiodes; wherein the first electrical signal comprises channel A and channel B signals, the channel A signal being proportional to the portion of the incoming reference beam incident on the first photodiode, the channel B signal being proportional to the portion of the incoming reference beam incident on the second photodiode; and wherein the second electrical signal provides a correction signal to the channel A signal and the channel B signal, and a focus error signal comprises the difference between the corrected channel A signal and the corrected channel B signal.