Patent ID: 7411667

Claim:
A level sensor for determining a height of a substrate, said level sensor comprising: a measurement beam generator that generates at least one measurement beam and direct said at least one measurement beam to a measurement spot on said substrate in order to produce at least one reflected measurement beam; a reference beam generator that generates at least one reference beam; a detector beam that detects both said at least one reflected measurement beam and said at least one reference beam, respectively, and produces at least one measurement signal and at least one reference signal, respectively, said measurement signal being indicative for said height at said measurement spot; and a processor that corrects said measurement signal based on said reference signal, wherein said level sensor comprises an optical arrangement arranged in a predetermined area close to where said substrate is to be located, and the level sensor is arranged to propagate said at least one measurement beam and said at least one reference beam along substantially equal optical paths of propagation in said predetermined area, said optical arrangement being arranged to deviate said at least one reference beam from said substantially equal optical paths of propagation in said predetermined area such that said at least one reference beam does not hit said substrate.