Patent ID: 8519314

Claim:
A method of establishing the focal position of a measurement sample relative to an imaging optical system, the method comprising the following steps: illuminating the sample with a light source producing a predetermined level of irradiance on a detector receiving an image of the sample; and finding said focal position by varying a distance between the imaging optical system and the sample and determining a focal distance corresponding to a minimum light intensity produced by said source while maintaining said predetermined level of irradiance on the detector; wherein said finding step is initiated by varying said distance between the imaging optical system and the sample by an initial step in a predetermined direction, comparing a new level of detector irradiance produced by said initial step with said predetermined level of irradiance; and (i) if said new level of detector irradiance is greater than said predetermined level of irradiance, continuing the finding step by varying said distance between the imaging optical system and the sample by translation in a continuing direction equal to said predetermined direction; or (ii) if said new level of detector irradiance is smaller than said predetermined level of irradiance, continuing the finding step by varying said distance between the imaging optical system and the sample by translation in a continuing direction opposite to said predetermined direction; (iii) and, in either case (i) or (ii), continuing the translation in said continuing direction to determine said a focal distance corresponding to said minimum light intensity produced by said source while maintaining said predetermined level of irradiance on the detector.