Method and apparatus for automatic optical focusing on an optica

Details Method and apparatus for automatic optical focusing on an optica Method and apparatus for automatic optical focusing on an optica

1983-10-28

1986-07-15

Westin, Edward P.

Radiant energy

Photocells; circuits and apparatus

Photocell controls its own optical systems

250234, 354402, G03B 300

Patent

active

046008322

ABSTRACT:
A method for the automatic fine focusing of an image scanning densitometer, micrometer, or television camera or the like, after an initial manual focus has been made on the edge of a discernible feature on the surface of an object of interest. A plurality of at least three, and preferably five, scans are made across the image of the feature at a corresponding plurality of focus positions, and the variations in light intensities are noted at each of a plurality of locations in each scan so that an edge value may be determined for each focus position. If plotted, edge values as a function of focus position appears as a second order curve of the form, edge value=ae.sup.2 +be+c, from which the values of the coefficients "a" and "b" may be determined by the least squares best fit method. When the derivative of the curve equation is made equal to zero to correspond to a zero slope or bottom of the curve, the best focus position is readily determined to that the precise focus may be adjusted to that position.

REFERENCES:
patent: 3912922 (1975-10-01), Lacotte et al.
patent: 4350884 (1982-09-01), Vollath
patent: 4373817 (1983-02-01), Coates
Tucker, "Improved Critical Dimension Measurement with Computer Aided Automatic Fine Focus", MMT Proceedings, Mar. 1981.

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