Patent ID: 7342608

Claim:
A focus detecting method of an image measuring device having an image pickup device arranged on an optical axis that crosses a measured object and picking-up an image of the measured object, and measuring said measured object from the picked-up image; wherein said image measuring device comprises an illuminating device having a light emitting source that irradiates illumination light toward said measured object; a grating filter arranged between the light emitting source and said measured object and projecting a grating pattern to the measured object by interrupting the illumination light from said light emitting source; a rotation driving device that rotates and operates the grating filter and changes the grating angle of the grating pattern projected to said measured object; and a distance driving device that operates at least one of said image pickup device and said measured object, and adjusts a mutual interval distance along said optical axis; and the focus detecting method comprises: an image input process in which said distance driving device and the rotation driving device are operated in association with each other, and the grating pattern of said grating filter is projected to said measured object for every predetermined rotating angle, and the image of said measured object for every predetermined interval distance according to said predetermined rotating angle is picked up by said image pickup device by multiple exposure so that image information is obtained; a contrast value calculating process that obtains a contrast value for every said predetermined rotating angle on a basis of frequency area information obtained by converting the obtained image information into a frequency area; and a focus position calculating process that obtains a focus position from the interval distance between said image pickup device and said measured object according to the rotating angle for maximizing said contrast value.