Three-dimensional shape sensor based on lateral focused-plane sectioning

This paper presents a practical shape-from-focus method for measuring three-dimensional shapes on production lines. A focused plane was inclined typically at 45° to an optical axis in an afocal optical system and was imaged on a CCD image sensor, which was also inclined at an angle of 45° to the optical axis. When the focused plane and surface of an object intersect, a contour of the object is defined at each position of the carrier conveying the object. Each contour can be obtained by detecting focused points in an image of the focused plane using a specific focus-measure. To correctly detect the focused points on the right focused-plane image, a sequence of focused-plane images was taken at each position of the carrier moving perpendicular to the optical axis. A focused image point representing a contour point was detected as a specific pixel in a focused-plane image that gave a maximum of focus-measure for the contour point from the focused-plane images taken contiguously while sweeping the object by the focused-plane. Both the detected pixel coordinates and the carrier position producing the focused-plane image, including the detected pixel, could determine the three-dimensional coordinates of the contour point. A minute hemisphere with a diameter of 4.8 mm could be measured with a standard deviation of 16 μm.