| 摘要 |
The invention defines a boundary in space and optically determines the crossing of it by an object and at the time of crossing measures the distance between a point located on the object and the boundary. More particularly, the invention establishes the boundary with respect to an amount of light eclipsed in a light beam positioned in space. In one embodiment, an examination zone containing the boundary is illuminated by two light beams, one of which is produced by inverting and reflecting the other after the other passes through the examination zone with the result that the rate of change in light intensity which occurs during the eclipsing is greater than that in the case of a single beam.
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