It works by scanning a laser beam across an object and manipulating the reflected light to produce an extremely narrow focal plane.

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If, for some reason, some of the convergence points meet in front of, or behind, the focal plane, aspects of the resulting picture will be out of focus.

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Instead of converging to a point on a focal plane behind the lens, the rays spread out so as to run in a thick stream roughly parallel to each other (see chart).

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