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Image Number 4 for United States Patent #5539990.
An optical levelling, plumbing and angle-calibrating instrument includes: a frame; a plumb body universally pendulously mounted on the frame and defining a vertical plumb line gravitationally; at least an illuminator electrically connected to a power supply and mounted on the plumb body for emitting laser light; and at least a cylindrical-surfaced lens mounted on the plumb body in front of the illuminator for planarly diverging the laser light as emitted from the illuminator through the lens to form a laser light plane transverse to a lens axis of the cylindrical-surfaced lens, whereby the laser light plane will projectively intersect an objective wall to form a straight line of optical image, serving as a reference line for levelling or plumbing use.
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