Abstract: Measurement of lengths for determining the outside diameter, wall thickness or inside diameter of tubes which, must be maintained to close tolerances. A mandrel of tungsten carbide having a coaxial projection of knife-edge width is postioned so that its transverse dimension intersects the beam and is parallel to the plane of the beam. A first length is measured between the end of the beam and the nearest point on the projection. A tube is mounted on the projection with its its longitudinal axis parallel to the longitudinal axis of the mandrel and its inner surface engaging this. A second length is measured from the end of the beam and the nearest point on the tube. The outside diameter is measured on the shadow of the tube produced by the laser between the shadow of the nearest point and the shadow of the most remote point from the end of the laser beam. The wall thickness is equal to the first length less the second length. The inside diameter is the outside diameter less twice the wall thickness.