Abstract: An apparatus and method to determine with a high degree of precision the position of the inside wall of a translucent or transparent tube comprises a laser beam at a wavelength for which the material being measured is transparent or translucent and a directing means for aiming the laser beam toward the tube and normal to the axis of the tube while moving the aimed beam to traversely scan the tube. A light detection means, positioned to receive light polarized normal to the axis of the tube which light, having been reflected by the inside wall and refracted by the tube, causes the detection means to generate signals. The signals are processed to analyze when the light beam is reflected by and when the light is refracted at the inside wall of the tube. The apparatus is also useful in connection with finding the position of the inside wall of curved material of generally uniform thickness.