A system for non-contact measurement of thickness of a test object. A
laser beam is split into two identical directly opposed input beams. A
calibration object of known thickness causes beams to be reflected from
sides of the test object. Each reflected beam passes through sensing
means including a pinhole aperture and a photodiode sensor. Maximum
sensor output defines first and second focal points a known distance
apart. The calibration object is removed, and the test object is inserted
into the path of the input beams, creating focus position intensity
curves for the reflected beams. By determining the deviation, at maximum
photodiode output, of the positions of the test object reflecting
surfaces from the positions of the calibration object surfaces, the test
object thickness can be readily and accurately determined.