Abstract: An in situ thickness change monitor for determining thickness change in opaque product material, such as silicon, in chamber apparatus, such as reactive ion etching apparatus, operative to produce such thickness change. Reference material having thickness change properties, such as etch-rate, correlatable to the product material thickness change properties is deposited upon a substrate having an index of refraction such as to form a monitor exhibiting an optical discontinuity. With the monitor positioned within the chamber with the product material, light directed thereto acts to provide reflected beams producing light having an intensity variation due to interference indicative of the thickness of the reference material. Changes in the thickness of the reference material are correlated to changes in thickness of the product material.