Abstract: Apparatus for determining the thickness of a configuration having flat, parallel surfaces that are transparent, or nearly so, to pre-specified types of energy. Embodiments comprise a mechanism for illuminating a front surface with an energy source and mechanisms for measuring reflections of the illumination from a parallel back surface. The energy is contained in a spectrum of wavelengths, the energy being refracted in components at unique wavelengths, e.g., different colored light bands, and similarly reflected from the back surface. The measuring mechanisms, e.g., spectrometers, determine the relative lateral displacement between two spectral lines in the refracted and reflected beams to enable determination of thickness. Other characteristics of the material of the configuration may be ascertained, e.g., chemical composition is ascertained by measuring the intensity of responses at multiple wavelengths and comparing this to responses of known materials.