Patent ID: 8139234

Claim:
A method of measuring the extinction of light in a coating as it is being deposited, comprising the steps of: depositing at least one pair of thin film layers comprising a first layer and a second layer with refractive index at least 0.10 lower than the refractive index of the first layer, each layer within any pair having optical thickness between about 1.4 and 2.5 quarter wavelengths, and sending at least one nominally collimated probe light beam from a substrate side of a substrate surface to be coated such that the beam encounters that surface at an angle of incidence for which the beam undergoes nominal total internal reflection prior to the deposition of the coating to be studied and such that the probe beam will, at a certain layer thickness less than the intended final thickness, be waveguided in that layer thus reducing the internal reflection for some limited period of deposition, and measuring any reduction(s) of the internal reflection during the deposition and calculating the extinction(s) in that deposited waveguiding layer that correspond(s) to the measured drop(s) in internal reflection.