Absorption gauge for determining the thickness, moisture content or other parameter of a film of coating

摘要

The apparatus includes means for directing a plurality of beams of radiation of different spectral compositions to a sample such as a film or coating. A part of the beams is transmitted through the sample and received by a radiation detector. Another part of the beams reflected by the sample is in turn reflected, preferably by a concave mirror, back through the sample onto another radiation detector. The signals 'a' and 'b' from the detectors are combined in accordance with the relationship a/(1-b/a) to suppress optical interference effects.