Absorption knife for the determination of the thickness, moisture or other parameters of a film or a 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.