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

Claim 1

Apparatus for determining or controlling a parameter or parameters which represent the thickness, moisture content, composition, or other property of a film or coating capable of transmitting radiation, comprising: (a) means for defining a sample zone in which a sample of a film or coating can be received; (b) means for deriving a plurality of beams of radiation of differing spectral compositions; (c) optical directing means for directing a plurality of said beams to said sample whereby said beam is divided by said sample into a transmitted part transmitted by said sample, and a reflected part reflected from said sample; (d) reflector means for reflecting said reflected part of said beams through said sample; (e) a first radiation detector which receives the transmitted part of said beams and produces a signal "a" in response thereto, and a second radiation detector which receives the reflected part of said beams, reflected only once by said sample, and produces a signal "b" in response thereto; (f) processing means for processing said signals "a" and "b" to provide a resultant signal a/(1-b/a) or a signal proportional thereto, said resultant signal being processed in order to provide a first set of electrical signals representing the separate intensities of the beams of different spectral compositions as received by said first and second radiation detectors; and (g) means for providing at least one second signal representing the ratio of at least two of the signals of said first set of electrical signals.