Apparatus and method for measuring thickness

Claim 1

A radiation thickness gauge comprising: a radiation source; a radiation detector spaced from said source and located in the beam emitted by said source, said detector comprising means translating the intensity of the radiation received by the detector into an electrical signal output; a first path extending through the space between said source and detector for the passage of a material whose thickness is to be measured; a second path in said space, parallel to said first path, for introducing one of several standards of known thickness of reference material on demand; shutter means of radiation absorbing material movable into said space to absorb the beam upon command; means for recording several values of detector outputs corresponding to several combinations of standards, and computer means comprising means for determining a thickness function that best fits the relationship between the recorded values of standards thicknesses and corresponding detector outputs and recording said function as a relationship between thickness and detector output; said computer means further comprising means for determining a value of thickness of the material to be measured by applying said thickness function to the detector output when the material whose thickness is to be measured is in the beam; memory means for storing as errors the difference between the known thickness of each standard combination and the thickness obtained by calculation by applying said thickness function to the detector output obtained by placing said standard combination in the radiation beam; and said computer means further comprising means for correcting detector outputs by: 1. calculating a thickness ratio as a fraction having as a numerator the known thickness of a standard combination minus the recorded error of said standard combination and having as a denominator the thickness as obtained by measuring the standard combination while using a previously stored thickness function; and 2. updating the thickness function by multiplying the previously stored function by said thickness ratio.