Abstract: A measuring apparatus is provided having an illumination unit including a source of electromagnetic radiation; fiber optic apparatus and sensing apparatus. The fiber optic apparatus includes first fiber optic structure having an input end for receiving at least a portion of electromagnetic radiation emitted from the radiation source and an output end for directing the received radiation to a web of material, and second fiber optic structure having an input end for receiving radiation reflected from the web of material and an output end for directing the reflected radiation to a sensing apparatus. The sensing apparatus includes a first detector for sensing electromagnetic radiation of a first wavelength band and generating a corresponding first output signal and a second detector for sensing electromagnetic radiation of a second wavelength band and generating a corresponding second output signal indicative of a first property to be measured of the web of material.

Abstract: Beta gauge composition correction is performed using signals from a plurality of detectors that are positioned so that the ratio of radiation received by the detectors depends on the composition of material through which the radiation passes before reaching the detectors. Radiation is measured at the detectors and the differences between radiation received by the detectors is used to compensate the beta gauge to correct for composition variations. An array of detectors is divided into inner detectors generally aligned with the central portion of a beta radiation beam and at least one set of outer detectors surrounding, at least in part, the inner detectors. Measurements are made including all the detectors, the inner detectors and the at least one set of outer detectors with the difference between the measurements made by the inner detectors and the outer detectors being used to compensate the total measurement made by all the detectors.