Spherical cavity method and apparatus for measuring a sheet material property using infrared radiation

Abstract

A property such as moisture content in a sheet of material such as paper that both absorbs and strongly scatters infrared radiation is measured by passing the sheet through the central region of a substantially spherical cavity formed between a pair of concave mirror-surfaced, specularly reflecting bodies that are spaced apart to form a pass gap of sufficient width to allow substantially free movement of the sheet therethrough while substantially enclosing a first portion of the sheet on both sides. A spot of infrared radiation is projected onto one side of the sheet in the center of the cavity to illuminate a second portion of the sheet included within and substantially smaller than the enclosed first portion. Thus infrared radiation scattered or transmitted by the second portion is reflected from the mirror-surfaced bodies and directed back to the second portion substantially within the area of the sheet defined by the spot. The relative sizes of the first and second portions in relation to the width of the pass gap determine the relative quantities of radiation being retained in the cavity and escaping through the pass gap. On the opposite side of the sheet relative quantities of both the projected radiation transmitted by the sheet and radiation reflected from the reflective bodies are detected so as to produce an output response indicative of the measured property. The relative quantities of the retained and escaping radiation and the detected transmitted and reflected radiation are selected to make the output response substantially independent of changes in other properties affecting the scattering power of the sheet material, such as the bone-dry weight or composition of paper.

Dependencies are inferred from the claim text.