Apparatus for measuring a characteristic of relatively thin sheet material

1367193 Photo-electric moisture content measurement MEASUREX CORP 20 April 1972 [6 May 1971] 18341/72 Heading G1A In an arrangement for measuring the moisture content of a sheet material 14, Fig. 2 by determining the ratio of the absorption of a pair of spectral bands that are respectively sensitive and insensitive to absorption by water, at least a part of the beam from a source 22 is applied to and modified more than once by the sheet before arriving at a detector 21, the reapplication being effected by a reflecting surface or reflecting surfaces of for example anodized aluminium, quartz or glazed ceramic which may be planar dished, hemispherical or highly irregular. A plurality of planar surfaces need not be parallel. The source and detector are axially offset, and may be on the same side or opposite sides of the sheet material, and in the case of the former, a single reflector need be provided only on one side. The invention is described as being particularly applicable to measuring the moisture content of tissue paper by using infrared radiation. '