Many attempts have been made to provide an on-line sensor using reflected light to determine the roughness and in some cases both roughness and gloss of a relatively moving paper sheet, however, to date none have been particularly successful.
One system, described in U.S. Pat. No. 4,019,066 issued Apr. 19, 1977 to Lucas et al, illuminates the web by a light directed at a low angle onto its surface and collects scattered light by a lens system which focuses the collected light as a sharp image through an aperture onto a detector. The detector and associated equipment determine the AC and DC components of the reflected light sensed by the detector and develops a roughness index based primarilty on the ratio of AC to DC signals. This device provides a reasonable correlation between a roughness index and the Sheffield Roughness however the results contain a significant amount of scatter.
U.S. Pat. No. 2,755,702 issued Jul. 24, 1956 Cooke also discloses a system for measuring the smoothness of a surface. In the device microscopic surface characteristics are examined. Light is directed at an oblique angle onto the surface to emphasize irregularities reflected (non-specular) light is viewed through a microscope objective lens and directed through an aperture that serves to restrict the area scanned to dimensions in the order of the microscopic particles. The light passing through the aperture is measured to generate a signal and the signal is analyzed in a spectrum analyzer. The theory of operation is based on the concept that the intensity of deflected light varies in accordance with the shadows formed due to the surface roughness and the oblique lighting.
Projection of a light beam at an angle on the web surface and collection of the specularly reflected component of the light beam has been suggested and applied as described for example in Optical Measurement Throws New Light on Paper Surface by Hansuebsai et al in Pulp Technology and Industry, August 1987, pages 563-573. This paper describes an investigation of the use of different incident angles and an assessment of the use of polarized light, polarized either parallel or perpendicular to determine the best mode of detecting surface properties of paper. Reasonable results were obtained but the sensitivity was lacking.
In a paper entitled `A Method to Measure Variations in Surface and Diffuse Reflectants of Printed and Unprinted Paper Sample` by Bryntse et al in TAPPI, April, 1976, Volume 59, No. 4, pages 102 to 106, a device similar to that invented by applicant is disclosed. In this device polarized laser light is projected onto a surface and then collected via a lens and projected as an `image` with a portion of the image passing through a pin hole and focused onto a prism which splits the beam into its perpendicular and parallel polarized components, each of which is independently measured and used to distinguish surface characteristics of the paper sheet. This device, as with the previous devices, has limited sensitivity.