The article describes how Jari Palviainen and colleagues at the Universities of Joensuu and Oulu in Finland, use what is known as a diffractive optical element-based sensor to investigate how laser-light interacts with paper before and after laser printing with color ink.

The physical properties of paper, such as color, brightness, gloss, opacity and roughness, affect its quality -- the way it looks and how suitable it is for printing. For example, the surface roughness affects the reflectance and scattering of light from the paper surface, contributing to the print quality of the paper.

Knowledge of these factors can be used to improve the suitability of the wood and other materials used to make paper pulp.

To meet the growing need for tools to measure paper quality, Dr. Palviainen and colleagues focused their work on the fibers in the paper -- the wood cells. The cell wall can be thought of as bundles of multilayered structures of microfibrils or nanotubes made up of cellulose, hemicellulose and lignin.

The cell walls influence the light by scattering it. By analyzing the scattered light, optical changes in the paper can be detected.

Using a diffractive optical element sensor (DOE), they investigated the laser-light interaction with commercial paper before and after printing with color ink. The DOE sensor detects changes in the optical properties of the paper such as surface roughness, reflectance and transmittance caused by the ink.

By shining a polarized laser beam on the surface of the paper sample, reflected and transmitted wavefronts of light are guided to the DOE aperture. These wavefronts are then focused onto different locations where a CCD-camera records the information. The scattering caused by the paper sample changes the perfect wavefront into a disturbed wavefront, giving information on the paper quality.

"We have found a correlation between optical signals and properties of paper such as surface quality, uneveness and quality of black and colored ink prints. These parameters will be very useful to make compact and robust optical measuring tools for laboratory and industrial applications," Jari Palviainen said.

The Institute of Physics is a leading international professional body and learned society with over 36,000 members, which promotes the advancement and dissemination of a knowledge of and education in the science of physics, pure and applied.

The Institute is a member of the Science Council, and a nominated body of the Engineering Council. The Institute works in collaboration with national physical societies and plays an important role in transnational societies such as the European Physical Society and represents British and Irish physicists in international organizations.

In Great Britain and Ireland, the Institute is active in providing support for physicists in all professions and careers, encouraging physics research and its applications, providing support for physics in schools, colleges and universities, influencing government and informing public debate.

(Reference: Measurement Science and Technology, Volume 12, Issue 4, N31-37)


[Contact: Dr. Jari Palviainen, Dr. Alice Bows]

01-Mar-2002