Lighting up paper

Researchers have developed a sophisticated way of measuring the print quality of paper. The work, published today in the Institute of Physics journal, Measurement Science and Technology, 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 colour ink.

The physical properties of paper such as colour, 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 sophisticated tools to measure paper quality, Dr Palviainen and colleagues focused their work on the fibres 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 analysing 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 colour 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 polarised 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 coloured ink prints. These parameters will be very useful to make compact and robust optical measuring tools for laboratory and industrial applications,” said Jari Palviainen.

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