Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE credits
In the process of recycling paper, printing inks are removed but even after the most efficient de-inking, some particles remain. These particles strongly affect the brightness and shade of printing papers made from the recycled pulp.
The reason for measuring the brightness without being disturbed by dirt specks is to more efficiently control the process of bleaching in a recycled fibre process. There is no use of trying to bleach the dirt specks, only the fibres can be bleached and therefore this is the material which is interesting to measure.
It is not possible to measure the fibre brightness (reflectance factor) in a conventional instrument such as the Elrepho or other corresponding instrument without including any dirt specks in the sample. This work aimed at implementing a method in which the brightness of paper is evaluated from an image, where dirt specks can be identified and removed by image processing, thus giving a measurement of the fibres only.
An existing instrument, Colour Dirt Speck Counter, was investigated with this purpose. A calibration routine was implemented. Several different light sources were tested, with the purpose of getting reliable measurements of the reflectance factor of paper at short wavelengths. Samples made of pure mechanical wood pulps were evaluated; the result from the Elrepho was regarded as a reference since it is the standard instrument. The Colour Dirt Speck Counter ranked the samples in the same order as the Elrepho if data were evaluated at the intensity peak of a white LED illumination, 470 nm. No other light source gave this result. A considerable variation between the darkest and brightest pixel was found in the images. It is suggested that this is due to the structure of the paper where fibres appear brighter than the spaces between them.
2005. , 69 p.