An overview of the theory describing light scattering in paper is given. It is investigated
experimentally and theoretically how the anisotropy of light reflected from paper depends on the
paper absorption and thickness. This is done by measuring the angular resolved reflectance from
a series of handsheets containing different amounts of dye and filler and varying in grammage.
The theoretical investigation is done by using the angular resolved model DORT2002.
Measurements and simulations both show that the anisotropy increases with increased
absorption and is higher for lower grammages. The relative amount of light scattered into larger
polar angles increases for these cases. It is shown that the range of exact validity of the Kubelka‐
Munk model is limited to a case where an infinitely thick non‐absorbing medium is illuminated
diffusely, since this is the only situation where the reflectance is isotropic. It is also shown that the
reflectance from what is intuitively thought to be a perfect diffusor strongly depends on the
illumination conditions, meaning that a bulk scattering medium that reflects light diffusely
independently of the illumination conditions does not exist.
It is investigated how the anisotropy affects d/0° measurements. The DORT2002 model is
adapted to the d/0° instrument to allow for inverse calculations starting from d/0° measurement
data. This gives access to the objective parameters used in the DORT2002 model through an
instrument originally not designed for this purpose. It is shown that this method can explain
more than 50 % of the widely investigated anomalous parameter dependence of the Kubelka‐
Munk model.
The causes of anisotropic reflectance are investigated and it is shown, using analytical methods
and the Monte Carlo model Grace, that it depends on the relative contribution from near‐surface
bulk scattering. The reflectance in larger polar angles is higher from near‐surface bulk scattering
than it is from scattering deeper inside the medium. Near‐surface bulk scattering dominates in
strongly absorbing media since the remaining light is absorbed and in optically thin media since
the remaining light is transmitted. Obliquely incident illumination causes the light to scatter
closer to the surface, and this also causes the relative contribution from near‐surface bulk
scattering to increase.
Comment
This master's thesis was done by Magnus Neuman as the final part of a Master of Science
program in Engineering Physics at Umeå University. The work was done at M‐Real Technical
Center in Örnsköldsvik and at Digital Printing Center (DPC), Mid Sweden University in
Örnsköldsvik. Supervisors were Nils Pauler, Senior Research Scientist at M‐Real and Assistant
Professor at Mid Sweden University, and Per Edström, Lecturer and Researcher at Mid Sweden
University.
Sundsvall: Mittuniversitetet, FSCN , 2006. , p. 37