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Angle resolved color of bulk scattering media
Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.ORCID iD: 0000-0002-0529-1009
2011 (English)In: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 50, no 36, 6555-6563 p.Article in journal (Refereed) Published
Abstract [en]

The angle resolved reflectance factor of matte samples is measured with a goniophotometer and simulated using radiative transfer theory. Both measurements and simulations display the same characteristic dependence of the reflectance factor on the observation angle. The angle resolved reflectance spectra are translated to CIELAB color coordinates and the angular color differences are found to be surprisingly large. A chromatic adaptation that is dependent on the observation angle is suggested, in which a nonabsorbing opaque medium is used as the reference white, and the angular color differences are then reduced. Furthermore, the use of an undyed paper as the reference white is evaluated. The angular lightness differences are then reduced further, but the angular differences in chroma are still large. It is suggested that smaller variations in perceived color could be explained by angle dependent chromatic adaptation and a limited sensitivity of the human visual system to changes in chroma.

Place, publisher, year, edition, pages
2011. Vol. 50, no 36, 6555-6563 p.
Keyword [en]
Angle resolved color; bulk scattering media
National Category
Other Physics Topics
Identifiers
URN: urn:nbn:se:miun:diva-15157DOI: 10.1364/AO.50.006555ISI: 000299166700002Scopus ID: 2-s2.0-84255190004OAI: oai:DiVA.org:miun-15157DiVA: diva2:463866
Projects
PaperOptOptics of paper and print: continuous models
Available from: 2011-12-12 Created: 2011-12-12 Last updated: 2016-04-19Bibliographically approved
In thesis
1. Whiteness and Fluorescence in Layered Paper and Board: Perception and Optical Modelling
Open this publication in new window or tab >>Whiteness and Fluorescence in Layered Paper and Board: Perception and Optical Modelling
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis is about modelling and predicting the perceived whiteness of plain paper from the paper composition, including fluorescent whitening agents. This involves psychophysical modelling of perceived whiteness from measurable light reflectance properties, and physical modelling of light scattering and fluorescence from the paper composition.

Existing models are first tested and improvements are suggested and evaluated. A colour appearance model including simultaneous contrast effects (CIECAM02-m2), earlier tested on coloured surfaces, is successfully applied to perceived whiteness. An extension of the Kubelka-Munk light scattering model including fluorescence for turbid media of finite thickness is successfully tested for the first time on real papers. It is extended to layered constructions with different layer optical properties and modified to enable parameter estimation with conventional d/0° spectrophotometers used in the paper industry. Lateral light scattering is studied to enable simulating the spatially resolved radiance factor from layered constructions, and angle-resolved radiance factor simulations are performed to study angular variation of whiteness.

It is shown that the linear CIE whiteness equation fails to predict the perceived whiteness of highly white papers with distinct bluish tint. This equation is applicable only in a defined region of the colour space, a condition that is shown to be not fulfilled by many commercial office papers, although they appear white to most observers. The proposed non-linear whiteness equations give to these papers a whiteness value that correlates with their perceived whiteness, while application of the CIE whiteness equation outside its region of validity overestimates perceived whiteness.

It is shown that the fluorescence efficiency of FWA is essentially dependent only on the ability of the FWA to absorb light in its absorption band. Increased FWA concentration leads accordingly to increased whiteness. However, since FWA absorbs light in the violet-blue region of the electromagnetic spectrum, the reflectance factor decreases in that region with increasing FWA amount. This violet-blue absorption tends to give a greener shade to the paper and explains most of the observed greening and whiteness saturation at larger FWA concentrations. A red-ward shift of the quantum efficiency is observed with increasing FWA concentration, but this is shown to have a negligible effect on the whiteness value. The results are directly applicable to industrial applications for better instrumental measurement of whiteness and thereby optimising the use of FWA with the goal to improve the perceived whiteness.

Place, publisher, year, edition, pages
Sundsvall: annat förlag, 2012. 158 p.
Series
Mid Sweden University doctoral thesis, ISSN 1652-893X ; 138
Keyword
Whiteness, Perception, Colour Appearance Modelling, Paper Optics, Light Scattering, Fluorescence, Lateral Light Scattering, White-Top Mottle, Kubelka-Munk, Radiative Transfer
National Category
Paper, Pulp and Fiber Technology Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:miun:diva-17782 (URN)978-91-87103-50-6 (ISBN)
Public defence
2013-01-23, Sal 0111, Mittuniversitetet, Holmgatan 10, Sundsvall, 10:31 (English)
Opponent
Supervisors
Projects
PaperOpt
Funder
The Knowledge FoundationVinnova
Available from: 2012-12-20 Created: 2012-12-17 Last updated: 2012-12-20Bibliographically approved
2. Applied problems and computational methods in radiative transfer
Open this publication in new window or tab >>Applied problems and computational methods in radiative transfer
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Light scattering in turbid media is essential for such diverse applications as paperand print, computer rendering, optical tomography, astrophysics and remote sensing.This thesis investigates angular variations of light reflected from plane-parallelturbid media using both mathematical models and reflectance measurements, dealswith several applications and proposes novel computational methods for solving thegoverning equations.Angular variations of light reflected from plane-parallel turbid media is studiedusing both mathematical models and reflectance measurements. It is found that thelight is reflected anisotropically from all media encountered in practice, and that theangular variations depend on the medium absorption and transmittance and on theangular distribution of the incident light. If near-surface bulk scattering dominates,as in strongly absorbing or highly transmitting media or obliquely illuminated media,relatively more light is reflected in large polar (grazing) angles. These results areconfirmed by measurements using a set of paper samples. The only situation withisotropic reflectance is when a non-transmitting, non-absorbing medium is illuminateddiffusely, and it is shown that this is the only situation where the widely usedKubelka-Munk model is exactly valid.A number of applied problems is studied, including reflectance measurements,angle resolved color and point spreading. It is seen that differences in instrumentdetection and illumination geometry can result in measurement differences. The differencesare small and if other sources of error — such as fluorescence and gloss— are not eliminated, the differences related to instrument geometry become difficultto discern. Furthermore, the dependence of point spreading in turbid mediaon the medium parameters is studied. The asymmetry factor is varied while maintainingconstant the optical response in a standardized measurement geometry. It isseen that the point spreading increases as forward scattering becomes more dominant,and that the effect is larger if the medium is low-absorbing with large meanfree path. It is argued that the directional inhomogeneity of the scattering mediummust be captured by the model to reproduce experimental results. Finally, the angleresolved color of a set of paper samples is assessed both theoretically and experimentally.The chroma decreases and the lightness increases as the observation polarangle increases. The observed differences are clearly large, and a modification ofthe L∗a∗b∗ formalism including angle dependent chromatic adaptation is suggestedhere to handle this situation.

The long standing issue of parameter dependence in the Kubelka-Munk modelis partially explained by recognizing that light reflected from paper samples in standardizedmeasurements has angular variations, and by using the appropriatemodelin the calculation of the scattering and absorption coefficients.The radiative transfer (RT) equation is solved with a recently proposed particlemethod (DFPM), both in standard cases and in cases previously considered intractable.Fluorescence is added to the RT equation, thus including wavelength asan additional dimension, and this equation is solved using DFPM. The discrete RTequation can be written as a system of linear equations, and a comprehensive analysisof the convergence properties of DFPM when solving this type of problems ispresented.

Place, publisher, year, edition, pages
Härnösand: Mittuniversitetet, 2013. 25 p.
Series
Mid Sweden University doctoral thesis, ISSN 1652-893X ; 151
National Category
Other Physics Topics
Identifiers
urn:nbn:se:miun:diva-19776 (URN)978-91-87103-80-3 (ISBN)
Public defence
2013-09-20, Alfhild Agrell-salen, Universitetsbacken 3, Härnösand, 10:00 (Swedish)
Opponent
Supervisors
Funder
Vinnova
Available from: 2013-09-16 Created: 2013-08-26 Last updated: 2013-09-16Bibliographically approved

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Neuman, MagnusG. Coppel, LudovicEdström, Per
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