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Measuring and modelling light scattering in paper
Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [sv]

Avhandlingen behandlar de teoretiska och praktiska aspekterna av att använda spektrala vinkelupplösta reflektansmätningar för optisk karakterisering av fiberbaserade material såsom papper och kartong. En spektral goniofotometer används för att mäta det reflekterade ljusets vinkelfördelning. En stor del av arbetet utgörs av att utvärdera instrumentets noggrannhet, samt utreda hur de vinkelupplösta mätningarna skall utföras på bästa sätt för att erhålla en så fullständig karakterisering som möjligt. Det reflekterade ljuset består av tre komponenter; ytreflektans, bulkreflektans samt fluorescens. En fullständig karakterisering förutsätter att dessa tre komponenter kan analyseras separat, vilket i detta arbete görs genom nyutvecklade metoder.

En metod har utvecklats för separation av ytreflektans och bulkreflektans. Metoden bygger på att analysera hur den totala reflektansen förändras vid ökande absorption i det reflekterande materialet. Absorptionen kontrolleras genom inkjet-tryckning där tryckfärg appliceras på substratet i sådan mängd att bulkreflektansen helt släcks ut. Genom att kombinera mätningar på tryckt och otryckt substrat kan de båda komponenterna separeras. Trots att ytreflektansen från ett matt papper är liten i förhållande till bulkreflektansen, så visar resultaten att den ökar markant med ökande betraktningsvinkel och kan därmed ha stor inverkan på den totala reflektansen. Bidraget från fluorescens kan kvantitativt analyseras genom att kombinera mätningar utförda med respektive utan UV-filter. Vinkelupplösta mätningar och Monte Carlo-simuleringar av fluorescensens vinkelfördelning visar att dess anisotropi är relaterad till det medeldjup vid vilket fluorescensen emitteras. Resultaten förklarar observerade skillnader och motstridigheter i tidigare rapporterade studier kring huruvida fluorescens kan anses vara Lamberskt fördelad.

Samtliga goniofotometriska mätningar är utförda med ett kompakt, kommersiellt tillgängligt, dubbelstråleinstrument. För att undersöka instrumentets lämplighet för absoluta reflektansmätningar utförs en analys av dess mätnoggrannhet. Resultaten visar att instrumentets kompakta storlek i kombination med den anisotropa reflektansen från papper introducerar systematiska fel av samma storleksordning som den totala mätnoggrannheten. Dessa fel uppstår på grund av den relativt stora detektorapertur som måste användas vid mätningar av diffus reflektans, vilket är karakteristiskt för papper och kartong. Resultaten visar även att felen är störst vid flacka mätvinklar och för prover med hög grad av anisotropisk reflektans, och en geometrisk korrektionsmetod för denna typ av systematiska fel föreslås.

Spektrala och vinkelupplösta mätningar medför per automatik stora mängder mätdata. Genom att använda strålningstransportteori som en matematisk modell för hur ljus sprids i papper kan mätdatat reduceras till en uppsättning beskrivande materialparameterar. Att uppskatta dessa optiska parametrar utifrån vinkelupplösta reflektansmätningar är i sig ett komplicerat problem, vilket dessutom är känsligt för mätfel och val av mätvinklar. Detta inversa problem analyseras i detalj, och speciellt hur valet av mätvinklar kan reduceras utan att försämra förutsättningarna för estimeringen. Simuleringar visar att mätningarna kan begränsas till infallsplanet, eller till och med enbart framåtriktningen, så länge tillräckligt flacka mätvinklar är inkluderade i mätsekvensen.

Abstract [en]

This thesis is about measuring and modelling light reflected from paper by using goniophotometric measurements. Measuring bidirectional reflectance requires highly accurate instruments, and a large part of the work in this thesis is about establishing the requirements that must be fulfilled to ensure valid data. A spectral goniophotometer is used for measuring the light reflected from paper and methods are developed for analyzing the different components, i.e. the fluorescence, surface reflectance and bulk reflectance, separately. A separation of the surface and bulk reflectance is obtained by inkjet printing and analyzing the total reflectance in the absorption band of the ink. The main principle of the method is to add dye to the paper until the bulk scattered light is completely absorbed. The remaining reflectance is solely surface reflectance, which is subtracted from the total reflectance of the undyed sample to give the bulk reflectance. The results show that although the surface reflectance of a matte paper is small in comparison with the bulk reflectance, it grows rapidly with increasing viewing angle, and can have a large influence on the overall reflectance.

A method for quantitative fluorescence measurements is developed, and used for analyzing the angular distribution of the fluoresced light. The long-standing issue whether fluorescence from turbid (or amorphous) media is Lambertian or not, is resolved by using both angle-resolved luminescence measurements and radiative transfer based Monte Carlo simulations. It is concluded that the degree of anisotropy of the fluoresced light is related to the average depth of emission, which in turn depends on factors such as concentration of fluorophores, angle of incident light and the absorption coefficient at the excitation wavelength.

All measurements are conducted with a commercially available benchtop sized double-beam spectral goniophotometer designed for laboratory use. To obtain reliable results, its absolute measurement capability is evaluated in terms of measurement accuracy. The results show that the compact size of the instrument, combined with the anisotropic nature of reflectance from paper, can introduce significant systematic errors of the same order as the overall measurement uncertainty. The errors are related to the relatively large detection solid angle that is required when measuring diffusely reflecting materials. Situations where the errors are most severe, oblique viewing angles and samples with high degree of anisotropic scattering, are identified, and a geometrical correction is developed.

Estimating optical properties of a material from bidirectional measurements has proved to be a challenging problem and the outcome is highly dependent on both the quality and quantity of the measurements. This problem is analyzed in detail for optically thick turbid media, and the study targets the case when a restricted set of detection angles are available. This is the case when e.g. an unobstructed view of the sample is not possible. Simulations show that the measurements can be restricted to the plane of incidence (in-plane), and even the forward direction only, without any significant reduction in the precision or stability of the estimation, as long as sufficiently oblique angles are included.

Place, publisher, year, edition, pages
Örnsköldsvik: Mid Sweden University , 2015. , p. 42
Series
Mid Sweden University doctoral thesis, ISSN 1652-893X ; 224
Keyword [en]
goniophotometry, anisotropy, angle resolved light scattering, reflectance measurements, turbid media
National Category
Other Physics Topics
Identifiers
URN: urn:nbn:se:miun:diva-24986ISBN: 978-91-88025-27-2 (print)OAI: oai:DiVA.org:miun-24986DiVA: diva2:814222
Public defence
2015-06-04, Digital Printing Center, Mediacenter-salen, Järnvägsgatan 3, Örnsköldsvik, 10:00 (Swedish)
Opponent
Supervisors
Available from: 2015-05-26 Created: 2015-05-26 Last updated: 2015-05-26Bibliographically approved
List of papers
1. Influence of finite-sized detection solid angle on bidirectional reflectance distribution function measurements
Open this publication in new window or tab >>Influence of finite-sized detection solid angle on bidirectional reflectance distribution function measurements
2014 (English)In: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 53, no 6, p. 1212-1220Article in journal (Refereed) Published
Abstract [en]

This paper deals with limitations and often overlooked sources of error introduced in compact double-beam goniophotometers. It is shown that relative errors in measured radiance factor, comparable to the total measurement uncertainty, can be introduced if recommended corrections are not carried out. Two different error sources are investigated, both related to the size of the detection solid angle. The first is a geometrical error that occurs when the size of the illuminated area and detector aperture are comparable to the distance between them. The second is a convolution error due to variations in radiant flux over the detector aperture, which is quantified by simulating the full 3D bidirectional reflectance distribution function (BRDF) of a set of samples with different degrees of anisotropic reflectance. The evaluation is performed for a compact double-beam goniophotometer using different detection solid angles, and it is shown that both error sources introduce relative errors of 1%–3%, depending on viewing angle and optical properties of the sample. Commercially available compact goniophotometers, capable of absolute measurements, are becoming more and more common, and the findings in this paper are therefore important for anyone using or planning to use this type of instrument.

Place, publisher, year, edition, pages
Optical Society of America, 2014
Keyword
Reflection, Scattering measurements, BRDF, turbid media
National Category
Natural Sciences
Identifiers
urn:nbn:se:miun:diva-20949 (URN)10.1364/AO.53.001212 (DOI)000331949600027 ()2-s2.0-84942366011 (Scopus ID)
Available from: 2014-01-03 Created: 2014-01-03 Last updated: 2017-12-06Bibliographically approved
2. Separation of surface and bulk reflectance by absorption of bulk scattered light
Open this publication in new window or tab >>Separation of surface and bulk reflectance by absorption of bulk scattered light
2013 (English)In: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 52, no 19, p. 4749-4754Article in journal (Refereed) Published
Abstract [en]

A method is proposed for separating light reflected from turbid media with a rough surface into a bulkand a surface component. Dye is added to the sample, thereby increasing absorption and canceling bulkscattering. The remaining reflected light is surface reflectance, which can be subtracted from the totalreflectance of an undyed sample to obtain the bulk component. The method is applied to paper wherethe addition of dye is accomplished by inkjet printing. The results show that the bulk scattered light isefficiently canceled, and that both the spectrally neutral surface reflectance and the surface topographyof the undyed paper is maintained. The proposed method is particularly suitable for characterization ofdielectric, highly randomized materials with significant bulk reflectance and rough surfaces, which aredifficult to analyze with existing methods. A reliable separation method opens up for new ways of analyzing,e.g., biological tissues and optical coatings, and is also a valuable tool in the development of morecomprehensive reflectance models.

Place, publisher, year, edition, pages
Optical Society of America, 2013
Keyword
Reflection, Scattering measurements, Turbid media
National Category
Other Physics Topics
Identifiers
urn:nbn:se:miun:diva-19640 (URN)10.1364/AO.52.004749 (DOI)000321289700043 ()2-s2.0-84879948231 (Scopus ID)
Available from: 2013-09-05 Created: 2013-07-26 Last updated: 2017-12-06Bibliographically approved
3. Angular variations of reflectance and fluorescence from paper - the influence of fluorescent whitening agents and fillers
Open this publication in new window or tab >>Angular variations of reflectance and fluorescence from paper - the influence of fluorescent whitening agents and fillers
2012 (English)In: Final Program and Proceedings - IS and T/SID Color Imaging Conference, Springfield, USA: The Society for Imaging Science and Technology, 2012, p. 236-241Conference paper, Poster (with or without abstract) (Refereed)
Abstract [en]

It has earlier been shown that light reflected from the bodyof paper exhibit anisotropic behavior. On the other hand, fluores-cence emission is often assumed to be distributed in a Lambertianmanner. The angular behavior of light reflected and fluorescedfrom paper is examined using measurements from a spectral go-niophotometer. The angular dependency of the radiance factorswas measured for a range of excitation wavelengths. Moreover,the influence of fillers and fluorescent whitening agents (FWA)on the anisotropy was studied. The measurements show that theanisotropy of the total radiance factor of paper decreases whenan increasing amount of FWA is added to the paper. The sameeffect was also observed when an increased amount of filler wasadded to the paper. In addition, it was shown that the presenceof fillers reduce the effect of the FWA. The results show that incomparison to the anisotropy of the total radiance factor from thepaper samples, the anisotropy of the fluorescence alone is negligi-ble. Hence, for paper samples containing FWA evenly distributedin the bulk, the fluorescence alone should not induce significantdifferences between color measuring instruments of different mea-surement geometries.

Place, publisher, year, edition, pages
Springfield, USA: The Society for Imaging Science and Technology, 2012
Keyword
reflectance, goniophotometer, fluorescence, anisotropy
National Category
Other Physics Topics
Identifiers
urn:nbn:se:miun:diva-18079 (URN)2-s2.0-84876737413 (Scopus ID)978-0-89208-303-9 (ISBN)
Conference
20th Color and Imaging Conference: Color Science and Engineering Systems, Technologies, and Applications, CIC 2012; Los Angeles, CA; United States; 12 November 2012 through 16 November 2012; Code 96727
Projects
Spektral Goniofotometer
Funder
The Knowledge Foundation
Available from: 2012-12-21 Created: 2012-12-20 Last updated: 2015-05-26Bibliographically approved
4. Angular dependence of fluorescence from turbid media
Open this publication in new window or tab >>Angular dependence of fluorescence from turbid media
2015 (English)In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 23, no 15, p. 19552-19564Article in journal (Refereed) Published
Abstract [en]

We perform Monte Carlo light scattering simulations to study the angular distribution of the fluorescence emission from turbid media and compare the results to measured angular distributions from fluorescing white paper samples. The angular distribution of fluorescence emission is significantly depending on the concentration of fluorophores. The simulations show also a dependence on the angle of incidence that is however not as evident in the measurements. A detailed analysis of the factors affecting this angular distribution indicates that it is strongly correlated to the mean depth of the fluorescence process. The findings can find applications in fluorescence spectroscopy and are of particular interest when optimizing the impact of fluorescence on e.g.the appearance of paper as the measured values are angle dependent.

National Category
Other Physics Topics
Identifiers
urn:nbn:se:miun:diva-24983 (URN)10.1364/OE.23.019552 (DOI)000361035300093 ()2-s2.0-84954492839 (Scopus ID)
Available from: 2015-05-26 Created: 2015-05-26 Last updated: 2017-12-04Bibliographically approved
5. The inverse radiative transfer problem - considerations for optically thick media
Open this publication in new window or tab >>The inverse radiative transfer problem - considerations for optically thick media
(English)Article in journal (Refereed) Submitted
National Category
Other Physics Topics
Identifiers
urn:nbn:se:miun:diva-24985 (URN)
Available from: 2015-05-26 Created: 2015-05-26 Last updated: 2016-12-09Bibliographically approved

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