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Spectrophotometric measurements and ray tracing simulations of mirror light pipes to evaluate the color of the transmitted light
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
Lawrence Berkeley National Laboratory, CA, USA.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
2014 (English)In: Solar Energy Materials and Solar Cells, ISSN 0927-0248, E-ISSN 1879-3398, Vol. 124, 172-179 p.Article in journal (Refereed) Published
Abstract [en]

Tubular daylighting systems are designed to guide light to the building's core using a highly reflective pipe. The intensity of the transmitted light is essential for the performance of the system. For the qualitative perception of the provided illumination, the color of the delivered light is also an important aspect. For highly reflective mirror light pipes, spectral variations are generally assumed not to affect the color of the transmitted light. Here, spectrophotometric measurements and ray tracing simulations of mirror light pipes are used to verify this commonly made assumption. The characterization methods employ spectral evaluations for both direct and diffuse incident light. The color properties are evaluated for mirror light pipes with a length to diameter aspect ratio of up to 16, using the CIE chromaticity diagram and CIELAB coordinates. For the xy chromaticity diagram, a larger color shift was noted for different illuminants than as a result of the optical properties of the reflective material. Using the CIELAB coordinates, a small color shift was noted for light incident at low solar altitudes. Overall, highly reflective films with spectral variations of a few percent do not markedly affect the color of the transmitted light.

Place, publisher, year, edition, pages
2014. Vol. 124, 172-179 p.
National Category
Energy Engineering
Research subject
Engineering Science with specialization in Solid State Physics
Identifiers
URN: urn:nbn:se:uu:diva-183305DOI: 10.1016/j.solmat.2014.01.049ISI: 000335111000024OAI: oai:DiVA.org:uu-183305DiVA: diva2:562360
Available from: 2012-10-24 Created: 2012-10-24 Last updated: 2017-12-07Bibliographically approved
In thesis
1. Daylighting Systems: Development of Techniques for Optical Characterization and Performance Evaluation
Open this publication in new window or tab >>Daylighting Systems: Development of Techniques for Optical Characterization and Performance Evaluation
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Successful integration of daylighting systems requires the ability to predict their performance for given climates. In this dissertation, a bottom-up approach is applied to evaluate the optical performance of a selection of daylighting systems. The evaluations are based on the optical properties of the included materials, and part of the dissertation focuses on developing new optical characterization methods.

The work on characterization techniques uses an integrating sphere method to characterize the transmittance of light scattering samples more accurately. The method's principle is to reduce the discrepancy in light distribution between the reference and the sample scans by using an entry port beam diffuser. For samples exhibiting distinct light scattering patterns, the benefits of improved uniformity outweigh the errors introduced by the diffusing material. The method is applicable to any integrating sphere instrument, and its simplicity makes it suitable for standard measurements.

In addition to normal-hemispherical properties, many daylighting applications require knowledge of the system's spatial light distribution. This dissertation presents a method combining experimental techniques and ray tracing simulations to assess the light distribution from a Venetian blind system. The method indicates that ray tracing based on simplified optical data is inadequate to predict the light distribution for slat materials exhibiting both specular and diffuse properties.

Ray tracing is a promising complement to experimental methods used to characterize light guiding or light redirecting systems. Here, spectrophotometric measurements of a scaled mirror light pipe validate a ray tracing model. The model shows excellent agreement with experimental results for both direct and diffuse incident light. The spectral evaluation shows no dramatic color changes for the transmitted light. The ray tracing model is used to evaluate four daylighting systems for a selection of Swedish locations. The percentage of occupied time when the studied systems achieve full design illuminance is relatively low, but the systems provide a valuable contribution to the required illuminance.

Additionally, this dissertation provides an overview of available energy efficient windows and illustrates the importance of including the solar energy transmittance when evaluating window energy performance.

Overall, this dissertation presents optical characterization techniques for improved performance evaluations of daylighting systems.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2012. 157 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 987
Keyword
Optical spectroscopy, integrating sphere, ray tracing, light scattering, window physics, Venetian blinds, core daylighting, optical characterization
National Category
Engineering and Technology
Research subject
Engineering Science
Identifiers
urn:nbn:se:uu:diva-183307 (URN)978-91-554-8512-2 (ISBN)
Public defence
2012-12-07, Polhemsalen, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2012-11-16 Created: 2012-10-24 Last updated: 2013-01-23Bibliographically approved

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Nilsson, Annica M.Roos, Arne

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