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daylight

Daylight in Buildings A visible factor contributing to comfort and energy demand

The effect of daylight on energy demand, individuel perception, visual comfort and personal well-being are as complex and variable as its impact on our perception of buildings is obvious. The Competence Center Building Envelopes and Civil Engineering conducts research in this field, aiming to develop solutions that improve energy efficiency and comfort in buildings alike.

Goniophotometer for measuring the spatial distribution of transmitted and reflected light (image: M. Vogel)
Spatial flow of daylight at Villa Müller (simulation radiance photon map)
Daylight redirection and absorption (simulation radiance photon map)

The building skin as a daylight filter

The term «Building Skin» underlines our understanding of a building envelope that does not isolate the building from its surroundings, but rather mediates between interior and exterior conditions. Applied to the context of daylight, this notion turns the building envelope into a filter, that admits natural lighting as far as possible, but mitigating negative effects, e.g. glare on the interior. 

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This filter modulates the incident daylight in the temporal domain, e.g. by shielding the interior from solar radiation on sunny days, and spatially, e.g. by redirection the transmitted light to balance the illumination of the interior. Established, but simplifying planning aids, such as Daylight Factor cannot account for the variable exterior conditions and therefore cannot provide insights into the eventual availability of daylight in buildings. Accordingly the Competence Centre applies climate-based metrics, that take for the local environmental conditions into account and allow for the prediction of absolute photometric quantities to estimate daylighting performance.
The Competence Centre conducts research in close collaboration with its international partners. Topics include the development, characterization, modelling and evaluation of daylighting techniques, that turn glazed facade components and roofs into daylight modulating building skin. With our know-how as well as our infrastructure, e.g. the gonio-photometer laboratory, we support manufacturers in the development and planners in the application of innovative daylighting.

Solar shading, glare control, and daylight redirection

The prevalence of highly insulated buildings, makes solar shading an increasingly indispensable feature of building envelopes, revealing conflicting effects of daylighting and the protection from overheating in the summer. All too often, solutions only take one aspect into account, for instance  by providing shade but performing poorly in terms of daylight supply and glare control, or by occluding the view to the outside.

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Our aim is to ensure solar shading and glare control at all times, without neglecting daylight supply or obstructing the view to the exterior. This can be achieved with well-tuned control of sun-shades, operating independently from glare protection and taking occupancy into account. Directionally selective and switchable shading devices are a valid alternative,in  that they adapt to both exterior conditions and interior requirements without the need for moving mechanical parts.
Conventional shade and glare control relies on reducing the building skin’s transmissivity. They either absorb excess irradiation, or reflect it back to the exterior. Daylight redirecting aims at the utilization of this excess irradiation by deflecting it toward building zones that receive insufficient illumination. It has the capacity to increase the depth of the naturally lit area, even out its otherwise unbalanced illumination, and in some cases, even act as a shading device.
The Competence Centre researches dedicated and complementary glare and solar control devices, as well as advanced solutions that aim to combine both funtionalities, while purposefully redirecting  the transmitted light.

The spatio-temporal description of light in buildings by light fields

The human eye captures an image of its environment from a given point of view in terms of the directionally resolved distribution of luminance. The light field extends this directionally resolved description light in the spatial domain by a variable location, or point of view. The Competence Center explores opportunities in research and application, that emerge from this complex but holistic description of daylight in buildings.

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 The light field is typically understood as a multidimensional but temporally static function of location, direction and wavelength. It comprises any possible image, that can be perceived by an observer within the extents of the light field representation. Moreover, it lends itself to the description and evaluation of the spatial distribution of light in architecture. The extension of the light field in the temporal domain moreover accounts for the continuously changing externior daylight conditions. Consequently, it lends itself to assess visual comfort and glare not only for few, selected locations and view directions, but to evaluate entire built-over spaces. The Center`s ongoing research with international partners is supported by the Swiss National Science Foundations and other public funding institutions.

Measurement and modelling of daylight

There are but few cases where daylight can be evaluated exclusively by means of on-site measurements. Daylight simulation allows reliable prognoses of daylight supply, visual comfort and glare already in the planning stage. It requires powerful and reliable algorithms as well as accurate descriptions of the optical properties of all building components, that might impact light transmission through the building skin.

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The representative assessment of daylight in buildings covers the spatial extent of the evaluated building zone as well as the temporal duration of its utilization. Given the required effort involved, this cannot usually be achieved with measurements and when planning new buildings, it is even impossible. Planning with daylight must therefore to a large extent rely on simulations.
The Competence Center uses Radiance, the de-facto standard in advanced daylight simulations. We continually maintain and extend its Photon Map extension, which is primarily used in modelling daylight redirection.
The precise characterization of optical properties of fenestration allows for the modelling of its impact on light propagation, both at product and building level. Methods developed e.g. with support of institutions such as Swiss National Science Foundation are used in the modelling of market available fenestration products, e.g. in using Radiance, as well as in product development by our partners.

projects

Adaptive Auflösung in der Erkennung von Blendungsquellen.

Spatial assessment of daylight in architecture

The Competence Center researchs the spatial propagation of daylight as an architectural element in collaboration with the Tokyo University of Sciences. A project on the evaluation of light fields, supported by the Swiss National Science Foundation, focusses on aspects such as glare and visual comfort.

Datenbasierte Modellierung der Tageslichtumlenkung durch eine prismatische Struktur.

Modelling of daylight redirecting systems

Daylight redirection is a persistent challenge in planning, that we address with improved simulation methods and models derived from measured data. The research project was conducted in close collaboration with our international partners and supported by the Swiss National Science Foundation.