Combined daylight systems for lightpipe
applications
A. Rosemann and H. Kaase
Institute for Lighting Technology, Technical University Berlin, Sekr. E6, Einsteinufer 19, 10587
Germany
E-mail: [email protected]
Website: www.lichttechnik.tu-berlin.de
Abstract The European research project ARTHELIO has examined the possibility of guiding daylight
into deeper building interiors using hollow light guides. Within this project, a goniophotometer was
developed and built up to measure the luminous intensity distribution of lightpipes. In two
demonstration sites, the daylight utilisation by the hollow light guides combined with heliostats has
successfully been demonstrated. Daylight is collected by heliostats and enters the building. It is guided
along hollow light guides and coupled out along its way to illuminate the building interior.
Measurements show that the prototypes help to decrease the energy consumption for electric lighting
of the building. The advantages of lightpipes in a daylight system can also be shown in a hybrid system.
The maintenance costs of lightpipe applications are inexpensive so that the variable costs of their
operation are reduced. Current research activities at Technical University of Berlin deal with this
topic.
Keywords daylight; lightpipes
1. Introduction
Light can be guided along hollow light guides or lightpipes with little loss. This
paper describes the combined utilisation of daylight and artificial light by lightpipes.
By this, artificial light is only needed if the amount of daylight is not enough to illuminate the building interior. This helps saving energy by daylight utilisation.
Two installations of the ARTHELIO system are described. In an outlook another
way for the combined utilisation of daylight and artificial lights is given in the socalled hybrid system.
The research carried out in the ARTHELIO project was funded by the European
Commission. Project partners were Ricerca et Progetto, Italy, Semperlux AG,
Germany, CelsiusTech, Sweden and University of Gothenburg, Sweden.
The work on the hybrid system is supported by the German Federal Ministry of
Economy and Work.
2. Lightpipes
Lightpipes are hollow tubes with a highly reflective inner surface so that light can
travel along the tube with little loss. Usually, light is coupled into the system by a
beamer at one end of the lightpipe. An end mirror reflects the light that has not been
coupled out back into the lightpipe.
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Combined daylight systems for lightpipe applications
Figure 1.
11
Luminous Transmittance of the prismatic film.
One of the most effective materials for good light propagation in lightpipes is the
prismatic film [1]. It allows light hitting its surface from certain incidences to
undergo total internal reflection if the angle criterion regarding Snell’s law is met.
Otherwise the light will leave the lightpipe system and is used for illumination. An
additional extractor film reflects light diffusely so parts of it do not meet the angle
criterion any more and are coupled out.
The performance of the prismatic film is shown in Fig. 1. The light areas mark
the light incidences with high luminous transmittance whereas the darker regions
indicate the light incidences that lead to total internal reflection [2].
2.1 Goniophotometer for Lightpipes
For measuring the photometric properties of a lightpipe system a goniophotometer
has been built up at Technical University Berlin [3]. The lightpipe is divided into
several segments and data can be achieved on the luminous flux and the luminous
intensity distribution of each segment.
2.2 Photometric Characteristics of Lightpipes
Several lightpipe systems have been tested to measure their performance. It could
also be shown that the luminous data achieved can be used in common software
packages for light simulation and interior lighting design.
The measurement data on lightpipe systems help optimising these systems for
their applications.
3. Daylight utilisation with lightpipes
Daylight utilisation does not only save energy. Positive psychological and photobiological effects on humans are given by the view out and melatonine depression
[4]. Lightpipes cannot meet all these positive purposes so they are not a substitute
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A. Rosemann and H. Kaase
Figure 2.
Figure 3.
The Goniophotometer for Lightpipes.
A typical luminous intensity distribution of a lightpipe system.
for windows or other daylighting systems in vertical facades. They can be seen as
systems that will lead daylight into deeper building interiors that daylight usually
does not reach. The prototype installations described in the following sections
provide certain rooms with daylight that did not have access to daylight before [5].
The prototype installed in Carpiano, near Milano is more focusing on visual comfort,
while the one installed in Berlin is more focusing on energy savings.
Since the light entering a lightpipe needs to be quite direct to meet the angle criterion for further propagation, daylight utilisation with lightpipes only works with
direct sunlight. It needs to be fed into a lightpipe by a heliostat following the sun.
By the optical path of the coupling system, light of the heliostat must be concentrated to quite small cross sectional areas. Typically, lightpipes have a diameter of
0.3 m or less.
For a high luminous flux entering the system, the area of the first element of a
heliostat needs to be rather large. The maximum luminous flux that can be used
depends on the geographical location and the size of the heliostat’s surface.
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Combined daylight systems for lightpipe applications
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Artificial light needs to be added when needed (i.e. when direct daylight is not
sufficient for good indoor illuminations regarding the requirements defined in standards). For this purpose, a mixing unit is part of the ARTHELIO system.
For energy considerations, the probability of direct sunshine has to be taken into
account. By computing the illuminance on the surface of the heliostat for different
sky conditions, the annual amount of daylight used for indoor illumination can be
determined.
3.1 Prototype in Carpiano, Italy
A heliostat unit is constructed so that it redirects the direct sunlight downwards into
a lightpipe. The light entering the system travels down into a daylight luminaire unit
that couples out the light diffusely, illuminating the surface below.
Two horizontal lightpipes each 20 m long are additionally fed by two sulphur
lamps. The sulphur lamps are powerful light sources with a maximum luminous flux
of 120 klm. They can be dimmed down to 20% of their maximum electrical power
consumption. The luminous flux drops quite linearly with the consumed electrical
power. By this, the illuminance level can be controlled depending on the amount of
daylight entering the room.
The room itself is a store with a long transportation band. The prototype installation illuminates this band and light is only switched on when the store is used. By
the high luminous efficacy of the lamps and the daylighting control, the amount of
electrical energy used to lighting could be lowered.
Figure 4.
Heliostat of the Carpiano Prototype.
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A. Rosemann and H. Kaase
Figure 5.
Transportation Band Illuminated by the ARTHELIO System.
Figure 6.
Heliostat of the Berlin Prototype.
3.2 Prototype in Berlin, Germany
A windowless staircase in the Semperlux AG headquarters has been illuminated by
artificial light only. The prototype consists of a heliostat mounted on the buildings
roof (see Fig. 6). The moving mirror has a size of 6.2 m2 and reflects the direct sunlight to a set of fresnel lenses that concentrate the light. Reflector systems close to
the focal points of the fresnel lenses re-direct the light into a mixing unit. A schematic
diagram shows the main components of the system (Fig. 7).
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Combined daylight systems for lightpipe applications
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Heliostat
Sulphur Lamp
Mixing Unit
Lightpipe
Figure 7.
Schematic Diagram of the Prototype Installation.
Source 1
Source 2
Figure 8.
Sketch of the Mixing Unit.
The mixing unit is a box with a highly reflective film in the inner surface to reduce
the loss. Half of the optical cross sectional area is covered by a mirror with an orientation of 45° to the vertical surface. Half of the light entering the box from two
orthogonal directions is re-directed and the other half goes through (see Fig. 8).
The artificial light is provided by a sulphur lamp which is also attached to this
mixing unit. Two lightpipes going down all the way to the ground floor (3.5 stories,
approximately 12.5 m in height) are fed by the mixing unit.
The size of the heliostat mirror has been chosen so that on an average sunny day
a luminous flux of 120 klm is guided into the lightpipe system. This is approximately
the luminous flux of a sulphur lamp.
The lightpipes are to spread out light in all directions so the extractor cannot be
put on the inner surface of the tube. Instead of an extractor foil extractor bodies are
put in the centre of the lightpipes.
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A. Rosemann and H. Kaase
Figure 9.
Staircase Illuminated by the ARTHELIO System.
3.3 Results
Carpiano
The horizontal illuminance level in the height (0.7 m) of the working plane (conveyor belt) is measured by 12 illuminance sensors. The modelling of faces and
objects is influenced very much by the vertical illuminance which is measured by 3
vertical illuminance sensors in the height of 1.6 m. The glare of the daylight output
areas of the diffuser module is controlled by 2 luminance sensors. The dim level of
the sulphur lamps is measured by voltage sensors. This is used to evaluate the energy
savings and presence of workers. The outdoor illuminance level is important for the
calculation of the caught daylight luminous flux by the heliostat and gives additional
information about the weather conditions. A special outdoor illuminance sensor is
placed horizontally on the roof near the heliostat. So the system efficiency could be
estimated.
At night, the artificial light extracted from the diffuser provides more light in the
areas which are besides the diffuser while the illuminance level is reduced but still
enough for security purposes (140 lx).
The overall efficiency of the system was 55%, i.e. 55% of the collected luminous
flux is extracted from the diffuser 20 m further down in the building.
Berlin
The horizontal illuminance level of the staircase is measured with two arrangement
series in the height of 0.2 m by 12 illuminance sensors in total. The modelling of
faces and objects is influenced very much by the vertical illuminance which is measured by 3 vertical illuminance sensors in the height of 1.2 m. The dim level of the
sulphur lamp is measured by a voltage sensor. This is used to evaluate the energy
saving. The actual weather conditions and luminous flux of the daylight captured by
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Combined daylight systems for lightpipe applications
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200
Mean Illuminance in lx
180
160
140
120
100
80
60
40
20
0
0.0
2.0
4.0
6.0
8.0
10.0
12.0
Udimm in V
Figure 10.
Mean Horizontal Illuminance in the Staircase for Different Sky Conditions.
the heliostat are evaluated by the measurement data of a luminance sensor directed
to the centre of the primary mirror.
It is necessary to check, whether the minimum requirements regarding the German
standard for Interior Lighting are met by the ARTHELIO-system in Berlin. The standard demands an average illuminance of 100 lx for staircases.
In Figure 10, the measurement data for a whole day is shown. As it can be easily
seen, the sunlight was not sufficient to illuminate the whole staircase during the
whole day (some data points at Udimm = 10 V. This occurs in the morning and the
evening. However, for most of the time the direct sunlight was used without the
addition of the sulphur lamp (many data points at Udimm = 0 V). For all cases, the
mean illuminance is greater than 100 lx. When there is no need for additional artificial lighting, the illuminances rise even higher (up to 190 lx) – in that case, there
is more direct sunlight than needed. The criteria defined in the German standard are
met.
For a sunny day, the luminance L and the dimming voltage Udimm is shown in
Figure 11. The luminance sensor is placed in the centre of the 4 fresnel lenses and
is directed to the centre of the primary mirror. Thus, the sensor measures the mean
luminance of a reflected small sky part containing the sun. The measured values give
the information about the actual sky condition and are nearly proportional to the
luminous flux coupled into the building when the sun is shining.
Note that the sulphur lamp was switched on in the morning of the sunny day
because necessary maintenance work on the bus system was carried out.
The peaks show disturbances by small clouds passing by. The control system had
been started at roughly 10 am. The amount of daylight spread into the system was
already sufficient to illuminate the staircase without artificial light at that time. As
soon as the control system started working, the sulphur lamp switched off. Around
3:30 pm one can sense an oscillating behaviour of the system. The sulphur lamp is
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A. Rosemann and H. Kaase
1.4E+07
10.0
1.2E+07
8.0E+06
6.0
6.0E+06
4.0
Udimm in V
Luminance in cd/m2
8.0
1.0E+07
4.0E+06
L
Udimm
2.0
2.0E+06
0.0E+00
08:00:32
0.0
09:12:32
10:24:32
11:36:32
12:48:32
14:00:32
15:12:32
16:24:32
True Local Time
Figure 11.
Luminance of the Heliostat Mirror and Dimming Voltage of the Sulphur Lamp
on a Sunny Day.
1.2E+07
10.0
1.0E+07
8.0E+06
6.0
6.0E+06
L_mixed
Udimm_mixed
4.0
Udimm in V
Luminance in cd/m2
8.0
4.0E+06
2.0
2.0E+06
0.0E+00
08:00:32
0.0
09:12:32
10:24:32
11:36:32
12:48:32
14:00:32
True Local Time
Figure 12.
Luminance of the Heliostat Mirror and Dimming Voltage of the Sulphur Lamp
on a Partly Cloudy Day.
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Combined daylight systems for lightpipe applications
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1.4E+06
10.0
8.0
1.0E+06
L_overcast
Udimm_overcast
8.0E+05
6.0
6.0E+05
4.0
Udimm in V
Luminance in cd/m2
1.2E+06
4.0E+05
2.0
2.0E+05
0.0E+00
08:00:32
0.0
09:12:32
10:24:32
11:36:32
12:48:32
14:00:32
True Local Time
Figure 13.
Luminance of the Heliostat Mirror and Dimming Voltage of the Sulphur Lamp
on an Overcast Day.
switched on and immediately switched off again since the control sensor indicates
that there is no need for artificial light. As soon as the lamp is switched off, the
sensor indicates the opposite. This behaviour occurs at a certain outside luminance.
As soon as the luminance drops below a certain level, the oscillation stops and the
lamp is dimmed up while the sun is settling down.
On a partly cloudy day there is unsteady behaviour of the control system. Figure
12 shows the behaviour of the control system for various, quickly changing situations. As soon, as the outside luminance rises, the dimming level drops. The time
delays in the control are very small. The system adapts to the given situation quickly
and correctly.
Since only direct sunlight can be used by the heliostat, the diffuse sky does not
provide enough light for the system to dim the sulphur lamp below 100%. However,
the sky clears up every now and again, but this does not affect the dimming level.
The small and necessary delay in the control path does not affect the system because
the sky becomes overcast again fairly quickly.
4. Hybrid system
The work in the field of hollow light guides continues and a hybrid system for both,
daylight and artificial light is now being developed. The advantages of daylight utilisation in building interiors can be used in assembly halls. The artificial light is provided by lightpipes when there is not enough natural light available. These systems
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A. Rosemann and H. Kaase
Daylight
Projector
Module
Sensor
Module
Comparison
Edemand/Eactual
Daylight
Module
Measurement
Edirect
Lightpipe Module
Artificial Light / Daylight Illumination System
Figure 14.
Overview of the Hybrid System.
allow the hall to be illuminated and their maintenance costs are low, since only a
few lamps are installed [6]. For changing the lamps, the production does not need
to be stopped – the projectors are only at one end of the lightpipes. An intelligent
control system will be switching the lightpipes depending on the outside situation
(Figure 14).
The low cost maintenance and the fact that the production does not need to be
stopped during lamp changes is an economic factor that can be considered for a
reduction of variable costs.
5. Conclusions
Lightpipes can guide daylight into deeper building interiors that were previously not
provided with daylight. They are not to replace existing windows with daylight
systems but they can be seen as an addition to them.
The ARTHELIO system fits into different building typologies and can therefore
be installed in existing buildings. The dimensions of the system components depend
on the mean luminous flux that should enter the building, the geographical location
and the meteorological situation.
Electrical energy for electrical lighting can be saved whenever direct sunlight is
available. Places with a high probability of sunshine have a higher system potential.
With the low maintenance costs, another advantage of lightpipes has been pointed
out. In combination with daylight systems, this combined illumination system does
not only allow the saving of electrical energy but can also reduce the variable costs
for a building. The work inside does not necessarily need to be stopped during lamp
changes.
The goniophotometer allows the photometric characterisation of lightpipes. The
measurement data can be used in simulation tools so that the visual environment
with lightpipe luminaires can be computed.
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Combined daylight systems for lightpipe applications
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References
[1] Whitehead, L. A., ‘Overview of Hollow Light Guide Technology and Applications’, International
Daylighting Conference, 1998.
[2] Rosemann, A., Hohllichtleiterbeleuchtungsanlagen für Tageslichtnutzung, PhD-thesis, TU Berlin,
2001.
[3] Kloss, S.-H., Ein Goniophotometer zur Messung des Lichtstromes und der Lichtstärkeverteilung von
hohlen Lichtleitern, PhD-thesis, TU Berlin, 2001.
[4] Piazena, H., ‘Circadiane Wirksamkeit der Solarstrahlung’, 2004, TAGESLICHT, 1/04, 41–45.
[5] Müller, T., Kloss, S.-H., Rosemann, A., Kaase, H., Mingozzi, A., Ehjed, J., ‘ARTHELIO-Zwei
Hohllichtleiterbeleuchtungssysteme mit kombinierter Einspeisung von Tageslicht und Kunstlicht der
Schwefellampe’, Proceedings of LICHT 2000, 2001, Goslar, Germany.
[6] CIE TC 3-30, Hollow Light Guide Applications, CIE TC 3-30, 1995.
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