F. VAJKAY et al.: ASSESSMENT OF TUBULAR LIGHT GUIDES WITH RESPECT TO BUILDING PHYSICS
409–412
ASSESSMENT OF TUBULAR LIGHT GUIDES WITH RESPECT TO
BUILDING PHYSICS
OCENA CEVASTIH VODNIKOV SVETLOBE GLEDE NA
GRADBENO FIZIKO
Franti{ek Vajkay, David Be~kovský, Vladimír Tichomirov
Brno University of Technology, Faculty of Civil Engineering, Institute of Building Structures, Veveøí 95, 602 00 Brno, Czech Republic
vajkay.f@fce.vutbr.cz
Prejem rokopisa – received: 2013-10-01; sprejem za objavo – accepted for publication: 2015-06-08
doi:10.17222/mit.2013.204
Architecture and the building industry consist of several minor fields, which come together when a building is designed and
erected. One of them is the field of building physics. It primarily focuses on the evaluation of constructions, structures and
spaces with respect to thermo-technical conditions, daylighting and many more. It is believed that among the above-mentioned
sub-fields of building physics, daylighting is the most important because it influences the health of every human being.
Daylighting enables people to see the colours and objects surrounding them. Therefore, buildings have to be equipped with a
kind of daylighting system. In the past, spaces located either in the centres of buildings or in the underground areas were
illuminated solely on the basis of luminaries. Nowadays, indirect daylighting systems like optical fibres or tubular light guides
may also be utilized.
In Central Europe, the application of tubular light guides in buildings increases every year. The manufacturers say that this is an
important and maintenance-free system. It is a system that brings light into any building, thus helping to save money. On the
other hand, the designing of tubular light guides is complicated as it has to deal with daylighting, thermo-technical aspects and
moisture. A huge amount of light guides is problematic due to the condensation of water inside the pipes, which is a side effect
of a non-air-tight solution, or just due to an incorrect thermal analysis. The paper focuses on different aspects of designing
light-guiding systems through computer simulations.
Keywords: tubular light guide, illuminance, luminance, building physics, computer simulations
Arhitektura in gradbeni{tvo sestojita iz ve~ glavnih podro~ij, ki se zdru`ijo, ko je objekt na~rtovan in zgrajen. Eno od takih
podro~ij je gradbena fizika. To je podro~je, ki je usmerjeno predvsem na oceno konstrukcije, zgradbe in prostora glede na
termo-tehni~ne pogoje, dnevno svetlobo in podobno. Med omenjenimi podpodro~ji gradbene fizike je najbolj pomembna
dnevna svetloba, ker vpliva na zdravje vsakega ~loveka. Dnevna svetloba omogo~a, da vidimo barve in predmete okrog sebe.
Zato morajo biti zgradbe opremljene s sistemom za dnevno svetlobo. V~asih so bili prostori, locirani v sredini zgradbe ali v
podzemlju, osvetljeni samo na osnovi svetlobnih teles. Danes pa lahko uporabljamo tudi posredne sisteme za dnevno svetlobo z
opti~nimi vlakni ali s cevastimi vodniki svetlobe.
V centralni Evropi uporaba cevastih vodnikov svetlobe v zgradbah iz leta v leto nara{~a. Proizvajalci trdijo, da je to pomemben
sistem, ki ne potrebuje vzdr`evanja. Sistem omogo~a svetlobo v zgradbi, kar pomaga pri zmanj{evanju stro{kov. Vendar pa je
na~rtovanje cevastih vodnikov svetlobe zapleteno, glede na to ali gre za dnevno svetlobo ali termo-tehni~no na~rtovanje in
vlago. Velik dele` vodnikov svetlobe ima te`ave zaradi kondenzirane vlage znotraj cevi, kar je stranski u~inek nevodotesne
izvedbe ali pa~ samo nepravilne termi~ne analize. ^lanek je usmerjen na razli~ne vidike ra~unalni{ke simulacije pri na~rtovanju
sistema za prevajanje svetlobe.
Klju~ne besede: cevast vodnik svetlobe, osvetljenost, svetilnost, gradbena fizika, ra~unalni{ka simulacija
1 INTRODUCTION
Lighting design of indoor spaces is a rarely discussed
discipline of the building industry. More often then not,
it is in the shadows of the fields related to the thermo-
technical processes organised in a building,1 although the
light makes it possible for humans to see their
surroundings, since the visible part of optical radiation
(i.e., the visible light) causes certain photochemical
reactions in the eye balls. The information perceived by
the eye balls is then processed in the brain.2 When more
light enters the eyes through the corneas we can
distinguish more details of the surroundings. This
property should be reflected on the daylighting design of
indoor spaces as well. Human beings spend about 80 %
of their lives indoors and it is necessary to think about
how natural light can enter a building. Usually architects
use one of the available direct daylighting systems, like:
• windows and their variants,
• roof lights.
However, as we need to make buildings more
compact because of land prices or we use the under-
ground areas, architects are required to use indirect day-
lighting systems as well.
Representatives of these are optic-fibre systems
(widely applied in the USA, but not in the EU) and
tubular light guides.
Tubular light guides are a type of indirect daylighting
system. Their history can be traced back to ancient
Egypt, when the predecessors of light guides were used
to illuminate sanctuaries.3 Only after their reinvention in
the 19th century and their further development that
Materiali in tehnologije / Materials and technology 50 (2016) 3, 409–412 409
UDK 628.981:628.9.021:004.94 ISSN 1580-2949
Professional article/Strokovni ~lanek MTAEC9, 50(3)409(2016)
started at the end of the 20th century, they become what
they are today.4 Each light guide consists of at least three
elements:
• the copula (the external cover made from a material
with high light transmission),
• the tube (the pipe with a special surface finish that
has a high light-reflectance value),
• the diffuser (the internal finish of light guides
distributing the light).
Tubular light guides can differ in every aspect. They
can have different lengths and diameters of the tube, and
also different copulas and diffusers. Howbeit, once the
elements ordered are assembled on a construction site
and later built into the building, some issues might arise
because of the unexpected physical properties of the
building. One of these may be associated with the day-
lighting of the building since light guides are difficult to
design. The other difficulty might be a result of the
thermal design, as water vapour often condenses inside
the pipe.
The paper is focused on the available computer tools,
which can be used to monitor the resulting properties of
the light guides within the selected test case.
2 EXPERIMENTAL SECTION
Indoor spaces in the centres of buildings or under-
ground areas can be illuminated with indirect daylighting
systems using natural light, or with luminaries. Tubular
light guides already proved to be efficient in this respect,
at least during daytime. The first half of this section is
focused on this particular topic.
The second half of the experimental section deals
with the thermo-technical simulations of the chosen
tubular-light-guide system, since it passes through the
envelope, especially through the roof.
The dimensions of the room assessed were 7 m ×
2.5 m and the room represented a corridor. The tubular
light guides were located along the longitudinal axis of
the space. The plan of the room including the light-guide
positions is shown in Figure 1.
2.1 Daylighting design
One of the most useful computer simulation tools is
HOLIGILM.5 It was developed to assess the amount of
the visible light transmitted by straight tubular light
guides. It is simple to use, fast and it provides a graphical
output. Nonetheless, it cannot evaluate the internally
reflected component of daylight in the same way as
Radiance or Velux Daylight Visualizer.
Although all 15 sky-type definitions described by
CIE6 are implemented in HOLIGILM, considering the
local codes of European countries including the Czech
Republic, the simulations were done only for the CIE
overcast-sky conditions.7
The properties of the tubular light guides input to
HOLIGILM were as follows:
• transparency of the copula: tc = 95 %,
• surface reflectance of the pipe (Spectralight Infinity
compound): rp = 99.7 %,
• transparency of the thermally insulating element
dividing the pipe into two segments: tte = 95 %,
• transparency of the diffuser (to be on the safe side of
the calculations): td = 75 %.
Seeing that HOLIGILM does not have any input
fields for additional light-transmitting parts like the
thermally insulating element, the total light transmittance
of the diffuser had to include that value as well; hence,
this value was set to 71 % instead of 75 %.
The total length l of the tubular light-guide pipe is
4.5 m and its diameter d is 0.530 mm.
Since the space is used as a corridor two distances
(l1) were set between the diffuser and the working plane:
• 2.15 m corresponding to the distance to the base
location of the working plane at 0.85 m above the
floor since in each room there may be some furniture;
• 3.0 m at the floor level so that the corridor is suitable
for movements.
2.2 Thermo-technical design
Thermo-technical calculations were carried out in
Agros2D. Agros2D is a free multi-platform alternative to
ANSYS Workbench and COMSOL Multiphysics.
The aims were:
• to find out whether water vapour can condense within
the pipe,
• to determine the point-wise thermal conductivity of
the structure.8
The thermal-conductivity coefficients of the structu-
ral elements were set as follows:
• acryl: la = 0.2 W m–2 K,
• the thermal insulation of mineral wool around the
pipe: lti1 = 0.040 W m–2 K,
• the thermal insulation of mineral wool as part of the
roof: lti2 = 0.040 W m–2 K,
• the pipe: lp = 0.220 W m–2 K,
• steel: ls = 50.0 W m–2 K,
• the lambda value of air depends on its volume.
F. VAJKAY et al.: ASSESSMENT OF TUBULAR LIGHT GUIDES WITH RESPECT TO BUILDING PHYSICS
410 Materiali in tehnologije / Materials and technology 50 (2016) 3, 409–412
Figure 1: Plan of the corridor including the positions of designed
tubular light guides
Slika 1: Na~rt hodnika, ki vklju~uje polo`aj na~rtovanih cevastih vod-
nikov svetlobe
3 RESULTS
3.1 Daylighting design
It was assumed that the luminous efficacy of a tubular
light guide of this length under the CIE Overcast Sky
conditions would prove to be insufficient to illuminate
the working plane at 0.85 m above the floor level. The
resulting illuminance levels were rather low. The values
varied from 0 lx to 100 lx (equivalent to 0 % and 1 % of
daylight factor, Figure 2). With an additional light guide,
it was possible to shift these values. For example, the
minimum value grew by 40 lx (Figure 3).
Seeing that the design only included the sky compo-
nent of the daylight factor and that the light-trans-
mittance value of the light guide was reduced by the
thermally insulating element, it can be concluded that the
values were extraordinarily high. The peaks beneath the
light guides, at a distance of 2.15 m, increased by 10 lx,
from 100 lx to 110 lx.
Since illuminance decreases with the increasing dist-
ance, when considering the results described previously,
the evaluation at the floor level was done under the
assumption that two tubular light guides should be used
to illuminate the designed space.
As anticipated, the resulting light levels dropped. The
peak values beneath the light guides receded to 58 lx
(Figure 4).
3.2 Thermo-technical simulations
The results presented on Figure 5 were obtained with
the simulations provided under common boundary con-
ditions. External and indoor air temperatures were set to
–15 °C and 20 °C and the corresponding relative air
humidity inputs were 84 % for the exterior and 50 % for
the interior.
As can be seen on Figure 5, the contact surface
temperature on the interface of the light guide and the
roof varies between 16 °C and 20 °C.
Other results can be connected to the overall thermal
conductivity of the light pipe, which is evaluated just like
in the case of windows.
The simulations pointed out that the U point-wise
thermal conductivity of the structure is 0.37 W K–1. This
value is smaller than the one required by the Czech
legislation; therefore, the light guide meets the necessary
conditions.
F. VAJKAY et al.: ASSESSMENT OF TUBULAR LIGHT GUIDES WITH RESPECT TO BUILDING PHYSICS
Materiali in tehnologije / Materials and technology 50 (2016) 3, 409–412 411
Figure 4: Illuminance-contour plot at 3.0 m beneath the diffuser for
two tubular light guides
Slika 4: Diagram obrisa osetljenosti pri 3 m pod difuzorjem pri dveh
cevastih vodnikih svetlobe
Figure 5: Temperature profile of the light guide
Slika 5: Temperaturni profil vodnika svetlobe
Figure 3: Illuminance-contour plot at 2.15 m beneath the diffuser for
two tubular light guides
Slika 3: Diagram obrisa osvetljenosti pri 2,15 m pod difuzorjem pri
dveh cevastih vodnikih svetlobe
Figure 2: Illuminance-contour plot at 2.15 m beneath the diffuser for
one tubular light guide
Slika 2: Diagram obrisa osvetljenosti pri 2,15 m pod difuzorjem pri
enem cevastem vodniku svetlobe
4 DISCUSSION
Tubular light guides are a type of daylighting system,
used to illuminate a room or a bigger space inside a
building or construction, in which natural light is hardly
available. However, an emphasis should be put onto the
ways a light guide can be designed with respect to
daylighting, thermo-technical requirements and other
factors.
The above results for daylighting were determined
solely with HOLIGILM. The software can draw lumi-
nance maps for the diffuser and determine the total
luminous flux beneath the optical interface. It is useful
for simulations when a tubular light guide is regarded as
a luminary, practically artificial light source. For the
artificial-lighting design, luminaries are represented as
IES data files. Light guides can also be designed in this
way instead of using the approach based on a natural-
light source. This approach is useful because HOLI-
GILM cannot determine the internally reflected compo-
nent of the daylight factor, but only the sky component.
Another disadvantage of HOLIGILM is that the room
used can only have a rectangular shape. On the contrary,
when the internally reflected component of the daylight
factor is neglected, the results may be obtained for rooms
of any shape. However, simulations for different sky
types created by the software tools using one of the
global illumination models can be important in future
since the use of photorealistic rendering slowly but
steadily rises year by year.
As for the thermo-technical design of light guides, it
can be said that they lack one crucial parameter. That is
the motion of the air inside and outside the pipe. The
pipe is commonly manufactured from plain metal sheets,
thus, when put together, the joints are not sealed by
sealants. Therefore, at some places, there can be leakage
of air, with a possibility of water vapour entering the
light-guiding system. This water can condense inside the
pipe at any time, especially in winter. The condensed
water may damage the surrounding structures. Also, the
fact that the thermal conductivity of the resulting struc-
ture can be evaluated just because it goes through the
envelope of a building leaves some space for improve-
ments. Another approach similar to that of the determi-
nation of the thermal conductivity may or may not be
correct and should be researched in detail via long-term
monitoring, not just by the means of computer simu-
lations.
5 CONCLUSION
The design and application of tubular light-guiding
systems have a flaw: most of their parameters regarding
building physics are determined solely by computer
simulations, which are not required to correspond to
reality.
This could be changed by the means of experimental
activities, which are planned within the Faculty of Civil
Engineering. Through a comprehensive long-term moni-
toring, several characteristics could be observed, such as
the movement of the air in the pipe, temperature rises
and the effects of air humidity in the light guides.
Acknowledgement
This paper was prepared within project
No. LO1408 "AdMaS UP – Advanced Materials, Struc-
tures and Technologies", supported by the Ministry of
Education, Youth and Sports under National Sustain-
ability Programme I.
6 REFERENCES
1 P. Charvat, T. Mauder, M. Ostry, Simulation of Latent-Heat Thermal
Storage Integrated with Room Structures, Mater. Tehnol., 46 (2012),
239–242
2 P. Boyce, P. Raynham, SLL Lighting Handbook, CIBSE, London
2009
3 F. Moore, Concepts and Practice for Architectural Daylighting, Van
Nostrand Reinhold, New York 1985, 290
4 J. Mohelnikova, F. Vajkay, Study of Tubular Light Guides Illumi-
nance Simulations, LEUKOS, 5 (2009), 267–277, doi:10.1582/
LEUKOS.2008.05.04.001
5 M. Kocifaj, S. Darula, R. Kittler, HOLIGILM: Hollow light guide
interior illumination method – An analytic calculation approach for
cylindrical light-tubes, Solar Energy, 82 (2008) 3, 247–259,
doi:10.1016/j.solener.2007.07.003
6 Commission Internationale de l’Eclairage, CIE DS 011.2/E: 2002
Spatial distribution of daylight – CIE standard general sky, Vienna:
Commission Internationale de l’Eclairage, 2002
7 Czech Standardization Institute, ^SN 73 0580 Daylighting in
buildings – Part 1: Basic Requirements, Prague, Czech Standardi-
zation Institute, 2007
8 Czech Standardization Institute, ^SN 73 0540 Thermal protection of
buildings – Parts 1,2,3,4 Prague, Czech Standardization Institute,
2005 and 2011
F. VAJKAY et al.: ASSESSMENT OF TUBULAR LIGHT GUIDES WITH RESPECT TO BUILDING PHYSICS
412 Materiali in tehnologije / Materials and technology 50 (2016) 3, 409–412