1) An experimental and simulation study was conducted to evaluate daylighting in an atrium building model in tropical conditions. Physical experiments using a scale model measured daylight illuminance levels and distributions in the adjoining space. 2) Simulation software (BESIM) that uses ray tracing and radiosity methods was used to calculate interior daylight levels and compared to measurements. Good agreement between simulations and measurements was found, validating the simulation approach. 3) Both experiments and simulations showed that daylighting in the atrium building was generally sufficient for interior spaces throughout the day due to strong tropical sunlight and sky luminance. Illuminance levels were highest in southern areas due to sun position, and decayed exponentially further from the light well.

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4th International Conference on Sustainable Energy and Environment (SEE 2011):
A Paradigm Shift to Low Carbon Society
23-25 November 2011, Bangkok, Thailand
1
AN EXPERIMENTALAND SIMULATION STUDY OF DAYLIGHTING IN
ATRIUM BUILDING IN THE TROPICS
Atitaya Saradphun1,
*, Pipat Chaiwiwatworakul1
and Surapong Chirarattananon1
1
The Joint Graduate School of Energy and Environment, King Mongkut’s University of Technology Thonburi, Bangkok,
Thailand, *
Corresponding Author: a.saartphun@gmail.com
Abstract: This study investigates an application of the daylight in an adjoining space of an atrium building in a tropical
climate. Physical experimentation using a scale model was performed under real skies to measure the daylight
illuminance and its distribution in the adjoining space and to validate the daylight calculation from a simulation
software namely “BESIM”. BESIM uses the ray-tracing method to deal with the beam light calculation and the flux-
transfer method for determining the interior light from the diffuse skylight. Using BESIM, the interior daylight in the
adjoining space was evaluated as functions of well index and interior surface reflectance. The simulation results show
that with the strong sunlight and the high luminosity of the tropical sky, daylighting in the atrium building is applicable.
The daylight is generally sufficient for circulation in the adjoining space throughout the day and all year round. For
Thailand where is located 5-20 degrees north of the earth equator, the influence of the sunlight to the northern area of
the space is larger than other areas. The sunlight also causes a high variance of the daylight in the area. The more
uniformity of the daylight distribution is observed for the southern area of the space. The use of interior shading helps
improve the daylight distribution and increase the daylight level for the lower floors of the atrium.
Keywords: Atrium building, Daylighting, Daylight factor, Well index, Illuminance
1. INTRODUCTION
Atrium offers a significant advantage of introducing natural light for use in building. It provides connection of the
interior adjoining space with the exterior and creates a focal contact point among people. The natural light delivered by
the atrium well does not only conserve electrical energy use from artificial lamps but also improve the interior on
psychological and ergonomic grounds. It brings more benefits for buildings in high latitude regions where more both
natural light and heat gains are needed in building especially during winter.
Atrium design for building involves analysis of several configurations and properties: orientation to the sun, shape of
the atrium, glaze transmittance of the atrium roof, reflectance of the atrium wall surfaces, etc. Daylight factor (DF) is
found to be an index typically used to describe the level and distribution of daylight in adjoining space in the atrium.
The daylight factor is expressed mathematically as a ratio of the interior daylight level to its corresponding exterior
daylight illuminance.
In the late 1980s, Kim and Boyer (1986) [1] developed a relationship between the shape of the atrium and the daylight
factor (DF) at the center of an open atrium. Gillette and Treado (1988) [2] carried out later a detailed thermal transport
and daylighting analysis of atria buildings. The results demonstrated the benefits of roof glazing on reducing the
lighting energy requirements.
Liu et al. (1991) [3] investigated the variations of daylight distribution in an atrium in relation to its geometric shape
index. Based on a scale-model experiment, Szerman (1992) [4] developed a monogram for calculating the mean
daylight factor in an adjoining space. In order to use the monogram, the information required were the fundamental
design parameters i.e. space position, atrium width, section-to-aspect ratio SAR (height/depth), atrium wall and floor
reflectance and glazing type. Although this monogram was relatively simple and easy to use, it still lacked some
validation and flexibility and it was hard to extend it to general applications [5]. Baker et al. (1993) [6] presented some
measured data of horizontal illuminances in the spaces of a square atrium in the form of curves relating daylight factor
to aspect ratio for three atrium wall surfaces. The results indicated that the spaces near the ground were mainly
illuminated by light reflected from the wall and floor whilst the top spaces received most light directly from the sky.
Several studies have illustrated that the reflectance of the atrium wall surfaces and the percentage of glazing in
comparison with the atrium wall surfaces are basic parameters that affect the transmission of the light in the adjoining
spaces. Cole (1990) [7] conducted experiments with scale models on the effects of varying the glazed area of the atrium
walls on daylight values in the adjacent atrium spaces. Aizlewood (1995) [8] reported that the daylight levels and
distributions in the adjoining spaces are significantly influenced by the vertical daylight levels on the atrium well
surfaces and the space properties (size and surface reflectances). The studies from Sharples (2007) [9]and from Du
(2009) [10] are additional evident the well geometries and surface reflectances are important factors determining atrium
characteristics which have a direct effect on the vertical daylight levels at the atrium wall. However, the reviews by

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4th International Conference on Sustainable Energy and Environment (SEE 2011):
A Paradigm Shift to Low Carbon Society
23-25 November 2011, Bangkok, Thailand
2
Wright (1998) [11] and Sharples (2007) [9] indicated that many of the researches investigating daylight in atria have
tended to focus upon illuminance levels on the atrium well floor.
Several of the works presented above were based on scale model-measurements in an artificial sky; certainly a
computer simulation could give a more rapid evaluation of the design choices [12], saving time and money provided
that the software is supported by validation studies. Radiance [13] has been in widespread use in current light research
and several studies have shown good agreement with the measured data confirming its scientific validity [14-16].
Those investigations showed that Radiance simulations could achieve a high accuracy in typical daylit spaces through
comparison with measurement and theoretical analysis. Radiance has become the most popular package for daylight
modeling in the built environment.
In the midst of the researches, it is found that the works are still limited in high latitude regions. The studies has
investigated particularly for overcast sky conditions as it represents a worst case scenario as well the most challenging
circumstances under which an atrium could be tested as being beneficial in a building.
2. COMPARISONS BETWEEN MEASUREMENT AND SIMULATION
2.1 BESIM Simulation Software
A computer program called BESim was used in the calculation of daylight in adjoining space in atrium that utilized
measurement data taken at the station. The program can be used for daylight as well as thermal calculation. The
program requires defining the coordinates of each flat interior section in a zone. The program utilizes the method of
Hien and Chirarattananon (2005)[17] in the calculation of view factors between all surfaces in each enclosed zone
created by a user. For daylight, it calculates sunlight illuminance through atrium using forward raytracing. For diffuse
daylight from the sky, it uses flux transfer, or the radiosity method, to calculate the inter-reflecting light. It uses
configuration factors to calculate illuminance at a given point on a work plane. In the present version, BESim uses the
ASRC-CIE sky luminance and sky irradiance models that utilize CIE clear and turbid clear sky models, partly cloudy
and cloudy sky models. The BESIM program was used to predict the daylight illumiances in the atrium adjoining space.
Comparison between the calculated values and that of measurements was made in order to validate the program.
2.2 Physical Model Measurement and Simulation
A 1:25 scale model of a ten-storey square-shape atrium building was constructed for this study. Figure 1 (a) illustrates a
photograph of exterior model configuration. The model made from plywood and has dimensions of width 1.6 m.,
length 1.6 m. and height 1.4 m. The floor-to-floor height of the model is about 0.14 m. The model is equivalent with an
atrium building with 40 m. width, 40 m. length and 35 m. height. The interior surfaces of the model were painted white
with a visible reflectance value of about 0.75.
Eight light sensors were used to measure daylight in the atrium model. Examine Fig. 1 (b), four light sensors were
placed along the centre line of the floor space toward south direction. The first sensor was located 8 cm. from the edge
of the light well that is equivalent to 2 m. for the actual. The second sensor was located 8 cm. apart from the first one
and so on for the third and the forth sensors. The position of the forth sensor is near the rear wall. The light
measurements were carried out at work plane level 0.75 m. above floor.
(a) Photograph of exterior view of the scale model (b) Position of sensors for light measurement
Fig. 1 A scale model of an atrium building
A light well was located at the center of the atrium model. The dimensions of the well are 0.4m. wide, 0.4 m. long, and
1.40 m. high, running from the top of the model to its base floor. The well index (WI) is about 3.5. The model has no
balcony wall around the well (assuming fully clear glazed surfaces) and no obstruction/partition in the adjoining space.
The aperture on the model roof top has no glazed or structural roof systems in order to exclusively study the effect of
specific parameters (geometry and reflectance) on daylight levels. Different roofs forms would have distorted the light

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4th International Conference on Sustainable Energy and Environment (SEE 2011):
A Paradigm Shift to Low Carbon Society
23-25 November 2011, Bangkok, Thailand
3
distributions in the atria. In the experimentation, the model was created on the roof deck of a seven-story building of
the school of Bioresearches and Technology in order to measure daylight in the atrium under real sky.
2.2.1 Experiment on 4th
July 2011
On this experimentation day, the sky can considered to be clear throughout the day. The global daylight illuminance
was about 100 klux during 10:30-15:00. The beam illuminance was also high in this day. Figure 2 exhibits a plot of the
variations of beam (Evb), diffuse horizontal (Evd), and global (Evg) daylight illuminance from 8.00-17.00 in the day (4
July 2011). The values of sky ratio index are smaller than 0.3.
Fig. 2 Variations of exterior daylight illuminance on 4 July 2011
Figure 3(a) shows the variation of the daylight illuminance at the measurement point #1 on the ninth floor of the model.
The illuminance values are within a range of 5,000-7,000 lux. From the plot, the illuminance values are comparatively
high during before- and after-noon. This might be explained by the penetrations of the sunlight to the western area of
the floor in the morning (the sun stays eastern direction) and to the eastern area in the afternoon would increase
extremely the illuminance in the areas which in turn increase the daylight illuminance in the measurement area by the
internal light reflection. It can be observed that at noon, most of the sunlight penetrates directly to the base floor thus
the illuminance is lower at this time.
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Fig. 3 Experimental results on 4 July 2011 (9th
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During the experiment period (June-August 2011), the sun stayed toward northern direction. The penetration of
sunlight (in a period of day) causes the interior daylight in the southern area of the adjoining space to be relatively
larger than that of the northern. This would also partially the result from the higher luminosity of the sky in circumsolar
region. The plot in Fig. 3(a) also shows the calculated values of the daylight illuminance from BESIM program. The
values from the measurement and the calculation are comparable.

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4th International Conference on Sustainable Energy and Environment (SEE 2011):
A Paradigm Shift to Low Carbon Society
23-25 November 2011, Bangkok, Thailand
4
The results of the illuminance measurements at Point #2, #3, and #4 on the ninth floor of the scale model are exhibited
respectively in Fig. 3 (b), (c), and (d). The daylight illuminances at Point #2 varied between 2,800-3,600 lux, Point #3
between 1,500-2,500 lux and Point #4 between 1,000-1,500 lux. The variations of the illuminance at these points are
similar to that at Point #1. The daylight on the upper floor in atrium can be higher than 1,000 lux throughout the day
(Fig. 3.(d)). It also observes that the illuminance decays exponentially from the light well to the rear walls.
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Fig. 4 Experimental results on 4 July 2011 (2nd
Floor)
Figure 4 exhibits similar plots with Fig. 3 but for 2nd
floor. From the plots, the simulation results quite agree with that
from the experiment. The daylight levels on the second floor are comparatively lower than that on the 9th
floor because
of the lower penetration of daylight from both the sun and the sky.
The daylight illuminance on this floor began with a very low magnitude (close to zero) and then arose to reach its peak
during noon. On the 2nd
floor, the maximum daylight level measured at point 1 is about 3,500 lux. The peak values
decrease when the measurement points were located in deeper area from the light well. It can be observed clearly that
the variation of the daylight on the 9th
floor and the 2nd
floor are totally different.
3. SIMULATION OF DAYLIGHT INATRIUM
This simulation study was performed to assess the daylight illuminance and distribution in the adjoining atrium space.
The simulation results from BESIM were employed next to investigate the influences of the light well configuration, the
depth of the floor space and the reflectance of the interior surface to the daylight illuminance and distribution.
3.1 Daylight Illuminance and Distribution in Atrium Space
A simulation was carried out for a ten-storey square-shape atrium building. The dimension of the building is 40m. wide
by 40m. long by 35m. high (the floor height is 3.5m.). A 10m. x 10m. light well is situated at the center of the building,
running from the roof deck to the base floor. The index of the well is 3.5 that represents a tall atrium building. The
reflectance of the interior surfaces including ceiling, floor and walls are all 0.5. The simulation was performed to
determine the daylight illuminance on a work plane level at points 2m., 4m., 6m., 8m., and 10 m. apart from the light
well edge. The work plane is 0.75m above the room floor. The results also illustrate the illuminances for the 4 areas of
the atrium space (northern, eastern, western and southern areas). It also assumed that each floor has the opening
balcony. Figure 5 illustrates the floor plan of the atrium building and the points the daylight illuminance to be
calculated.
Figure 6 illustrates the plots of monthly average daylight illuminance on the tenth floor of the atrium space. It can be
observed that the daylight illuminance is direction dependency. For the northern area, the average daylight illuminances
are comparable for each month except November and December. The illuminance at 2 m. apart from the well edge is

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4th International Conference on Sustainable Energy and Environment (SEE 2011):
A Paradigm Shift to Low Carbon Society
23-25 November 2011, Bangkok, Thailand
5
about 5,000 lux and decrease exponentially to about 500 lux at 10m. apart from the well edge. However, due to the
penetration of the direct sunlight to the floor, the daylight at 2m. increases sharply to 10,000 lux and 25,000 lux in
November and December, respectively. The similar trends can be observed as well for other direction but less effect
from the sunlight penetration.
Fig. 5 Floor plan of the atrium building and the points the daylight illuminance to be calculated
(a) Northern area of the tenth floor (b) Southern area of the tenth floor
(c) Westhern area of the tenth floor (d) Easthern area of the tenth floor
Fig. 6 Variation of daylight on the tenth floor (interior surface reflectance 0.5)
Figure 7 illustrates the similar plots but for the base floor. The illuminance valves of the daylight are in a range of 200-
800 lux and seems decay linearly from the well edge to the rear walls of the space.
Figure 8 compares the annual average daylight illuminance with surface reflectance. The comparison is made for the
tenth floor. Apparently, the annual average daylight illuminances are quite similar for the northern, eastern and western
areas. That of the southern area is comparatively low. For the year round, the difference of the interior surface
reflectance 0.5 and 0.7 does not cause much difference of the daylight level on the floor. However, the lower surface
reflectance of 0.3 exhibits a distinguish drop of the daylight illuminance. The plots also imply the dominance of direct
component of the daylight transmitting through the light well aperture.

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4th International Conference on Sustainable Energy and Environment (SEE 2011):
A Paradigm Shift to Low Carbon Society
23-25 November 2011, Bangkok, Thailand
6
(a) Northern area of the base floor (b) Southern area of the base floor
(c) Westhern area of the base floor (d) Easthern area of the base floor
Fig.7 Variation of daylight on the base floor (interior surface reflectance 0.5)
(a) Northern area of the tenth floor (b) Southern area of the tenth floor
(c) Westhern area of the tenth floor (d) Easthern area of the tenth floor
Fig. 8 Comparison of the annual average daylight on the tenth floor with different surface reflectance
Figure 9 summarizes the plots of the annual average daylight on the base floor with different well index. In the

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4th International Conference on Sustainable Energy and Environment (SEE 2011):
A Paradigm Shift to Low Carbon Society
23-25 November 2011, Bangkok, Thailand
7
calculation, the values of well index were varied from 0.35 to 3.5. The smaller is the value of well index then the
shallower is the atrium building. The well index 3.5 represents a tall atrium building. The plots present the calculation
results for the interior surface reflectance 0.5.
Examining its value on the base floor, the daylight illuminance is relatively high for the atrium space with small value
of the well index. The distribution of daylight also varies largely from the light well to the back walls of the space. For
the atrium with high value of the well index, the distribution is much more uniform.
(a) Northern area of the base floor (b) Southern area of the base floor
(c) Western area of the base floor (d) Eastern area of the base floor
Fig.9 Comparison of the annual average daylight on the base floor with different light well index
4. CONCLUSIONS
The daylight in atrium building in the tropics was investigated through the experimentation under real skies and the
simulation using the BESIM software. The comparison between the measurements of the daylight in the scale model
and that from the calculation validated the BESIM is capable of determining the interior daylight in the atrium space
with an acceptable degree of accuracy.
The study results show that light well configuration, space depth from the well, reflectance of the interior surfaces and
position of the apparent sun are the main factors influencing the daylight illuminance and distribution in the atrium
space. Regarding the studied atrium model with the well index 3.5 (tall building), opening balcony and the interior
surface reflectance 0.5, the tropical daylight is sufficient for illumination for circulation area on the base floor of the
atrium space all year round. The annual average of the daylight reaches 1,000 lux at the well edge and 600 lux at 10 m.
departure. As the component of the internally-reflected light is dominant, the daylight distribution is rather uniform for
the base floor (the lower floor as well). The direct sunlight also penetrates to the floor with shorter period of time.
On the top and the upper floors of the atrium, the interior light is influenced largely from the diffuse skylight and the
direct sunlight. It can be observed the pronounced exponential decay of the daylight from the well edge to the deeper
areas. The average daylight illuminance on the southern, eastern and western areas of the adjoining space can be high
upto 6,000-8,000 lux at the well edge but that at 10m. apart is about 500 lux. The monthly average daylight for the
northern area of the space close to the well is quite high upto 25,000 lux due to the direct sunlight. For Thailand, the
sun transverses toward south for 8 months in a year. The internal shade seems to be required to shade direct sunlight
from the space and to improve the light distribution. The shade would help deliver more light into the lower floors as
well.

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23-25 November 2011, Bangkok, Thailand
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