ARTICLE IN PRESS
Building and Environment ] (]]]]) ]]]–]]]
www.elsevier.com/locate/buildenv
Photometry and colorimetry characterisation of materials in daylighting
evaluation tools
M. Bodarta,, R. de Peñarandab, A. Deneyerc, G. Flamantc
a
Fonds de la Recherche Scientifique (FNRS), Université Catholique de Louvain, Unité d’Architecture, Place du Levant, 1,
B-1348 Louvain-La-Neuve, Belgium
b
Université Catholique de Louvain, Unité d’Architecture, Place du Levant, 1, B-1348 Louvain-La-Neuve, Belgium
c
Department of Building Physics and Equipments, Belgian Building Research Institute (BBRI), Avenue Pierre Holoffe, 21, B-1342 Limelette, Belgium
Received 3 October 2007; received in revised form 6 December 2007; accepted 9 December 2007
Abstract
This paper presents a methodology for evaluating the photometric and colorimetric characteristics of internal building materials, for
daylight evaluation. The assessment of these characteristics is crucial both for modelling materials accurately in daylight simulation tools
and for building correct daylight mock-ups. The essential photometric and colorimetric parameters that influence the reflection of light
from and its transmission through building materials are identified and described. Several methods for evaluating these parameters
qualitatively and quantitatively are then proposed and discussed. Our new methodology was fused to create a database of materials in a
freely accessible web tool which compares full-size materials to scale-model materials in order to help architects and lighting designers
choose materials for building daylight scale models.
r 2007 Elsevier Ltd. All rights reserved.
Keywords: Daylighting; Photometry; Simulation; Scale models; Materials; Colorimetry
1. Introduction
In the context of global warming and decreasing natural
energy resources, it is essential to focus on reducing the
consumption of energy in buildings. Lighting is an
important part of the overall energy consumption in
buildings, especially in tertiary buildings. One of the most
efficient ways to reduce electric light consumption is to take
advantage of the free natural source of light called daylight.
In order to promote the use of daylight in buildings,
effective tools have been developed in recent decades; these
include accurate simulation programs and sky simulators
which allow precise lighting measurements to be made in
scale models.
Accurate prediction of daylight levels in buildings need
precise information, whatever evaluation tool is used.
Thanks to the extremely small size of light wavelengths,
properly constructed scale models can replicate the
Corresponding author. Tel.: +32 10 47 91 52; fax: +32 10 47 21 50.
E-mail address: [email protected] (M. Bodart).
distribution of daylight in a room almost exactly in a scale
model [1]. Many books contain advice or rules for building
such scale models [1–4]. The reflection and transmission
characteristics of the model walls and transparent materials
should be as close as possible to those of the full-size
building. According to Cannon-Brookes [5] and Thanachareonkit et al. [6], the correct evaluation of the
photometric properties of materials is one of the two core
issues in scale modelling (the other is the accuracy of the
dimensions).
In simulation tools, accurate information on the materials
used for internal surfaces is also fundamental. For example,
the well-known daylight-simulation software RADIANCE
needs information on the specularity (directional–directional
reflectance) as well as precise colour and roughness values as
inputs to its calculations [7].
In Fig. 1, RADIANCE-simulation luminance views
show that a minor change in specularity can lead to very
different distributions of internal luminance and hence
visual impression in the room. On the left, the walls are
perfectly diffused; in the centre, the walls have a specularity
0360-1323/$ - see front matter r 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.buildenv.2007.12.006
Please cite this article as: Bodart M, et al. Photometry and colorimetry characterisation of materials in daylighting evaluation tools. Building and
Environment (2008), doi:10.1016/j.buildenv.2007.12.006
ARTICLE IN PRESS
2
M. Bodart et al. / Building and Environment ] (]]]]) ]]]–]]]
Fig. 1. Simulated view of a room with walls made of diffuse oak (specularity 0.0%), varnished checkmate oak (specularity 0.2%) and varnished shining
oak (specularity 2.3%).
of 0.2% and on the right, the walls have a specularity of
2.3%. The specularity of the walls also influences the
quantity of light reflected to the back of the room. For
example, for an overcast day, the average illuminance of
the back table in Fig. 1 will decrease by 1% and 2%, if the
diffuse walls are replaced by walls with specularity of 0.2%
and 2.3%, respectively. On a CIE (Commission Internationale de l’Eclairage) clear day, when direct sun hits the
west wall, the difference is higher: a 4% decrease if the
specularity of the walls is medium and 3% if it is high. For
this reason, precise information is needed on the usual
interior building materials.
The first part of this paper presents the parameters
that need to be evaluated to characterise the colour
and light reflection or transmission of a material. Some
simple evaluation methods are compared and we show
that these parameters are difficult to evaluate without
accurate and expensive measurement devices. We made
measurements on traditional internal-building materials
and on scale-model materials. The results of these
measurements, which are presented in the second part of
the paper, allow us to propose a new classification of
materials according to their dispersion angle. In addition to
this new classification, we propose a quick and simple
method for evaluating the class of every material using a
simple light source. The materials we measured were
supposed to be isotropic, but some of them presented
unexpected anisotropic properties that we discuss in this
part of the paper. Specific daylight systems with bidirectional behaviour are more complex and cannot easily
be scaled for use in scale models. They are not considered
in this paper.
For mock-up design, once the photometric and colorimetric properties of the full-size material are known, it is
essential to find a scale-model material which replicates
these properties as closely as possible. A methodology of
comparison was developed to find, in the material
database, the most appropriate material for the objectives
of the study. This comparison is carried out automatically
by a free web tool, taking several criteria into account. The
web tool and methodology of comparison are presented in
the last section of the paper.
2. Photometric and optical parameters
The behaviour of the light hitting a surface depends on
the colour and texture of that surface. The colour
determines the quantity of reflected radiation, in the range
of visible wavelengths. The texture influences the way the
light is reflected or transmitted. This section presents the
parameters we chose to characterise the light reflected from
and transmitted through a material, and to give a precise
definition of each of them. Laboratory and in situ
measurements of these parameters are then discussed and
a simple new method for evaluating the materials
reflectance is proposed.
2.1. Selection and description of parameters
In order to characterise an opaque surface by its
reflected- and transmitted-light comportment, we used the
following parameters:
the
the
the
the
colour and lightness of the reflected light,
directional–hemispherical reflectance,
directional–directional reflectance and
reflection mode.
For transparent materials, additional parameters characterise the transmission of light through the material.
They are:
the
the
the
the
colour and lightness of the transmitted light,
directional–hemispherical transmittance,
directional–directional transmittance and
transmission mode.
These parameters are evaluated according to the
methods described below.
As the objective of the colour characterisation was to
quantify the exact difference in colour between the full-size
and the scaled material, it was crucial to measure the
surface colours in a system that could express colour
differences. For this reason, we chose the CIE L*a*b*
colour system, which is widely used for non-luminous
Please cite this article as: Bodart M, et al. Photometry and colorimetry characterisation of materials in daylighting evaluation tools. Building and
Environment (2008), doi:10.1016/j.buildenv.2007.12.006
ARTICLE IN PRESS
M. Bodart et al. / Building and Environment ] (]]]]) ]]]–]]]
objects such as paints and plastics [8]. In the L*a*b* colour
space, L* is the lightness and varies from 0 (black) to 100
(white). a* and b* are the chromaticity coordinates: +a* is
the red direction, a* is the green direction, +b* is the
yellow direction and b* is the blue direction. The centre is
achromatic. As a* and b* increase, and the point
moves out from the centre, the saturation of the colour
increases [9].
The colour difference DE* between two samples is
defined by the equation
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
DE ab ¼ ðDL Þ2 þ ðDa Þ2 þ ðDb Þ2 ,
(1)
where DL*, Da* and Db* are the difference between the two
samples in L*, a* and b*, respectively. The perception of
the colour difference DE* varies according to the observed
colour and the sensitivity of the human eye. The human eye
only distinguishes colour difference if DE* is larger than
1–3. For some colours, mainly blues, DE* values of 1 can
be detected, but for other colours, such as red, the same
DE* may not be perceptible.
The directional–hemispherical reflectance of a surface is
the ratio of the total reflected luminous flux to the
directional incident flux [10]. The hemispherical reflectance
corresponds to the Y tri-stimulus value defined by the CIE
in 1931 [8]. The Y is linked to the L* value by
1=3
Y
L ¼ 116
16,
(2)
Yn
where Yn ¼ Y ¼ 100 for a white object, under the standard
illuminant D65, at an incidence angle of less than 101. Y
can thus be directly deduced from the L*a*b* values of the
material.
The directional–directional reflectance of a surface is the
ratio of the specularly reflected flux to the directional
incident flux [10]. The difference between the directional–
hemispherical reflectance and directional–directional reflectance is the directional–diffuse reflectance. These three
values of reflectance give information about the quantity of
light reflected by a surface, but no information about how
the light is reflected. Another parameter, the reflection
mode, is needed to describe the ‘how’.
Two well-known reflection modes are:
perfectly diffuse, also called Lambertian reflection, for
which the radiation reflected by the surface is distributed
angularly according to the Lambert’s cosine law,
proposed in 1760 [11] and
specular (or regular), for which the radiation is reflected
in accordance with the laws of geometrical optics,
without diffusion.
Light reflected by objects is usually neither completely
specular (or regular) nor completely diffuse but lies
somewhere between these two extremes [12]. The surfaces
may be termed ‘glossy’ (significant specular reflection),
3
‘semi-matt’ (low specular reflection) or ‘complex’ (the light
is randomly reflected and distributed).
The description of the surface reflectance mode is often
associated with its gloss. Gloss perception expresses the
way an object reflects light, particularly the way that light
is reflected from the surface of the object at and near the
specular direction. Gloss is normally perceived independent
of colour; although it may be affected by the underlying
colour of the object and may affect the perceived colour of
the object. It is usual for the perception of gloss to be
abstracted from the total visual experience as separate from
colour.
The limits of gloss measurements have been recognised
for a long time and the tendency today is to exploit the bidirectional reflectance distribution function (BRDF)
measured by spectrogoniophotometers that gives the shape
of the whole reflected light (peak specular and diffuse
parts) [13].
The angle of illumination of a spectrogoniophotometer
is often variable, and if several samples are to be compared,
some method must be devised for simplifying the information. The simplest value is the peak height intensity, Imax,
for a given value of illumination angle. However, it has
been found that this does not give a significantly better
correlation with subjective assessment than a simple
measurement with a gloss meter [14]. One useful way of
summarising the information for various angles of reflection is to use the material dispersion angle concept
proposed by Baker et al. [2]. The dispersion angle is the
angle between the direction of maximum intensity (Imax) of
the reflected light, and the direction of intensity with a
value of Imax/2. The intensity-distribution curve is assumed
to be symmetrical about the direction of Imax, as shown in
Fig. 2. The dispersion angle is thus easily deduced from
BRDF functions and can be precisely measured with a
spectrogoniophotometer.
Baker et al. [2] proposed classifying materials according
to their dispersion angle. After observing the reflection of
light from several materials, we decided to split the ‘narrow
scatter’ class of their classification into two. We did this
because the image of the reflected light can be very different
for materials in this class. Baker et al.’s classification and
the new one are illustrated in Fig. 3.
As already explained, additional information is needed
to characterise transparent materials. Like the reflected
light, the colour and lightness of the transmitted light is
evaluated in the L*a*b* colour space. By analogy to the
directional–hemispherical reflectance, the directional–
hemispherical transmittance of a surface is the ratio of
the total transmitted luminous flux to the directional
incident flux.
The directional–directional transmittance is the
ratio of the directly transmitted flux to the directional
incident flux [10]. The difference between the directional–
hemispherical transmittance and the directional–
directional transmittance is the directional–diffuse
transmittance.
Please cite this article as: Bodart M, et al. Photometry and colorimetry characterisation of materials in daylighting evaluation tools. Building and
Environment (2008), doi:10.1016/j.buildenv.2007.12.006
ARTICLE IN PRESS
4
M. Bodart et al. / Building and Environment ] (]]]]) ]]]–]]]
Like the reflection, the distribution of the light transmitted through a material has an impact on the view
through this material. The haze and the see-through
quality, which are two well-known parameters which
depend on the dispersion angle value, are described in
Fig. 4. Haze occurs when light is diffused in all directions
causing a loss of contrast. The see-through quality is
affected when light is diffused in a small angle range with
Fig. 2. Definition of the dispersion angle.
high concentration. This parameter describes how well very
fine details can be seen through the material [15].
2.2. Laboratory measurement
The most accurate instrument for measuring the photometric and colorimetric characteristics of a surface is a
spectrophotometer. This measures, for each wavelength,
the light reflectance or transmittance of the material, at
several incident and observation angles (with the angular
accessory, see below). Devices can differ from one
laboratory to another. The spectrophotometer we used
was a ‘Perkin-Elmer Lambda 900 UV–VIS–NIR’. This has
a double beam and double monochromator configuration
and belongs to the Belgian Building Research Institute
(BBRI). We used it to measure the materials described in
Section 3 and included in the web tool. The spectrophotometer can be fitted with two accessories: a 150 mm
diameter integrating sphere (PELA 1000) and an angular
accessory (PELA 1030). The integrating sphere is used to
measure the directional–hemispherical reflectance and
transmittance. The directional–directional reflectance and
Fig. 4. Two parameters depending on the dispersion angle: haze (large d)
and view-through quality (low d).
Fig. 3. Baker et al.’s (top) and new classification (bottom) of a material according to its dispersion angle.
Please cite this article as: Bodart M, et al. Photometry and colorimetry characterisation of materials in daylighting evaluation tools. Building and
Environment (2008), doi:10.1016/j.buildenv.2007.12.006
ARTICLE IN PRESS
M. Bodart et al. / Building and Environment ] (]]]]) ]]]–]]]
Fig. 5. Angular accessory principle. 1. Reference beam; 2. measurement
beam; 3. integrating sphere; 4. mirrors; 5. sample.
transmittance, and the dispersion angle were measured
with the angular accessory.
Fig. 5 illustrates the double beam spectrophotometer
equipped with the angular accessory. The accessory
includes two symmetric mirror paths for the reference
and measurement beams. These beams are collected in a
small integrating sphere through small apertures, one, for
the reference beam, on the top of the sphere, and the other,
for the measurement beam, on the side of the sphere. The
sample can rotate independently of the integrating sphere.
The sample position can be varied from 01 to 3601 while the
sphere position can be varied from 201 (near-normal
reflectance) to 1801 (direction of incidence beam). The
sphere can only be moved in the horizontal plan. The
samples to be measured are assumed to be isotropic.
The angular accessory was originally designed for
measurements on regular samples. A new methodology
was therefore developed in order to measure the dispersion
angle on all kinds of isotropic materials. For measurements
in reflection mode, the incidence angle of the light should
ideally be 01. However, it would then be impossible to
measure the reflected light without interfering with the
incident light. The incident angle was thus set at 101, and
observations were made from 101 to 701, in steps of 2.51
(between 101 and 301), 51 (between 301 and 501) and 101
(between 501 and 701). In this way, the sensor covers a half
circle around the sample, in successive steps, for each
measurement. The luminous intensity of the reflected light
was measured, at each step, for every wavelength of the
visible spectrum (from 380 to 780 nm, at intervals of
10 nm). An overall intensity value was then calculated by
integrating the spectral values according to the EN 410
standard [16]. A complete measurement of the intensity
curve requires 16 steps, with measurements at 40 different
wavelengths at each step.
For the transmission-mode measurements, the angle of
incidence was 01. The observation angle varied from 01 to
601 (where 01 is normal to the sample).
2.3. In situ material characterisation
When it is impossible to get samples of the full-size
material and/or to measure it with a spectrophotometer, in
5
situ measurements have to be made. Several solutions to
determine the colorimetric and photometric characteristics
of a material in situ exist.
The most accurate way to measure the colour and
reflectance of a material in situ is to use a portable
colorimeter. Some portable colorimeters can also measure
specular reflectance. However, only specialised laboratories
have such instruments. Therefore, researchers have developed several methodologies for on-site material characterisation using a luminance meter, an illuminance meter, or a
simple colour card. Three methods were studied and tested
on several internal walls and furniture. Their results were
compared to colorimeter measurements.
Fontoynont [17] suggests evaluating the hemisphericalhemispherical reflectance of opaque materials with a
luminance meter. He proposes the same method for
evaluating the normal–normal transmittance of clear
glazing. Moreover, combining a luminance measurement
with an illuminance measurement allows the evaluation of
the hemispherical–hemispherical transmittance of clear or
translucent glazings. The CIBSE guide [18] suggests two
methods for evaluating the hemispherical–hemispherical
reflectance of materials. The first is based on a luminancemeter measurement, coupled with an illuminance-meter
measurement. The second uses a sample card for comparisons between colour samples and on-site materials to
determine the diffuse reflectance of these materials.
Fig. 6 shows the measured reflectance according to each
of these methods, for nine walls or pieces of furniture. In
this figure, the method proposed by Fontoynont is named
‘ENTPE’, the CIBSE method using a luminance and an
illuminance meter is called ‘CIBSE lum/illum’ and the
CIBSE method based on the sample card is called ‘CIBSE
sample card’.
Table 1 shows the relative mean-bias error (MBE) and
the relative root-mean-square error (RMSE) for each
method, taking the values measured by the colorimeter as
the reference standard. The RMSE, which is a good
measure of the overall magnitude of errors, is defined as
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
u
n u1 X
X i;meth X i;ref 2
t
RMSE ¼
,
(3)
N i¼1
X i;ref
where N is the number of samples. It reflects the size of the
errors and the amount of scatter, but does not reflect any
overall bias in the data. The MBE is defined as
MBE ¼
n
1X
ðX i;meth X i;ref Þ
.
N i¼1
X i;ref
(4)
The positive and negative errors in the MBE cancel each
other out, so the MBE is an overall measure of how biased
the data is.
Table 1 and Fig. 6 show that, for the materials tested
here, the differences between the various measurements
and the colorimeter standard are considerable. The MBE
value indicates that all three methods overvalue the
Please cite this article as: Bodart M, et al. Photometry and colorimetry characterisation of materials in daylighting evaluation tools. Building and
Environment (2008), doi:10.1016/j.buildenv.2007.12.006
ARTICLE IN PRESS
M. Bodart et al. / Building and Environment ] (]]]]) ]]]–]]]
6
Fig. 6. Determination of hemispherical reflectance by different methods.
Table 1
Relative mean-bias error (MBE) and relative root-mean-square error
(RMSE) for the difference between simplified evaluation methods and
precise colorimeter measurements of the characteristics of materials
Mean-bias error (%)
Root-mean-square
error (%)
CIBSE
lum/illum
ENTPE
CIBSE
sample chart
0.59
25.69
19.28
36.58
32.24
54.49
visible, the material can be classified as widely scattering. If there is no visible patch of light, the material can
be classified as diffuse.
This method is not very precise, and does not determine
the dispersion angle of the material accurately, but it will
help the user choose a scale-model material belonging to
the same class as the full-size material.
3. Full-size and scale-model surface materials
3.1. Opaque surfaces
reflectance of the materials. The CIBSE method based on
luminance and illuminance measurement gives the best
results, but should be used with care for materials with low
reflectivity.
For in situ evaluation of the reflection mode, we propose
a new method for a qualitative appreciation of materials.
This method is empirical, and based on our observations of
162 materials. The procedure is as follows:
Take a light source and fix a baffle with sharp edges on
it, for example in an L form, as shown in Fig. 7.
Darken the room, illuminate the material with the light
source and observe the form of the reflected image on
the material.
Follow Fig. 8 to classify the material. If the reflected
image is a well-marked patch of light, the material can
be classified as ‘nearly specular’. If the patch of light is
not well marked, the material can be classified as
narrowly scattering. If the patch of light is hardly
Little information is available on the photometric
characteristics of internal building materials. In most
lighting-simulation algorithms and programs most indoor
surfaces are assumed to be diffused. However, our
measurements showed that, of the 46 indoor building
materials we tested, only 28% were diffuse, 11% of them
were narrowly scattering, 28% widely scattering and 33%
were nearly specular. Among the diffuse materials, were
carpets, bricks, unpolished stones and some paints. In the
widely scattering category, were unvarnished wood and
wood covered by a matt varnish, linoleum, waterproof
multiplex and other kind of paints. In the narrowly
scattering category, we found some paints, polished stones
and multiplex wood covered by Bakelite. Notice that the
manufacturers’ classification of paint does not correspond
to any accurate categorisation. According to our measurements and classifications, satin paints can have diffuse,
widely scattering or narrowly scattering properties. Matt
Please cite this article as: Bodart M, et al. Photometry and colorimetry characterisation of materials in daylighting evaluation tools. Building and
Environment (2008), doi:10.1016/j.buildenv.2007.12.006
ARTICLE IN PRESS
M. Bodart et al. / Building and Environment ] (]]]]) ]]]–]]]
7
Fig. 7. Light source method.
Fig. 8. Classification of a material according to the clarity of a reflected image.
Fig. 9. Nearly specular—full-size materials.
paintings can have diffuse or widely scattering properties.
Finally, we placed polished stones, faience and mirrors,
shining varnished woods, some metals (copper brasses) and
diffuse materials covered by a surface coating in the nearly
specular category.
Figs. 9–16 show the L reflection on scale-model and fullsize materials for a selection of 89 of the 162 materials we
measured. The names and characteristics of these materials
are listed in Table 2. Cardboard is very easy to use in a
mock-up and is available in many different colours, so we
tested many different cardboards in the scale-model
category.
Notice that many of the materials classified as ‘nearly
specular’ behave differently depending on their orientation.
For example, full-size Materials 17, 18 and 42 clearly show
greater dispersion of light in one direction (horizontal for
Materials 17 and 18 and vertical for Material 42) than in
another. This behaviour, which is not visible in diffuse
light, was unexpected, as we selected materials which we
thought were isotropic. Photographs and measurements
show that these materials were not totally isotropic. For
example, the dispersion angle for Material 42 varied from
2.51 to 51, according to the position of the sample in the
spectrophotometer.
Many scale-model materials have similar characteristics
(see Fig. 10), which is probably due to the material
building process. Most classical cardboards are diffused,
while matt silvered cardboard and matt aluminium
foils are narrowly scattering and shiny aluminium foils
are specular.
We tried to modify the reflection mode of several
cardboards by two different processes in order to get
Please cite this article as: Bodart M, et al. Photometry and colorimetry characterisation of materials in daylighting evaluation tools. Building and
Environment (2008), doi:10.1016/j.buildenv.2007.12.006
ARTICLE IN PRESS
8
M. Bodart et al. / Building and Environment ] (]]]]) ]]]–]]]
Fig. 10. Nearly specular—scale-model materials.
Fig. 11. Narrowly scattering—full-size materials.
Fig. 12. Narrowly scattering—scale-model materials.
Fig. 13. Widely scattering—full-size materials.
scale-model materials belonging to other classes. The first
process was to paste a transparent pressure-sensitive film
on cardboards. For the four cardboards tested, this process
changed the category from diffuse to nearly specular. The
specular component added varied from 4.1% to 4.7%,
while the colour difference (DE) varied from 1.0 to 6.2. The
second idea was to spray ink-jet-print fixative spray on the
cardboard. Only glossy fixative spray induced the same
change of class on the five cardboards tested: it altered their
class from diffuse to widely scattering. The average
specular component added was 0.8% and the colour
difference varied from 7.6 to 16.9. This is greater than
the colour modification induced by the transparent
sensitive film.
Please cite this article as: Bodart M, et al. Photometry and colorimetry characterisation of materials in daylighting evaluation tools. Building and
Environment (2008), doi:10.1016/j.buildenv.2007.12.006
ARTICLE IN PRESS
M. Bodart et al. / Building and Environment ] (]]]]) ]]]–]]]
9
Fig. 14. Widely scattering—scale-model materials.
Fig. 15. Diffuse—full-size materials.
Fig. 16. Diffuse—scale-model materials.
In general, internal building materials have unsaturated
colours, while cardboards can be found in a large variety of
colours.
3.2. Glazings
The scale models built to study internal daylight are, in
1
1
practice, on scales between 10
and 50
. For technical reasons
(costs and manufacturing), it is impossible to construct
glazing on this scale. For example, 6 mm glazing would
have to be reduced to between 0.6 and 0.012 mm thickness.
For scale-model measurements, some authors suggest using
unglazed models and applying a weighting factor, depending on the visible transmittance of the glazing. However, if
the main source of daylight enters the apertures at
incidence angles greater than about 601, the glazing
Please cite this article as: Bodart M, et al. Photometry and colorimetry characterisation of materials in daylighting evaluation tools. Building and
Environment (2008), doi:10.1016/j.buildenv.2007.12.006
ARTICLE IN PRESS
10
M. Bodart et al. / Building and Environment ] (]]]]) ]]]–]]]
material must be included in the model to establish the
proportion of daylight reflected off the glazing. For
diffusing glazing, the glazing itself or a corresponding
material should be used in the scale model. Several
glazings and materials that could be used in scale
models (plexiglass or glazings) have been characterised in
laboratory measurements. If the glazing is tinted or
covered by a reflective coating, then a corresponding
material should be used as the colour of the light is
influenced by the glazing [2]. Using double glazing in a
scale model is very difficult: the best solution is to use a
coloured pane of the real glazing. Manufacturers can
provide glazings for measurement, but as the coatings
are very delicate, special care has to be taken in the
measurement procedure.
comparison is made according to the objective of the
study and the importance attached by the user to each of
the parameters. For example, if the objective of an
illuminance study is the determination of accurate illuminance values, the hemispherical reflectance of the scalemodel material should be as close as possible to the
hemispherical reflectance of the full-size material, even if
the tint of the material does not match exactly. If necessary,
grey material can be used. However, if the objective of the
designer is the visual impact of the room, colour and
reflection mode would be more important than the exact
hemispherical reflectance. The web tool allows the user to
choose among three criteria: colour, reflectance and
reflection mode, or to select a combination of them as
the objective.
4. The photometric database and its web tool
4.2. Database operation
The objective of the web tool is to provide architects with
the scaled material that best matches their on-site material.
This database also gives precise information on many
indoor surface materials. This information is also useful for
the characterisation of materials in daylight-simulation
software. At present, the database only contains opaque
and transparent material (glazing), but we plan to test
translucent and shading materials (fabrics). The web tool is
available at http://www-energie.arch.ucl.ac.be/materiaux/
defaulte.asp.
The difference between the scale-model material and the
full-size material is calculated for three photometric criteria
(colour, lightness and hemispherical reflectance). An index
value between 0 and 100 is then derived for each criterion.
The index value is 0 when the difference between the scalemodel material and the full-size material is minimal and
100 when the difference between the scale-model material
and the full-size material is as large as it can possibly be, in
comparison with each material included in the database.
When the user chooses a full-size material in the list, the
dispersion angle, d, is known and a fourth index can be
calculated and considered for the selection of the best
overall match. For a user-defined material, the reflectance
class should be evaluated by the light source method. In
that case, the tool proposes a scale-model material that
belongs, if possible, to the same class as the full-size
material.
4.1. Web-tool input
Users of the web tool can either choose a full-scale
material in the database or characterise their own material.
If the user chooses a material in the database, the tool
automatically provides its photometric characteristics:
colour, lightness, hemispherical and specular reflectance
and reflection mode. It also displays pictures of the
material.
The web tool works with the L*a*b* colour space but
users who do not know these values can input the values in
other coordinate systems; (H*,L*,C*), (X,Y,Z) or (Y,x,y).
The RAL or NCS code can also be introduced, as can
the RGB value. The tool calculates the L*a*b* values
automatically. The lightness is calculated automatically
from these colour characteristics. The hemispherical
reflectance is also calculated automatically from the colour
characteristics, but the material’s hemispherical transmittance must be provided by the user. This value is available
from the manufacturers. Users who describe a material
not included in the database are unlikely to know the
dispersion angle. In this case, they need to perform the light
source test described above in order to classify the material
qualitatively according to Fig. 8. The objective of the web
tool is to propose, for each full-size material, a scale-model
material that has similar photometric and colorimetric
characteristics. This is achieved through a multi-criteria
analysis: several parameters are determined and the
4.3. Database output
The database output can be simplified or detailed. The
simplified results page gives the following information:
photometric and colorimetric characteristics of the fullsize material;
best scale-model material for all the criteria taken
together;
best scale-model material for each criteria separately
and
the closest RAL or NCS value, together with the colour
difference with the full scale material.
The detailed results page gives the following information:
photometric and colorimetric characteristics of the fullsize material;
best scale-model material for all the criteria together;
best scale-model material for each criteria separately;
Please cite this article as: Bodart M, et al. Photometry and colorimetry characterisation of materials in daylighting evaluation tools. Building and
Environment (2008), doi:10.1016/j.buildenv.2007.12.006
1. Clear oak
2. Varnished clear oak checkmate
3. Varnished clear oak shining
4. Red brick
5. Red-orange brick
6. Beige brick
7. Freestone
8. Belgian petit granit
9. Stone crema marfil
10. Cardboard Cansons Mi-Teinte 101
11. Cardboard Cansons Mi-Teinte 470
12. Cardboard Cansons Mi-Teinte 480
13. Cardboard Cansons Mi-Teinte 345
14. Cardboard Cansons Silvery
15. Cardboard CANSONs Astralux
16. Balsa wood
17. Aluminium—smooth face
18. Aluminium—brushed face
19. Aluminium—satin finished
20. Evergreen plates
21. Plywood
22. Foam core board—grey
23. Foam core board—black
24. Canson cardboard Mi-Teinte 101+pressure
25. Canson cardboard Mi-Teinte 470+pressure
26. Canson cardboard Mi-Teinte 480+pressure
27. Canson cardboard Mi-Teinte 345+pressure
28. Foam core board—white
29. Bluestone—surface 1
30. Bluestone—surface 2
31. Bluestone—surface 3
32. Multiplex
33. Plastic plate
34. Fibre building board+white melamine
35. Aluminium—satin finished
36. Rose faience
37. Red linoleum
38. Beige linoleum
39. Grey carpet
40. Multiplex
41. Stainless steel
42. Brushed aluminium
43. Plexiglas
44. Wood Quicksteps
45. Brown faience
Name
sensitive
sensitive
sensitive
sensitive
film
film
film
film
Table 2
The photometric characteristics of the materials we tested
66.99
65.74
63.86
39.13
42.00
65.66
79.40
34.11
80.63
92.26
80.38
64.43
36.34
77.53
40.50
79.70
90.15
88.27
88.37
57.21
72.96
63.00
0.09
90.65
79.58
64.54
42.37
95.00
43.70
43.34
32.14
10.46
93.60
92.96
86.22
59.48
50.11
76.23
38.68
47.72
67.89
89.18
20.49
64.02
60.84
L*
Colour
5.78
7.26
7.40
11.62
21.09
2.59
1.79
0.24
1.96
0.76
5.54
9.96
3.11
0.81
20.54
5.01
0.38
0.18
0.73
3.98
7.87
2.63
0.06
0.61
5.80
8.67
2.44
0.64
1.05
1.53
1.08
11.83
0.97
0.77
0.70
11.72
23.23
7.59
1.07
12.66
0.57
0.29
0.28
12.48
7.09
a*
20.54
26.39
26.74
12.16
19.36
18.01
17.08
0.05
16.32
18.91
29.60
25.34
5.35
2.21
48.83
16.48
2.35
2.50
1.01
13.63
26.43
0.04
0.08
21.01
30.23
23.35
3.94
3.33
3.96
0.78
2.08
11.80
2.39
3.83
0.60
12.27
20.39
26.14
5.82
23.57
6.14
3.03
0.28
26.20
18.39
b*
36.6
35.0
32.6
10.7
12.5
34.9
55.6
8.1
57.8
81.3
57.7
33.3
9.2
52.4
11.6
56.2
76.6
72.6
72.8
25.1
45.1
31.6
0.0
77.7
55.9
33.5
12.7
87.6
13.6
13.5
7.3
1.2
84.3
82.9
68.4
27.6
18.5
50.3
10.7
16.6
37.8
74.5
3.1
32.8
29.1
rdir–h (%)
36.4
34.8
30.3
10.7
12.5
34.9
55.6
7.8
53.9
81.3
57.4
33.3
9.2
41.5
9.1
56.1
6.0
26.4
60.2
24.6
44.7
30.8
0.0
73.0
51.6
29.0
8.4
87.5
13.6
13.5
7.3
0.3
82.2
81.1
56.9
23.9
18.0
50.1
10.7
16.6
27.7
12.8
0.0
30.5
28.6
rdir–dif (%)
0.2
0.2
2.3
0.0
0.0
0.0
0.0
0.3
3.9
0.0
0.2
0.0
0.0
10.9
2.4
0.1
70.6
46.2
12.7
0.6
0.4
0.8
0.0
4.8
4.4
4.4
4.3
0.1
0.0
0.0
0.0
0.9
2.1
1.8
11.6
3.7
0.5
0.3
0.0
0.0
10.1
61.7
3.1
2.3
0.5
rdir–dir (%)
39.5
40.5
5.5
55
54.5
54
53.5
7.5
4
53.5
58
58
62
9
4
57
4
4
9.5
25
55
20
8
6.5
4.5
4
4.5
52
41
42
54
10.5
5
25
22
4
25
46
60
38
4.5
3.5
4
7
27
Dispersion angle d
ARTICLE IN PRESS
M. Bodart et al. / Building and Environment ] (]]]]) ]]]–]]]
11
Please cite this article as: Bodart M, et al. Photometry and colorimetry characterisation of materials in daylighting evaluation tools. Building and
Environment (2008), doi:10.1016/j.buildenv.2007.12.006
Please cite this article as: Bodart M, et al. Photometry and colorimetry characterisation of materials in daylighting evaluation tools. Building and
Environment (2008), doi:10.1016/j.buildenv.2007.12.006
glossy
glossy
glossy
glossy
78.69
37.62
4.33
94.16
92.75
92.63
0.9291
75.64
72.53
35.85
34.54
91.99
90.35
95.67
32
81.95
74.71
37.51
92.65
94.52
92.7
90.85
56.23
77.04
56.81
39.45
78.68
25.05
35.63
68.97
90.64
59.26
77.61
46.91
40.4
91.36
57.51
77.75
58.05
41.63
88.68
73.84
93.79
46.71
47.94
1.80
0.38
0.11
1.36
0.39
0.4
1.7
19.07
0.08
41.8
35.68
1.7
0.11
0.44
17.3
0.3
18.83
0.02
0.1
0.16
0
1.35
1.45
9.04
8.58
13.67
0.55
0.2
10.45
7.82
1.4
1.16
7.83
10.64
13.8
1.85
1.1
8.51
10.88
14.42
0.55
12.37
0.86
60.01
7.51
a*
3.41
0.13
0.12
2.94
5.85
7.89
7.08
76.04
1.88
17.43
14.76
7.3
19.17
1.75
8.76
47.02
26.45
1.3
0.61
0.1
0.69
26.39
4.72
36.78
26.07
21.74
2.41
0.54
43.03
32.88
26.01
4.42
34.94
24.44
22.78
25.26
4.29
35.73
25.02
24.57
45.18
19.56
3.19
32.65
–44.9
b*
54.4
9.9
86.0
85.7
82.4
82.1
82.8
49.3
44.5
8.9
8.3
80.7
77.1
89.2
7.1
60.2
47.8
9.8
82.2
86.5
82.3
78.2
24.2
51.6
24.7
10.9
54.4
4.4
8.8
39.3
77.7
27.3
52.6
24.8
11.5
79.3
25.5
52.8
26.0
12.3
73.5
46.5
84.8
15.8
16.7
rdir–h (%)
Directional–hemispherical reflectance: rdirh; directional–diffuse reflectance: rdirdif; directional–directional reflectance: rdirdir.
Faience—marble imitation
Dark grey faience
Mirror
Levis paint—enamel satin
Levis paint—eggshell—mat
Levis paint—eggshell—mat—2 layers
Levis paint—stone of Paris—mat—2 layers
Levis paint—gold—mat—2 layers
Levis paint—grey dolphin—2 layers
Levis paint—bordeau—2 layers
Combinaison 56+57
De Keyn paint—satin—whipped cream
De Keyn paint—satin week-end
De Keyn paint—mat—white
Red PVC sheet
Brass sheet
Cooper sheet
Clear transparent sheet
Mirror-silver styrene
Aluminium foil—mat
Aluminium foil—shining
Cardboard Cansons 101+fixative spray Ghiant glossy
Cardboard Cansons Mi-Teinte 122+fixative spray Ghiant
Cardboard Cansons Mi-Teinte 470+fixative spray Ghiant
Cardboard Cansons Mi-Teinte 480+fixative spray Ghiant
Cardboard Cansons Mi-Teinte 502+fixative spray Ghiant
Cardboard Cansons Astralux 34 silver
Cardboard Cansons Astralux 30 Dark
Cardboard Cansons Astralux 40 Marine
Cardboard Cansons Astralux 36 Gold
Cardboard Cansons 101+fixative spray Ghiant matt
Cardboard Cansons 122+fixative spray Ghiant matt
Cardboard Cansons 470+fixative spray Ghiant matt
Cardboard Cansons 480+fixative spray Ghiant matt
Cardboard Cansons 502+fixative spray Ghiant matt
Cardboard Cansons 101+fixative spray Ghiant satin
Cardboard Cansons 122+fixative spray Ghiant satin
Cardboard Cansons 470+fixative spray Ghiant satin
Cardboard Cansons 480+fixative spray Ghiant satin
Cardboard Cansons 502+fixative spray Ghiant satin
Cardboard Cansons Vivaldi paille (yellow)
Cardboard Cansons Vivaldi ciel (light blue)
Cardboard Cansons Astralux 01 white
Cardboard Cansons Astralux 32 red
Cardboard Cansons Astralux 39 cyan
L*
Colour
50.6
8.9
0.4
83.2
82.2
80.6
80.7
48.5
44.4
8.6
8.0
80.2
76.5
89.0
2.5
25.5
23.3
0.5
1.3
68.8
26.8
77.3
23.3
50.4
24.0
10.2
42.5
1.2
5.1
30.5
77.2
26.9
52.2
24.5
11.2
78.6
25.2
52.3
25.6
11.8
73.2
46.3
81.2
11.9
13.4
rdir–dif (%)
3.8
1.0
85.7
2.4
0.2
1.6
2.1
0.8
0.1
0.3
0.3
0.5
0.5
0.2
4.6
34.7
24.5
9.3
80.9
17.7
55.5
0.8
0.8
1.2
0.7
0.7
11.9
3.2
3.7
8.8
0.5
0.4
0.4
0.4
0.3
0.7
0.3
0.5
0.5
0.5
0.3
0.2
3.5
3.9
3.4
rdir–dir (%)
3.5
4.5
3.5
8.5
54.5
15.5
10
25
54.5
22
28.5
50
56
54.5
6.5
7
5.5
3
3
11
3.5
42
24
26.5
23
21
9
3.5
3
8.5
51
42
50
39.5
30.5
49
36.5
43.5
31.5
27.5
50
52
3.5
3.5
2.5
Dispersion angle d
12
46.
47.
48.
50.
51.
52.
53.
54.
55.
56.
57.
58.
59.
60.
61.
62.
63.
64.
65.
66.
67.
68.
69.
70.
71.
72.
74.
75.
76.
77.
78.
79.
80.
81.
82.
83.
84.
85.
86.
87.
88.
89.
90.
91.
92.
Name
Table 2 (continued )
ARTICLE IN PRESS
M. Bodart et al. / Building and Environment ] (]]]]) ]]]–]]]
ARTICLE IN PRESS
M. Bodart et al. / Building and Environment ] (]]]]) ]]]–]]]
the closest RAL or NCS value, together with the colour
difference with the full scale material;
for a majority of materials, a view of a reference office
with walls made of the chosen material;
three animations showing the impact of the wall
reflectance on the light distribution;
the possibility of converting the characteristics of the
full-size material into RADIANCE input.
Moreover, for both the possibilities, the user has access
to the results of black-and-white RADIANCE simulations
in a reference room. It enables the user to either visualise
the room with illuminance isocurves or see the illuminance
distribution on a section in the middle of the room,
perpendicular to the window.
13
Widely scattering mock-up material are currently very
uncommon. However, we explored two techniques to
modify the reflection mode of diffuse cardboard to create
widely scattering mock-up materials.
In order to help the model builder in his or her choice of
materials for the scale model, we developed a web tool that
automatically compares the photometric characteristics of
a real internal building material with many scale-model
materials, for several possible objectives of the daylighting
study. This comparison is based on precise laboratory
measurements on 162 tested materials (the 92 materials
presented in the Appendix and 70 additional scale-model
materials). The web tool is regularly updated and the
photometric characteristics of shading devices will soon be
added to it.
4.4. Future developments
References
The database is constantly developed by the addition of
opaque materials and the material introduced by users. A
record of the name of the material is also entered manually
in the database. In the future, we intend to characterise
other glazings and, in particular, translucent (diffusing)
materials. It is also planned to work on shading devices.
5. Conclusion
Whatever the technique used, correct evaluation of the
photometric characteristics of internal building materials is
critical for the evaluation of internal daylight luminances
and illuminances. In this paper, we have selected the
minimal parameters that should be evaluated to achieve
accurate simulations or measurements of daylight illumination in scale models.
The paper shows that the only way to evaluate these
parameters accurately is to use precise measurement
devices. When this is not possible, several simpler methods
can be used with care. The best method for the evaluation
of the reflectance of the material is the CIBSE
method based on luminance and illuminance measurements. We developed a simple method for evaluating the
reflection mode, but this needs further validation. The
transmission mode could probably be evaluated using
the same procedure, but more research is needed to
confirm this.
Measurements on 92 full-size and scale-model materials
have shown that, while many simulation programs assume
that internal materials are diffused, less than one-third of
them are in reality perfectly diffuse. Around 10% were
narrowly scattering, while about 30% were widely scattering and the same proportion were nearly specular. Most of
the classical cardboards used in scale models are diffused.
Special shiny cardboards are usually narrowly scattering.
[1] Baker N, Steemers K. Daylight design of buildings. London: James
and James; 2002.
[2] Baker N, Fanchiotti A, Steemers K. Daylighting in architecture: a
European reference book. London: James and James; 1993.
[3] Robbins CL. Daylighting: design and analysis. New York: Van
Nostrand Reinhold; 1986.
[4] Schiler M. Simulating daylight with architectural models. US
Department of Energy, 1987.
[5] Cannon-Brookes SWA. Simple scale models for daylighting design:
analysis of sources of error in illuminance prediction. Lighting
Research and Technology 1997;29(3):135–42.
[6] Thanachareonkit A, Scartezzini JL, Andersen M. Comparing daylighting performance assessment of buildings in scale models and test
modules. Solar Energy 2005;79:168–82.
[7] Ward G, Shakespeare R. Rendering with RADIANCE; the art of
science of lighting visualization. Davis: Space and Light; 2003.
[8] Commission Internationale de l’Eclairage (CIE). Colorimetry.
Technical Report. 2nd ed. Wien; 1986.
[9] Minolta. Precise colour communication: colour control from feeling
to instrumentation, 1994.
[10] European Committee for Standardisation. Blinds and shutters:
thermal and visual comfort—test and calculation methods. CEN/
TC 33 N 1825, 2006.
[11] Lambert JH. Photometria 1760 [in Latin].
[12] Christie JS. Evaluation of the attribute of appearance called gloss.
CIE Journal 1986;5(2):41–56.
[13] Obein G, Knoblauch K, Viénot F. Difference scaling of gloss:
nonlinearity, binocularity and constancy. Journal of Vision
2004;4:711–20.
[14] Pointer MR. Measuring visual appearance: a framework for the
future. Project 2.3: measurement of appearance. Teddington:
National Physical Laboratory; 2003.
[15] Gardner B. Product catalogue on appearance measurement,
2005–2006.
[16] BS EN 410. Glass in building: the determination of the luminous and
solar characteristics of glazing, 1998.
[17] Fontoynont M. Daylight performance of buildings. London: James
and James; 1999.
[18] Chartered Institution of Building Services Engineers (CIBSE).
Lighting guide 11: surface reflectance and colour. London: Chartered
Institution of Building Services Engineers; 2001.
Please cite this article as: Bodart M, et al. Photometry and colorimetry characterisation of materials in daylighting evaluation tools. Building and
Environment (2008), doi:10.1016/j.buildenv.2007.12.006