Standards and Performance Indicators for Surgical Luminaires

Abstract

The illumination performance of surgical luminaires is quantified by performance indicators defined in an international standard (IEC 2000). The remaining maximum illuminance in relevant situations, the light-field size, and the spectral characteristics are performance indicators used by hospitals as input for luminaire opting processes. Industry however focuses on illuminance when communicating with health care professionals. The aim of this study is to evaluate whether these standards are sufficient to describe luminaire performance, especially for modern LED lighting technology. Illuminance distribution and spectrum measurements were performed on 5 different state-of-the-art (LED) surgical luminaires. The results showed that changing situations not only changed the maximum illuminance but also changed the light-field sizes and shapes, introducing substantial differences between luminaires. Moreover, colored cast shadows and light color variations across the light-field were observed for 3 luminaires using differently colored light emitting diodes (LEDs). Both the changing light-field sizes and shapes, and the cast shadows and light color variations for LED luminaires are not covered by the current standard. The standard should therefore be extended to incorporate these aspects, especially for such a high-end application as surgical lighting.

Full text

Standards and Performance Indicators for
Surgical Luminaires
Arjan J. Knulst
1
, Laurents P. S. Stassen
1,2
, Cornelis A. Grimbergen
2,3
, and
Jenny Dankelman
1
Abstract—The illumination performance of surgical luminaires is
quantified by performance indicators defined in an international
standard (IEC 2000). The remaining maximum illuminance in relevant
situations, the light-field size, and the spectral characteristics are
performance indicators used by hospitals as input for luminaire opting
processes. Industry however focuses on illuminance when
communicating with health care professionals. The aim of this study is
to evaluate whether these standards are sufficient to describe
luminaire performance, especially for modern LED lighting technology.
Illuminance distribution and spectrum measurements were performed
on 5 different state-of-the-art (LED) surgical luminaires. The results
showed that changing situations not only changed the maximum
illuminance but also changed the light-field sizes and shapes,
introducing substantial differences between luminaires. Moreover,
colored cast shadows and light color variations across the light-field
were observed for 3 luminaires using differently colored light emitting
diodes (LEDs). Both the changing light-field sizes and shapes, and the
cast shadows and light color variations for LED luminaires are not
covered by the current standard. The standard should therefore be
extended to incorporate these aspects, especially for such a high-end
application as surgical lighting.
Keywords—standards, performance, surgical luminaires, LED,
shadow.
1. Delft University of Technology, Faculty of Mechanical, Maritime and Materials Engineering,
Department of BioMechanical Engineering, Mekelweg 2, 2628CD Delft, The Netherlands; 2.
Reinier de Graaf Hospital Delft, Reinier de Graafweg 3 - 11, 2625 AD Delft, The Netherlands; 3.
Academical Medical Center, Biomedical Engineering & Physics, Meibergdreef 9, 1105AZ
Amsterdam, The Netherlands
Correspondence: Arjan J. Knulst, MSc., Delft University of Technology, Faculty of Mechanical,
Maritime and Materials Engineering, Department of BioMechanical Engineering, Mekelweg 2,
2628CD Delft, The Netherlands, A.J.Knulst@TUDelft.nl, 0031 15 2785625
Grant support: This research is supported by the Dutch Technology Foundation STW, applied
science division of NWO and the Technology Program of the Ministry of Economic Affairs.
LEUKOS VOL 6 NO 1 JULY 2009 PAGES 37– 49
© 2009 The Illuminating Engineering Society of North America
doi: 10.1582/LEUKOS.2009.06.01002
37

1 INTRODUCTION
Surgical luminaires are supposed to provide high quality, bright, comfortable
and true color illumination of a wound, even in difficult situations like deep
cavities, and with the surgeons’ heads and hands situated between the light
source and the surgical site (Beck 1978, 1981; Dain, Hood and others, 1998;
Gregory 1987; Hadrot 1999; Loonam and Millis 2003; Quebbeman 1993).
Traditionally, these luminaires are most commonly designed as a large hemi-
spherical reflector that contains a halogen or high intensity discharge light
source. The reflector focuses the light to the desired focal point at the surgical
field, provides sufficient deep cavity penetration, and minimizes the effect of
shadows cast by objects between the luminaire and the surgical field. A relative
new lighting technology is the white Light Emitting Diode (LED). Since white
light LED technology has made major improvements, many manufacturers
implement state-of-the-art LED technology in surgical luminaires. Although
white LED technology is fairly new, many fundamentally different luminaire
designs have been developed. However, the concept of these designs has not yet
been fundamentally tested.
Performance measures for the illumination characteristics of surgical lumi-
naires are defined by the international standard for surgical luminaires (IEC
2000). This standard describes a series of worldwide accepted measures that
define the illumination characteristics at the position of the surgical site, in
different well-defined scenarios. The different scenarios are simulated and
simplified representations of situations like deep wounds or surgeon’s heads
obstructing the light beam between the luminaire and the wound. Typical
illumination characteristics that are defined in the standard are the maximum
illuminance at the center of the light-field and the light-field size. The light-field
size needs to be measured in the unobstructed scenario only (IEC 2000), with
the luminaire set to the smallest and largest illuminated field possible. The
remaining maximum illuminance in these different scenarios as percentage of
the unobstructed scenario is defined as a measure for luminaire performance
(IEC 2000). The standard further describes colorimetric tests to be performed at
maximum illuminance, in the center of the light-field. The light beam’s corre-
lated color temperature, color rendering index (CRI) R
a
, and chromaticity
co-ordinates should lie within defined boundaries (IEC 2000). Ideally, all param-
eters mentioned above are presented in the product files by the manufacturers,
to be used by hospitals as input for luminaire opting processes. Remarkably, in
communication with health care professionals, manufacturers and their repre-
sentatives often only mention maximal illuminance.
The primary aim of this study is to evaluate whether the change in maximum
illuminance in different scenarios plus the smallest and largest available light-
field diameter in the unobstructed situation are sufficient parameters to de-
scribe the illuminance performance of the luminaire. For example, it might be
that the whole illuminance distribution changes during varying scenarios,
thereby changing the size and even the shape of the light-field. Therefore, we
measured the whole illuminance distribution during different scenarios to
obtain the changes in light-field size and shape.
The secondary aim of this study is to evaluate whether measuring the spectral
characteristics at the light-field center is sufficient to describe the spectral
characteristics for LED luminaires. As LED luminaires contain multiple light
sources, the spectral properties might vary across the illuminated field. There-
LEUKOS VOL 6 NO 1 JULY 2009 PAGES 37– 49
38

fore, spectral properties were measured at different locations in the illuminated
field.
2 MATERIALS AND METHODS
2.1 SETUP
The IEC standard defines illumination measurements that should be performed
in the measurement plane, a horizontal plane 1 m below the luminaire, centered
on the point of maximum illuminance, the Light Field Centre “LFC”. The
illuminance at the LFC is called the central illuminance “E
c
”. The illuminance
distribution on the measurement plane should be measured along four lines
through the LFC lying 45° apart. On each of these lines, the points where the
illuminance reaches 10 percent of the central illuminance should be determined.
The distance between those points is by definition the light-field diameter.
Ideally, the illumination distribution should be radially decreasing, from the LFC
outward.
A setup has been developed (Fig. 1) to measure the illuminance distribution
according to the IEC standard. A flat plate (60x60 cm) was used as illumination
measurement plane. Four lines (numbered 1 to 4) were drawn through the
center of the plate, 45° apart. Along each of the four lines an aluminum ruler
could be fixated. A calibrated photometer with a 13 mm sensor surface was
Fig. 1. The setup designed for
illuminance measurements. Di-
mensions are in cm’s. The Light
Field Centre (LFC) is located in
the measurement plane at the
intersection point of the lines 1
to 4.
LEUKOS VOL 6 NO 1 JULY 2009 PAGES 37– 49
39

secured to a slider that could be moved along the ruler to measure the
illuminance. The illuminance was measured along each line, from the center
outward with a radial sample distance of 10 mm. A deep wound was simulated
by placing a black PVC tube over the photometer as specified in the standard
(IEC 2000). The heads of two surgeons were simulated by two circular masks,
dimensions as specified in the standard (IEC 2000), placed 60 cm above the
photometer head, and above line 3. The lowest light emitting surface of the
luminaire was placed 1 m above the photometer head, and the vertical axis of the
luminaire was centered above the measurement plane center such that the
luminaire’s LFC coincides with the measurement plane center, and the axis of
the boom-luminaire connection was placed in parallel to line 1.
A calibrated spectroradiometer was standing by for spectral measurements in
case unexpected colorimetric observations would happen. The luminaire was
positioned 1 m above the sensor head during the spectral measurements. Both
measured data from the spectroradiometer and the photometer were stored in a
computer.
2.2 SCENARIOS AND LUMINAIRES
To evaluate the illuminance distribution, eight scenarios were defined as differ-
ent configurations of the measurement setup (Fig. 1). Scenarios S1 to S4 had the
luminaire set to the smallest light-field, and scenarios L1 to L4 had the luminaire
set to the largest light-field. The scenarios and their definition are listed in Table
1. The illuminance distribution is measured over each line for each scenario. The
standard (IEC 2000) requires reporting of the absolute maximum illuminance
for S1, the maximum illuminances relative to E
c
in Scenario S1 for S1 to S4, and
the absolute light-field diameters for S1 and L1.
Five different luminaire types were measured in operating rooms of two
hospitals. One luminaire had a halogen light source, one had single color LEDs,
one had multiple color chips in one LED unit, and two luminaires had a mixture
of colored LEDs. During the measurements the surgical luminaire was the only
activated light source. The measured luminaires are shown in Fig. 2, and are
listed in Table 2.
2.3 DATA PROCESSING
For each luminaire the measured illuminances were processed using software
developed in Matlab 7.5 (The Mathworks Inc, 2007). The data processing
contained the following steps for each luminaire:
1. For Scenario S1: Determine the maximum illuminance E
c
and normalize the
illuminance distribution such that the point of maximum illuminance is 100
TABLE 1.
Definition of the Eight
Scenarios. (a) A Tube was
Placed, (b) Two Masks were
Placed, (c) the Luminaire’s
Light Field Diameter was Set
to the Smallest or the Largest
Possible
Scenario Tube (a) Two Masks (b) Light Field Diameter (c)
Small 1 (S1) No No Smallest
Small 2 (S2) Yes No Smallest
Small 3 (S3) No Yes Smallest
Small 4 (S4) Yes Yes Smallest
Large 1 (L1) No No Largest
Large 2 (L2) Yes No Largest
Large 3 (L3) No Yes Largest
Large 4 (L4) Yes Yes Largest
LEUKOS VOL 6 NO 1 JULY 2009 PAGES 37– 49
40

percent E
c
(IEC 2000). This normalized E
c
is the reference value for the Scenarios
S2 to L4.
2. For each scenario: Interpolate the illuminance distribution along each line
to determine the radial locations where the illuminance is 50 percent or 10
percent E
c
(IEC 2000). The distances between those points of 50 percent and 10
percent E
c
are the light-field diameters d
50,i
and d
10,i
along line irespectively.
Calculate the light-field diameters d
50
and d
10
in each scenario by averaging d
50,i
and d
10,i
over the 4 lines (IEC 2000).
3. For each scenario: Calculate the variation of the light-field shape by dividing
the maximum diameter by the minimum diameter in the light-field found on any
of the 4 lines.
4. For each scenario: Normalize the light-field diameters with respect to the
light-field diameters found in Scenario S1.
The CIE 1931 (x,y) chromaticity co-ordinates, the CRI R
a
(CRI Ra), and the
correlated color temperature (CCT) were computed from the measured spectral
data, according to their definitions (Schanda 2007).
Fig. 2. An overview of the lu-
minaires A
1
to E
1
measured in
this study, as defined in Table
2. Both D
1
and E
1
consist of
four or five segments, slightly
rotatable to adjust the lumi-
naire’s focus. The colored
LEDs of luminaires D
1
and E
1
are clearly visible.
TABLE 2.
Overview of the Measured
Luminaires and Four
Relevant Properties
Type Label Light Source
Technology
Adjustable Colour
Temperature Focusable Rotational
Symmetric
Berchtold C950 A
1
Halogen No Yes Yes
Maquet
PowerLED
700
B
1
Cool white LEDs No No Yes
KLS-Martin
MarLED V16
C
1
Multicolour chip
LEDs
Yes: min. 3800K
max. 4800K
Yes, and option
to create oval
shaped light
field
No
Trumpf iLED 5 D
1
Mixture of white
and coloured
LEDs
Yes: min. 3500K
max. 5000K
Yes No
Trilux Aurinio
L160
E
1
Mixture of white
and coloured
LEDs
No Yes No
LEUKOS VOL 6 NO 1 JULY 2009 PAGES 37– 49
41

3 RESULTS
3.1 ILLUMINANCE MEASUREMENT RESULTS
Figure 3 presents the influence of the different scenarios on the maximum
illuminance E
c
, the light-field diameter d
10
, and the light-field diameter d
50
for
each luminaire relative to that luminaire in Scenario S1. Fig. 3a shows that
changing scenarios resulted in changing maximum illuminances for the differ-
ent luminaires. The maximum illuminance is therefore shown to be scenario
Fig. 3. (a). The normalized
maximum illuminance E
c
, (b)
the normalized light-field di-
ameter d
10
and (c) the normal-
ized light-field diameter d
50
in
various scenarios. The legend
labels the data points to lumi-
naire types. As luminaire B
1
had no option to focus the
light beam, its values for Sce-
nario L1 to L4 are set to zero.
The legend labels the data to
luminaire types (Table 2).
LEUKOS VOL 6 NO 1 JULY 2009 PAGES 37– 49
42

dependant. Fig. 3b visualizes that the d
10
light-field diameters did change during
these varying scenarios, and moreover that, especially in the large light-field
scenarios (L1 to L4), significant differences in light-field sizes between lumi-
naires were introduced. The d
10
light-field diameter was shown to be scenario
dependant. Fig. 3c displays the change of the d
50
light-field diameters. Note that
from scenario S4 to L4 hardly any luminaire was able to generate a light spot
having at least 50 percent of the maximum illuminance measured in scenario
S1. It is illustrated clearly in Fig. 3 that the maximum illuminance is not the only
parameter that changes during the various measured scenarios and that also
the light-field diameters d
10
and d
50
are scenario dependant.
Figure 4 shows the normalized changes of the light-field diameters d
10
and d
50
as function of the corresponding normalized changes of illuminance E
c
. The
standard (IEC 2000) only requires measuring and reporting the change of
illuminance in the different scenarios. This implies that change in light-field
diameter is either zero or equal to the change of illuminance. Therefore, Fig. 4
shows the line of equal changes in light-field diameter to changes in illuminance
d⫽E
c
and the line of constant light-field diameters d⫽100 percent. Fig. 4a and
4b visualize the relation between the changing illuminance and the changing
light-field diameter d
10
for both the small and large light-field size. The figure
clearly shows that the d
10
light-field diameter did change as the data points are
not on the line d
10
⫽100 percent, and that the diameter changes were less than
the illuminance changes, as all data points are above the line d
10
⫽E
c
. These
changes were found for both the small and the large light-field size. Fig. 4c and
4 days visualize the same aspects as Fig. 4a and 4b but now for the d
50
light-field
diameter. Here it is shown that d
50
was also subject to changes under varying
scenarios. The changes of the d
50
light-field diameter were larger than the
changes in illuminance, as most data points are situated below the line d
50
⫽E
c
.
As Fig. 3 supported the finding that the maximum illuminance E
c
and the
light-field diameters d
10
and d
50
were scenario dependant, Fig. 4 demonstrates
that there was no evident relation between the illuminance and the light-field
diameters.
Figure 5 represents for each measured luminaire the variation of the light-field
geometry for varying scenarios, displayed as the ratio of the maximum diameter
over the minimum diameter found within the d
10
light-field during that scenario.
The variation of the light-field shape and the ratio differences between lumi-
naires are most prominently visible for the large diameter Scenarios L1 to L4.
The differences between the luminaires are the largest for Scenario L4, with the
smallest and the largest ratio being 1.02 and 3.27 respectively.
3.2 UNEXPECTED RESULTS
Besides the measured illuminance distributions a number of unexpected obser-
vations were noted. Fig. 6 shows a photograph of the illumination distribution of
luminaire E
1
. Multiple illuminance peaks at the illuminated measurement
surface are clearly visible. This phenomenon occurred when the focus of both
luminaires D
1
and E
1
were adapted to the large light-field. Luminaires D
1
and E
1
both consist of (four or five) segments that are completely packed with LEDs (Fig.
2). Adapting the luminaire’s focus is done by slightly adapting the orientation of
those individual segments. Each luminating segment creates its own illumi-
nance distribution. For the smallest light-field, the summated illuminance
peaks of all individual segments overlap, thereby creating a single illuminance
peak. Adapting the luminaire’s focus shifts the illuminance peaks of each
LEUKOS VOL 6 NO 1 JULY 2009 PAGES 37– 49
43

segment outward from the LFC, thereby creating the observed multiple illumi-
nance peaks.
Light color variations in the illuminated field were noticed when using
luminaires that contain differently colored LEDs, either as separate LED units or
as differently colored chips in one LED unit. During the first encounter with
such a luminaire (C
1
) the color variations were noticed with the bare eye,
showing reddish and bluish areas within the illuminated field. Table 3 shows the
spectral characteristics of the luminaires with differently colored LEDs, derived
from the measured spectra. These luminaires were measured at different
Fig. 4. (a) Light-field diameter
d
10
vs. the maximum illumi-
nance, for the smallest light-
field size, (b) light-field diam-
eter d
10
vs. the maximum
illuminance, for the largest
light-field size, (c) light-field
diameter d
50
vs. the maximum
illuminance, for the smallest
light-field size, (d) light-field
diameter d
50
vs. the maximum
illuminance, for the largest
light-field. The dashed line is
the relation dⴝE
c
. The legend
labels the data to luminaire
types (Table 2).
LEUKOS VOL 6 NO 1 JULY 2009 PAGES 37– 49
44

Fig. 5. Variations of the light-
field geometry in different sce-
narios, represented as ratio of
the maximum and minimum
light-field diameter found within
the d
10
light-field. A ratio of one
represents an identical sized
light-field shape in all mea-
sured directions.
Fig. 6. Photograph of multiple
illuminance peaks on the mea-
surement surface, as seen
with luminaires D
1
and E
1
when using the large light-
field setting of the luminaire.
Just visible are the 4 measure-
ment lines drawn at the
lighted surface. Note that the
centers of the illuminance
peaks lie next to these lines.
LEUKOS VOL 6 NO 1 JULY 2009 PAGES 37– 49
45

luminaire settings and at different locations within the illuminated field. Note
that these spectral characteristics were different when the measurements were
performed at different locations in the light-field, or in different settings of the
light-field size.
Another aspect of using differently colored LEDs in luminaires is shown in Fig.
7. This phenomenon was observed when shadow casting objects were placed
between the luminaire and the illuminated field. The cast shadows showed a
wide variation of colors, varying from reddish for C
1
, from violet to orange for D
1
,
and from green to red for E
1
(Fig. 7). The variation in colors of cast shadows
depends on the selected colored LEDs in the luminaire.
4 DISCUSSION AND ANALYSIS
The primary aim of this study was to evaluate whether presenting the maximum
illuminance E
c
for Scenario S1, the remaining relative illuminances for Scenar-
ios S2 to S4, and the absolute light-field diameters d
10
and d
50
for both
Scenarios S1 and L1, as laid down in the current standard (IEC 2000), results in
a sufficient description of surgical luminaire performance. It was shown that,
besides the maximum illuminance, also the light-field diameters and even the
shape of the light-field did vary considerably in the various measured scenarios,
both for the small and the large light-field. Measuring and presenting only the
illumination characteristics, as currently required by the international stan-
dard, is therefore not enough to assess the performance of a luminaire for the
scenarios measured in this study. In order to supply proper and complete
information to users, the international standard should require measurement
and publication of the light-field diameters and shapes in the different scenarios
in addition to the reported change in maximum illumination, both for the small
and large light-field size. Stating these issues directly in the standard provides
more incentive to address these in the optical design.
The secondary aim of this study was to evaluate whether measuring the
spectral characteristics at the light-field center is sufficient to describe the
spectral characteristics for LED luminaires. In this study it was demonstrated
that the spectral characteristics may vary across the illuminated area. These
variations are shown to be present in luminaires that contain combinations of
different colored LEDs. The variations occurred as slight color differences across
the illuminated area or as clearly distinguishable differently colored cast shad-
ows. Since true color rendering of tissue is an important aspect of surgical
TABLE 3.
Light Colour Variations for
Luminaires Containing
Differently Coloured LEDs.
All Luminaires were
Measured in the Small Light
Field Diameter, and at
Maximum Power, Unless
Stated Otherwise. Shown are
the Correlated Colour
Temperature (CCT), the
Colour Rendering Index (CRI
R
a
), and the Colour Co-
ordinates According to the
CIE 1931 (x, y) Chromaticity
Co-ordinate System
Luminaire Setling Location CCT [K] CRI R
a
CIE 1931 Colour
Co-ordinates
(x, y)
C
1
3800K 2 cm outside LFC 3740 93.3 (0.3798, 0.3450)
C
1
3800K, large
light field
As previous 3906 92.8 (0.3722, 0.3372)
C
1
3800K, oval
along Line 1
As previous 3802 92.6 (0.3760, 0.3394)
C
1
3800K, oval
along Line 3
As previous 4021 93.2 (0.3688, 0.3367)
D
1
5000K LFC 5292 93.2 (0.3369, 0.3394)
D
1
5000K 5 cm outside LFC 4600 94.2 (0.3564, 0.3570)
E
1
LFC 4713 93.5 (0.3547, 0.3681)
E
1
Large light field
diameter
As previous 4992 85.9 (0.3471, 0.3748)
LEUKOS VOL 6 NO 1 JULY 2009 PAGES 37– 49
46

Fig. 7. An example of colored
shadows projected on a white
sheet of paper, (a) violet, blue,
green and orange colored
shadows by luminaire D
1
, and
(b) red and green shadows by
luminaire E
1
. The shadow
casting object was a human
hand.
LEUKOS VOL 6 NO 1 JULY 2009 PAGES 37– 49
47

lighting (Dain, Hood and others, 1998; Hadrot 1999) the spectral properties of
the light source are important factors for luminaire quality. Tissue color
rendering and correct tissue recognition might be more difficult in those
situations. Ideally, the standard should provide color variation limitations, both
under various shadow conditions and across the full pattern area, to set specific,
generally accepted design goals for manufacturers, and to raise awareness with
potential customers.
During the illuminance measurements multiple illuminance peaks were ob-
served for luminaires D
1
and E
1
. Apparently, the standard that requires a
radially tapered illuminance distribution (IEC 2000) is not met for the large
light-field. Note that not all these illuminance peaks were located on one of our
four measurement lines. This might have influenced the outcomes of the
computed light-field diameters and the maximum illuminances for these lumi-
naires in Scenarios L1 to L4. Although luminaires D
1
and E
1
were marketed as
having a small and large light-field adjustment option, this feature might be
better suitable to provide variable focus depths of the light beam.
The international standard requires multiple measurements with the two
masks placed above each of the four measurement lines, to average the
differences that might occur when asymmetric luminaires are measured (IEC
2000). The standard also requires placement of one mask above the center of the
light-field. For this study, only 2 masks above Line 3 were used. Luminaires that
are not rotational symmetric, like C
1
,D
1
or E
1
, may have been penalized or
favored because of such simplifications. However, a quick check on luminaire C
1
with the masks placed above Line 3 and above Line 1 showed hardly any
difference in light-field diameter d
10
. A 20 percent difference was observed for the
d
50
light-field diameter and the shape of the light-field was slightly affected. The
presented data should not be used to make a definite judgment between
luminaires, but as a starting point to discuss the current standard on surgical
lighting.
5 CONCLUSIONS
The results of this study show that the current international standard for
surgical luminaires is not covering all possibly relevant aspects that provide
useful information on the illumination performance of these luminaires. Infor-
mation on the light-field diameters and light-field shape in different simulated
surgical conditions as well as information on the spectral variation and the
appearance of colored shadows using colored LEDs is currently insufficiently
provided. Adding obligatory measurements to the standard to quantify the
change of the light-field diameters and the light-field geometry would provide
more complete information for hospitals that are opting for new surgical
luminaires. Furthermore, the standard should be extended with guidelines on
the use of colored LEDs in surgical luminaires to minimize the negative effects
(like colored shadows and light color variations) that are introduced by using
this technology. Manufacturers should be forced to minimize these effects before
launching new products on high-end markets like operating room lighting.
ACKNOWLEDGMENTS
The authors would like to thank the Academic Medical Centre in Amsterdam for
the opportunity to perform measurements on the LED luminaires, and would
like to thank VSL, (the National Metrology Institute of the Netherlands, formerly
LEUKOS VOL 6 NO 1 JULY 2009 PAGES 37– 49
48

known as NMi Van Swinden Laboratory), for the metrological support to this
study.
This research is supported by the Dutch Technology Foundation STW, applied
science division of NWO and the Technology Program of the Ministry of Economic
Affairs.
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