490 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 59, NO. 3, JUNE 2012
Reﬂectivity Spectra for Commonly Used Reﬂectors
Abstract—Monte Carlo simulations play an important role in
developing and evaluating the performance of radiation detection
systems. To accurately model a reﬂector in an optical Monte Carlo
simulation, the reﬂector’s spectral response has to be known.
We have measured the reﬂection coefﬁcient for many commonly
used reﬂectors for wavelengths from 250 nm to 800 nm. The
reﬂectors were also screened for ﬂuorescence and angular distri-
bution changes with wavelength. The reﬂectors examined in this
work include several polytetraﬂuoroethylene (PTFE) reﬂectors,
Spectralon, GORE diffuse reﬂector, titanium dioxide paint, mag-
nesium oxide, nitrocellulose ﬁlter paper, Tyvek paper, Lumirror,
Melinex, ESR ﬁlms, and aluminum foil. All PTFE ﬁlms exhibited
decreasing reﬂectivity with longer wavelengths due to transmis-
sion. To achieve
reﬂectivity in the 380 to 500 nm range,
the PTFE ﬁlms have to be at least 0.5 mm thick—nitrocellulose
is a good alternative if a thin diffuse reﬂector is needed. Several
of the reﬂectors have sharp declines in reﬂectivity below a cut-off
(420 nm), ESR ﬁlm (395 nm), nitro-
cellulose (330 nm), Lumirror (325 nm), and Melinex (325 nm).
PTFE-like reﬂectors were the only examined reﬂectors that had
reﬂectivity above 0.90 for wavelengths below 300 nm. Lumirror,
Melinex, and ESR ﬁlm exhibited ﬂuorescence. Lumirror and
Melinex are excited by wavelengths between 320 and 420 nm and
have their emission peaks located at 440 nm, while ESR ﬁlm is
excited by wavelengths below 400 nm and the emission peak is
located at 430 nm. Lumirror and Melinex also exhibited changing
angular distributions with wavelength.
Index Terms—Fluorescence, Lambertian reﬂection, reﬂection
coefﬁcient, specular reﬂection.
CINTILLATING crystals emit light in a broad range of
wavelengths, with many of them having their peak emis-
sions located between 375 and 480 nm –. In order to con-
vert this optical signal into a sizable electrical signal, the light
has to be directed into a photodetector by the means of sur-
rounding the scintillating crystal with a reﬂective material. This
reﬂector should maximize the light collection and the reﬂector
has to be chosen to provide high reﬂection coefﬁcients at the
scintillator’s emission wavelengths.
Monte Carlo simulations play an important role in developing
and evaluating the performance of radiation detection systems.
The simulations offer a way of evaluating the system as a whole
Manuscript received September 23, 2011; revised December 14, 2011; ac-
cepted January 04, 2012. Date of publication March 12, 2012; date of current
version June 12, 2012. This work was supported by the Director, Ofﬁce of Sci-
ence, Ofﬁce of Biological and Environmental Research, Biological Systems
Science Division of the U.S. Department of Energy under Contract DE-AC02-
The author is with the Lawrence Berkeley National Laboratory, Berkeley, CA
94720 USA (e-mail: email@example.com).
Color versions of one or more of the ﬁgures in this paper are available online
Digital Object Identiﬁer 10.1109/TNS.2012.2183385
Fig. 1. Schematic of an integrating sphere. Note that the light enters at a slight
angle in relation to the examined sample
in order to be able to detect the
specular reﬂection from the sample. The inside surface of the integrating sphere
is coated with a diffuse reﬂector creating a uniform angular distribution at the
photodetector. To force the light to reﬂect multiple times within the integrated
sphere, bafﬂes (i.e., light barriers) are located between the illuminated sample
and the photodetector as well between the in-port and the photodetector, elimi-
nating any direct optical paths.
by exploring system parameters without the added cost of con-
structing the system or any of its components. For these simula-
tions, the reﬂection’s angular distribution  and the reﬂection
coefﬁcient (as a function of wavelength) has to be known for the
results to be accurate.
Reﬂectivity measurements are generally performed with
an integrating sphere, where the entire angular distribution
of the reﬂected light is collected and thus contributes to the
photodetector response. As the light is reﬂected many times
within the integrating sphere before detection, it is important
to coat the inside of the sphere with a highly reﬂective “white”
material. The reﬂection data acquired using this technique do
not always provide sufﬁcient information to perform an accu-
rate Monte Carlo simulation—some reﬂectors are ﬂuorescent
or change their angular reﬂectance distributions with incidence
angle. Integrating sphere materials include barium sulfate
–, magnesium oxide (MgO) , –, and
polytetraﬂuorethylene (PTFE) based reﬂectors –, and
these reﬂector materials have been extensively studied. Other
reﬂectors that are frequently used in optical systems include
paint , , , Tyvek
, , ESR (Enhanced Specular Reﬂector) ﬁlm , ,
and Spectralon , . In addition, some commonly used
reﬂectors have not been reported on in the literature, including
, and Melinex
The aim of this work is to measure the reﬂection coefﬁcients
as a function of wavelength for the most common reﬂectors used
in the radiation detection ﬁeld, and to screen them for ﬂuores-
cence and angular distribution changes with wavelength.
We previously measured the angular distribution of the
reﬂected light at a ﬁxed wavelength (440 nm)  and found
0018-9499/$31.00 © 2012 IEEE
JANECEK: REFLECTIVITY SPECTRA FOR COMMONLY USED REFLECTORS 491
ESR ﬁlm and aluminum foil to be specular reﬂectors. The
angular distributions of Spectralon, PFTE ﬁlms, GORE
fuse reﬂector, magnesium oxide, titanium dioxide paint, and
nitrocellulose ﬁlter are best described as Lambertian (diffuse)
reﬂectors. All Lambertian reﬂectors that were measured had
Lambertian component at low incidence
, but a specular component appeared at high
incidence angles. Lumirror
, and Tyvek
were measured to have reﬂection distributions that could not
be described by a linear combination of specular and diffuse
reﬂection distributions .
The angular reﬂection distribution measurements for com-
monly used reﬂectors  were performed at 440 nm, dictated
by the use of a 440-nm solid-state laser [CrystaLaser
NV], and we determined the reﬂection coefﬁcient of the reﬂec-
tors by integrating the reﬂected light collection over the full
of solid angle. However, the reﬂection coefﬁcient was only de-
termined at one single wavelength. In this work, we measure the
relative reﬂection coefﬁcient for commonly used reﬂectors over
a large range of wavelengths. The results are normalized to the
reﬂectivity of white reﬂection standards made out of Spectralon.
There are several confounding effects when measuring the re-
ﬂection coefﬁcient as a function of wavelength for a reﬂector: 1)
the reﬂector can exhibit ﬂuorescence, shifting some of the inci-
dent light to a longer wavelength, and 2) the angular distribution
can change with wavelength, so measuring the reﬂectivity at a
speciﬁc angle can give misleading results. Because of this, we
have in this work 1) screened all reﬂectors for ﬂuorescence, 2)
compared the reﬂectors’ specular and diffuse reﬂection behavior
over wavelength, and 3) measured the reﬂectivity with an inte-
grating sphere (which is insensitive to changes in the angular
distribution of the reﬂectivity).
The reﬂectors we examined in this work are summarized in
Table I. The reﬂectors’ reﬂection coefﬁcients at 440 nm were de-
termined from literature values or manufacturer data—if avail-
able—or from our previous measurements . The thicknesses
of the reﬂectors were measured and are reported on in the table.
To achieve adequate thickness for the reﬂectivity measure-
ments, several of the reﬂectors were measured in multiple
layers, including ACE Teﬂon
tape, glossy PTFE tape,
ﬁlm, and Tyvek
paper. The magnesium oxide
powder was painted in a 1-mm thick layer onto a black plastic
holder by ﬁrst dissolving the powder with ethanol. The ni-
trocellulose reﬂector was examined in the same manner as
described in  (i.e., by measuring the angular distribution over
of solid angle and integrating over the entire light
distribution), however, after the original paper had already been
published. The conclusions from these measurements were that
the nitrocellulose reﬂector is an excellent reﬂector with
of the light being reﬂected in a Lambertian light distribution,
and the reﬂection coefﬁcient was determined to be 103% of the
reﬂection coefﬁcient for four layers of ACE Teﬂon
ESR ﬁlm was examined for both the front and the back surfaces
after all protective ﬁlms and glue had been removed (with
ethanol). The ESR ﬁlm front-side is in this work deﬁned as the
side of the ﬁlm that originally had a protective ﬁlm covering
it, but with no glue layer, and the backside is deﬁned as the
side that originally contained a glue layer as well as a protec-
tive ﬁlm. Measurements were performed with both VM2000
and VM3000 ESR ﬁlms. The Spectralon sample, which was
bought from Ocean Optics as a white reﬂection standard for
this project, is manufactured by Labsphere
and has the same
model number as Labsphere’s SRS-99 (Spectralon Reﬂection
Standard) white reﬂection standard. The only physical differ-
ence between these two standards is their diameters—25.4 mm
for the WS-1-LS versus 50.8 mm for the SRS-99.
The excitation light for our ﬂuorescence measurements
was produced by a 75W Xenon lamp [Oriel Research Arc
Lamp, Oriel Instruments, Stratford, CT], which has a usable
wavelength range of
to 2500 nm. A narrow band of
wavelengths was selected through a SP-2155 monochromator
[Princeton Instruments, Trenton, NJ] before being collected
at the exit-slit of the monochromator by 19 optical ﬁbers (ar-
ranged in a vertical column). The monochromator was set to
entrance and exit slits, which is equivalent to
dispersion. The optical ﬁbers were UV/VIS ﬁbers with a 0.22
numeric aperture (NA). One of the optical ﬁbers illuminated
a calibrated solid-state photodetector [S2281, Hamamatsu,
Japan], and this light was used to correct for the light intensity
variations across the wavelength spectrum of the Xenon lamp
and monochromator as well as any temporal variations. We
used a Keithley 6517A (Keithley Instruments, Inc., Cleveland,
OH) digital multimeter (DMM) to monitor the current on
the solid-state photodetector. The other 18 optical ﬁbers are
bundled together and illuminated the sample, as illustrated in
Fig. 2. Since the light exiting the optical ﬁbers will diverge
due to the limited numeric aperture (NA) of the optical ﬁbers,
492 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 59, NO. 3, JUNE 2012
Fig. 2. Fluorescent measurement setup for reﬂector samples. The sample is po-
sitioned with stepper motors, and the reﬂected light from the sample is collected
and focused into the spectrometer with two quartz lenses. The reﬂected light is
measured over a
wavelength spectrum with a CCD. The ﬁgure is
not to scale.
a collimating lens was placed at the exit of the optical ﬁber
bundle to collimate the light onto the reﬂector.
The reﬂectors were attached to a ﬂat surface, which was cov-
ered with black tape to minimize the back surface’s impact on
the ﬂuorescence measurements. The tape was analyzed in the
setup and veriﬁed to be non-ﬂuorescent. The collection beam
angle (in relation to the reﬂector) was equal to the incidence
angle. The light reﬂected off the sample and any ﬂuorescent
signal that was produced was collimated into a parallel beam
by a 50.8-mm diameter, 50-mm focal length quartz lens, lo-
cated 50 mm from the sample. The parallel light beam was then
focused by a second quartz lens into a SP-2156 spectrometer
[Princeton Instruments, Trenton, NJ] with a 100B charge-cou-
pled device (CCD) [Acton Research Corporation, Acton, MA]
. The setup is illustrated in Fig. 2. The oper-
ation of the setup and all data collection was controlled by a
LabVIEW program [National Instruments, Austin, TX].
We illuminated the reﬂector samples one wavelength at a
time, stepping from 220 nm to 600 nm in 5-nm steps. At each ex-
citation wavelength, an optical emission spectrum from 200 nm
to 1000 nm was collected. Since the CCD can only record
in a single acquisition, and since we could eliminate any
second or higher order reﬂections from the emission spectra by
using order-sorting ﬁlters, the spectrum was collected in three
separate spectra: 200–360 nm (blue), 360–620 nm (green), and
620–1000 nm (red). The blue spectra were acquired with no
order-sorting ﬁlters, while the green and red spectra used cut-off
ﬁlters located at 320 nm and 590 nm, respectively.
C. Reﬂectivity Measurements at Fixed Angles
To measure the reﬂectors’ specular and diffuse reﬂectivity
components as a function of wavelength, we modiﬁed the ﬂu-
orescence setup described in the previous section. The illumi-
nation side of the setup was identical to the ﬂuorescence setup,
with the exception of that the monochromator was set to
entrance and exit slits. These slit widths are equivalent to
dispersion. The reﬂectors were attached to a ﬂat sur-
face, which was covered with black tape to minimize the back
surface’s impact on the reﬂectivity measurements. For the spec-
ular samples (i.e., the ESR ﬁlms, Aluminum foil, and Tyvek
Fig. 3. Modiﬁed reﬂectivity measurement setup. The Xenon lamp’s light is
focused with two lenses into a monochromator, where a narrow band of wave-
lengths is selected. Most of the light is then guided to the reﬂector, where a
collimating lens (CL) focuses the light onto the reﬂector sample, and a smaller
portion of the light is monitored in a single strand of the optical ﬁbers with
a photodiode for normalization purposes
. The sample is attached
to an Aluminum block, and the reﬂected light is measured in a second photo-
. The photodiode currents are measured with digital multimeters
(DMMs). The ﬁgure is not to scale.
paper), the reﬂector sample was placed in relation to the inci-
dent light beam so the incidence angle (the angle between the
optical ﬁbers’ direction to the normal of the reﬂector) was equal
to the reﬂection angle (the angle between the normal of the
reﬂector to the centerline of the collection lens), making sure
that all specular light was directed onto the center of a second
S2281 photodiode. This photodiode was read out by a Keithley
485 DMM (Keithley Instruments, Inc., Cleveland, OH). For the
diffuse samples, measurements were performed both with the
reﬂection angle equal to the incidence angle (i.e., a specular
setup), and a setup where no specular light was allowed to make
it onto the photodiode (i.e., a diffuse setup). In practice, the
sample was placed at 45
and 60 (relative to the incident beam)
for the specular and diffuse setups, respectively, with the inci-
dent light beam and the photodetector being placed 90
For the specular measurements, the photodetector was placed
at a distance that created a cone with a 6.0
half-angle for the
photodetector, which is equivalent to
of the solid
angle, and for the diffuse measurements the distance between
the sample and the photodetector was decreased, which led to
photodetection half-angle, which is equivalent to a 5.5%
solid angle coverage.
The readout of the DMMs and the movement of the
monochromator were controlled by a LabVIEW program, and
the acquired data was saved to a text-ﬁle for post-processing.
Each current measurement was calculated as an average of
10 individual current measurements, and the value was only
accepted if the standard deviation within the ten samples was
below 1% of the average value. If the standard deviation of the
current was above 1%, indicating transient currents and not
a steady-state current, the current was re-measured until the
condition was met.
We illuminated the reﬂector samples one wavelength at a
time, stepping from 230 nm to 800 nm in 2-nm steps. No order-
sorting ﬁlters (to remove higher order light) were used. We esti-
mate the absence of order-sorting ﬁlters to produce an error for
our results of
The measured reﬂected light intensity was normalized at
each wavelength against a white standard (SRS-99), i.e., a
sample that has a well-deﬁned reﬂection coefﬁcient at certain
wavelengths, enabling us to translate our relative reﬂectivity
JANECEK: REFLECTIVITY SPECTRA FOR COMMONLY USED REFLECTORS 493
Fig. 4. Control panel (screen shot) for the reﬂectivity measurements performed
with the Lambda 950 UV/VIS spectrophotometer and an integrating sphere. See
article text for details.
values to absolute values across the wavelength spectrum.
Since the reﬂection coefﬁcient for the white standard is deﬁned
from 250 nm to 2500 nm, we only report on our measured
reﬂection coefﬁcients down to 250 nm. We had access to two
white standards, and we compare the results obtained from
both these standards. Both white standards are made out of
Spectralon, a polytetraﬂouroethylene (PTFE) based Lambertian
reﬂector , . Each reﬂection spectrum measurement was
performed at least ﬁve times (with different illumination spots
on the sample) to evaluate consistency.
Since these measurements do not collect all the light (i.e., the
of solid angle), the reﬂection coefﬁcient curves were
normalized to literature values at 440 nm. If the normalized
specular and the diffuse measurement curves were equal, we as-
sumed that the angular distribution does not change with wave-
D. Reﬂectivity With an Integrating Sphere
We also measured all the reﬂector samples in a Lambda 950
UV/VIS spectrophotometer [PerkinElmer, Inc., Waltham, MA].
The screen shot of the control panel illustrating the setup is
shown in Fig. 4. A deuterium lamp is used for the shorter wave-
and a Tungsten lamp is used for the longer
wavelengths when illuminating the reﬂector sample. The en-
trance and exit slits on the monochromator were set to pro-
duce 1.0 nm dispersion. The sample is placed at
angle, and all the reﬂected light is collected by the integrating
sphere to a photomultiplier tube. A second beam, created from
the incidence beam with a beam splitter, is used to correct for
temporal intensity ﬂuctuations in the measurements. The reﬂec-
tion coefﬁcient was measured every 2 nm. The measurements
were normalized to the white standard (SRS-99). Each measure-
ment was performed three times to evaluate consistency.
The only reﬂectors listed in Table I that exhibited ﬂuores-
cence were the ESR ﬁlms, Lumirror
, and Melinex
. All of
these three reﬂectors exhibited strong ﬂuorescence and their ﬂu-
orescence spectra are shown in intensity plots in Fig. 5. Fig. 5(a)
shows the ﬂuorescence plot for the ESR ﬁlm, Fig. 5(b) shows
ﬂuorescence for the glue that was originally attached to the
backside of the ESR ﬁlm, and Fig. 5(c) shows the ﬂuorescence
, respectively. All the ESR ﬁlms exhibited very
similar ﬂuorescence, and the only noticeable difference in all
the ESR ﬁlm measurements was that the VM3000 ﬁlm had a
slightly stronger ﬂuorescent signal for excitations below 280 nm
compared to the VM2000 ﬁlm. There was virtually no differ-
ence between the front- and backsides of each ESR ﬁlm. The
ﬂuorescence were virtually identical,
and only the Melinex
ﬂuorescence plot is therefore displayed.
A ﬂuorescence signal was detected from both sides (and for
both versions) of the ESR ﬁlm. The emission peak for the ESR
ﬁlm’s ﬂuorescent signal is located at 430 nm, and is produced
by wavelengths shorter than 400 nm. The glue that was removed
from the backside of the ESR ﬁlms was measured separately
for ﬂuorescence on a (non-ﬂuorescent) black tape. The glue’s
ﬂuorescent signal has its emission peak located at 290 nm, and
the glue is excited by wavelengths between 250 and 285 nm.
have their emission peaks located at
440 nm, and are excited by wavelengths between 320 and 420
nm. Proﬁles through the emission (and excitation) maxima are
shown in Figs. 6 and 7; Fig. 6 shows the proﬁles for the ESR ﬁlm
and the ESR glue, and Fig. 7 shows the proﬁles for Lumirror
B. Specular Reﬂectors
The reﬂection coefﬁcients as a function of wavelength are
displayed in Fig. 8 for the specular reﬂectors, i.e., the ESR
ﬁlms and the Aluminum foil, as well as for the Tyvek
The protective ﬁlms from the ESR ﬁlms and the glue layer
from the ESR backsides were removed (using ethanol) prior to
any reﬂection (or ﬂuorescence) measurements. As can be seen
in Fig. 8, the VM3000 front-side and the VM2000 backside
are virtually identical. In the same way, the VM3000 back-
side and the VM2000 front-side are virtually identical above
380 nm—below 380 nm the VM2000 front-side has a lower
reﬂection coefﬁcient. The difference between the front- and
backsides for both ﬁlms above 380 nm is minimal, though we
did measure a 5 to 6 nm difference in the cut-off reﬂection
wavelength between the two sides (at
paper displays high reﬂection coefﬁcients for all mea-
sured wavelengths, staying above 95% for wavelengths above
355 nm, and above 90% for wavelengths above 300 nm. Alu-
minum foil displays a ﬂat reﬂectivity curve, with reﬂection co-
efﬁcients between 70% and 80% above 265 nm.
C. Diffuse Reﬂectors
The reﬂection coefﬁcients as a function of wavelength for
Spectralon (WS-1-LS), GORE
diffuse reﬂector, MgO powder,
paint, and nitrocellulose ﬁlter paper are displayed in
494 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 59, NO. 3, JUNE 2012
Fig. 5. Fluorescence for A) ESR ﬁlm (VM2000 front-side is shown), B) ESR glue (from VM2000) and C) Melinex
. The excitation wavelengths are displayed on
the vertical axis, and the resulting intensity in the emission is shown along the horizontal axis. A reﬂection line is clearly visible in all plots, where the excitation
and emission wavelengths are equal. The highest intensity in each plot (excluding the reﬂection line) has been used to normalize the intensity scales.
Fig. 6. Excitation and emission proﬁles for ESR ﬁlm and ESR glue. The pro-
ﬁles are taken from data from Figs. 5(a) and 5(b) and pass through the maximum
value in the ﬂuorescence spectra.
Fig. 7. Excitation and emission proﬁles for Lumirror
. The pro-
ﬁles are taken from data from Fig. 5(c) and the Lumirror
data (not shown) and
pass through the maximum value in the ﬂuorescence spectra.
Fig. 9. A zoom-in image of the wavelength range from 360 to
500 nm is also shown in the ﬁgure. The GORE
exhibits an excellent reﬂectivity
for wavelengths be-
tween 250 and 500 nm, while MgO and Spectralon—two
materials commonly used as white reﬂectors—drop below 90%
reﬂectivity for wavelengths shorter than 280 nm. Nitrocellulose
paint exhibit sharp declines in their reﬂectivity for
wavelengths below 330 and 420 nm, respectively.
The reﬂection coefﬁcients as a function of wavelength
for PTFE tapes (glossy PTFE tape, ACE Teﬂon
ﬁlm) are displayed in Fig. 10. As described in de-
tail in the following paragraph, all ﬁlms have a decreasing
reﬂectivity with increasing wavelength due to transmission.
The transmission for a single layer of glossy PTFE ﬁlm is also
presented in the ﬁgure, as well as the sum of the reﬂected and
transmitted signal, i.e., 1 - absorbed. The transmission was
measured by placing the reﬂector in the incident beam’s path
into the integrating sphere.
The PTFE reﬂectors are bulk reﬂectors (as opposed to sur-
face reﬂectors) and longer wavelengths penetrate deeper into (or
though the material). For instance, a single layer of the
thick glossy PTFE tape allows a signiﬁcant amount of light
Fig. 8. Reﬂection coefﬁcient for the front- and backsides of two versions of
ESR ﬁlm, Tyvek
paper, and Aluminum foil as a function of wavelength. The
insert shows a zoom-in of the reﬂection data between 360 and 500 nm. The data
were acquired in the specular setup of the
Reﬂectivity measurements at ﬁxed
angles setup and normalized to the reﬂectivity values in Table I. The maximum
error was measured to be
, and the average error for all the data points
presented above was measured to be
Fig. 9. Reﬂection coefﬁcient for several diffuse reﬂectors as a function of wave-
length. The insert shows a zoom-in of the reﬂection between 360 and 500 nm.
The data were acquired in the Reﬂectivity with an Integrating Sphere setup. The
measurements have not been normalized to the reﬂectivity values in Table I since
the Lambda 950 instrument is calibrated.
transmitted for wavelengths longer than 380 nm. By
increasing the number of layers, the reﬂectivity increases, and
at for instance 440 nm, goes from a reﬂection coefﬁcient of 85%
(1 layer) to 92.6% (2 layers), to 94.4% (4 layers), and to 96.2%
(8 layers). In order to achieve at least 95% reﬂectivity for scin-
tillator light emissions (i.e., 380–500 nm), the examined PTFE
JANECEK: REFLECTIVITY SPECTRA FOR COMMONLY USED REFLECTORS 495
Fig. 10. Reﬂection coefﬁcient for PTFE tapes as a function of wavelength. The
insert shows a zoom-in of the reﬂection between 360 and 500 nm. The data were
acquired in the
Reﬂectivity with an Integrating Sphere setup. The measurements
have not been normalized to the reﬂectivity values in Table I since the Lambda
950 instrument is calibrated.
Fig. 11. Reﬂection coefﬁcient as a function of wavelength for Lumirror
a variety of reﬂection angles. The 45
-setup measured the specular component
of the reﬂectivity spectra, while the other angle-setups measure various compo-
nents of the diffuse spectra. The data were acquired in the Reﬂectivity measure-
ments at ﬁxed angles setup. The measurements have not been normalized to the
reﬂectivity values in Table I.
ﬁlms will need to be at least 0.5 mm thick. A good alternative,
if a thin diffuse reﬂector is needed, is to use nitrocellulose.
D. Changing Angular Distributions
reﬂection coefﬁcients as a func-
tion of wavelength were measured to be different in the spec-
ular and diffuse reﬂection measurements. Because of this, we
decided to measure the reﬂection over a larger range of reﬂec-
tion angles. The results from these measurements are displayed
in Fig. 11 for Lumirror
exhibited similar behavior.
The results in Fig. 11 have not been normalized to the reﬂec-
tion coefﬁcient at 440 nm and hence show the intensity vari-
ations with reﬂection angle. Since the incident beam and the
photodetector are place 90
apart, the 45 -setup is a specular
measurement, while all the other angle-setups measure various
components of the diffuse spectra. As can be seen, the slope
above 400 nm, as well as the intensity of the reﬂection peak at
260 nm (in relation to the baseline of the reﬂection spectrum)
varies with reﬂection angle. Since these two reﬂectors are ﬂuo-
rescent, a measurement in the integrating sphere produces arti-
ﬁcially high reﬂection coefﬁcients in the 320 to 420 nm wave-
length range, where the quantum efﬁciency for the photomulti-
Fig. 12. Reﬂection coefﬁcient as a function of wavelength for Lumirror
. The data were acquired in the Reﬂectivity with an Integrating Sphere
setup, and the ﬂuorescence contribution to the signal between 320 and 420 nm
(shown in grey shading) produces an artiﬁcially high signal since the quantum
efﬁciency for the photomultiplier tube is higher for the emitted wavelength than
for the incident wavelength. The measurements have not been normalized to the
reﬂectivity values in Table I.
Fig. 13. Reﬂection coefﬁcient as a function of wavelength for the two white
standars, WS-1-LS and SRS-99 (both Spectralon). Note that the “Reﬂectivity”-
axis scale is from 0.85 to 1.0 (and not 0 to 1) in order to enhance the differences
between the curves. The data for the SRS-99 were supplied by the manufacturer.
The data for the WS-1-LS were supplied by the manufacturer and acquired in the
Reﬂectivity with an Integrating Sphere setup. The measurements have not been
normalized to the reﬂectivity values in Table I, since the Lambda 950 instrument
plier tube is higher for the ﬂuorescent light compared to incident
light, see Fig. 12. Lumirror
have a decreased re-
ﬂectivity below a cut-off wavelength of 325 nm.
E. White Standards
The manufacturers’ reﬂection coefﬁcient data for the two
white standards are displayed in Fig. 13. The measured reﬂec-
tion coefﬁcients for the WS-1-LS standard, when normalized
to the SRS-99 standard, are also displayed in the ﬁgure.
The high reﬂectance value of nitrocellulose presented in
Table I indicates that the reﬂectivity of the ACE Teﬂon
sample—the reﬂector the nitrocellulose sample was normalized
against—is lower than reported values –. We do not be-
lieve that this is due to improper care of the samples (they were
stored at stable temperatures and out of sunlight, the surfaces
were clean, contact with chemicals and exposure to UV light
was avoided, etc.) but attribute it instead to transmission of light
through even thick layers of PTFE tape. The measurements
presented in Fig. 10 show that there is a very large amount of
transmission through PTFE ﬁlms, and that the values reported
in the literature must assume very thick or dense samples. The
496 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 59, NO. 3, JUNE 2012
reﬂection coefﬁcients at 440 nm were in this work measured
to be 0.944 for ACE Teﬂon
tape and 0.962 for nitrocellulose,
respectively, which gives nitrocellulose a 1.9% higher reﬂection
coefﬁcient at 440 nm than four layers of ACE Teﬂon
The intensity scales shown on the right of each ﬂuorescence
plot in Fig. 5 should not be used as absolute scales, as the size of
the reﬂectors and the reﬂectors positioning in relation to the in-
cident beam and the light collection lens play a very signiﬁcant
role in the detected light intensity. Although the ﬂuorescence
setup can be used to measure the reﬂection coefﬁcient by ﬁl-
tering out any unwanted ﬂuorescence and higher order signals,
the alignment of the collimated light from the optical ﬁbers, the
sample, and the “sweet spot” from which the light collection
lenses focus the light into the spectrometer (see Fig. 2), is a
very delicate operation and small variations in the positions of
any of these three parameters easily lead to inconsistent (i.e.,
inaccurate) results in intensity. For this reason, although this
setup is able to detect and characterize ﬂuorescence emission
and excitation wavelengths, it should not be used for absolute
intensity measurements. For instance, Lumirror
exhibit nearly identical ﬂuorescence spectra, and Lumirror
only slightly brighter compared to Melinex
under a black light
(excited at 365 nm), yet the measured intensity in Fig. 5(c) for
exhibited a four times weaker signal than for the Lu-
ﬂuorescence data (not shown).
Fluorescent light is isotropic in its nature, and the direction-
ality of the light will therefore be lost for the light that inter-
acts with the ﬂuorescent reﬂector and is reemitted. Therefore,
the ﬂuorescence exhibited by the ESR ﬁlms, Lumirror
can be beneﬁcial if the directionality of the light is not
an issue and if the photodetector has higher quantum efﬁcien-
cies at the emission wavelengths compared to the incident wave-
lengths. This effect is clearly demonstrated for the Lumirror
reﬂectors, as shown in Fig. 12.
The temporal behavior of the ﬂuorescent emissions was mea-
sured in an IBH FluoroHub [HORIBA Jobin Yvon Inc, Edison,
NJ] by exposing the reﬂectors to pulsed LED light close to the
reﬂectors’ maximum excitation wavelengths. The pulsed LEDs
we had access to have emission peaks located at 268 nm (used
for the ESR glue), 311 nm (ESR ﬁlm), and 370 nm (Lumirror
). The ﬂuorescent light emission as a function of
time was measured for each reﬂector through a monochromator
in which the signal was ﬁltered into a narrow bandwidth (1 nm)
centered at the emission peak at 430 nm (ESR ﬁlm) or 440 nm
). The ESR glue was measured at 300
nm (instead of 290 nm) to minimize the light contributions from
the LED’s emissions. Each measurement was performed until
we accumulated 10,000 counts in the peak bin. The count rate
for the ESR glue signal was several orders of magnitude smaller
due to lower efﬁciencies of the light and the photodetector
and due to the lower ﬂuorescence, and only 5,000 counts were
acquired for this material. Since the background signal is pro-
portional to the acquisition time, the ESR glue also exhibited
a much lower signal-to-noise ratio (by
). The temporal
emission curves are displayed in Fig. 14. The ESR ﬁlm’s ﬂu-
orescence was measured to have a half-life of 14 ns, the ESR
glue’s ﬂuorescence was measured to have a half-life of 7 ns,
while the Lumirror
ﬂuorescent half-lives were
measured to be
Fig. 14. Temporal behavior of the ﬂuorescent light emitted from the reﬂectors
that exhibited ﬂuorescence. Note that the vertical scale is logarithmic.
Several reﬂectors exhibited “cut-offs” for the reﬂectivity
for shorter wavelengths, including
(420 nm), ESR ﬁlm
(395 nm), nitrocellulose (330 nm), Lumirror
(325 nm) and
(325 nm). The lower reﬂection coefﬁcients below
the cut-off wavelengths have to be taken into consideration
when pairing up a scintillator with a reﬂector, by taking into
account the scintillator’s emission spectrum and the reﬂector’s
reﬂection coefﬁcients at these wavelengths.
Our measurements showed great repeatability, where the re-
ﬂection coefﬁcient between several runs in the Lambda 950
UV/VIS spectrophotometer were typically within 0.1% of each
other, and never more than 1%, and the accuracy in our mea-
surements are thus closely tied to the accuracy of the reference
standard. The data in Fig. 13 indicate that the white reﬂection
standards often used for these measurements can be a source
of errors, as their true reﬂectivity can differ signiﬁcantly from
the calibration provided by the manufacturer. For example, the
measured reﬂectivity of WS-1-LS is 2% lower at 380 nm and
5% lower at 300 nm than the values provided by the manufac-
turer. The manufacturer-provided reﬂectivity values for SRS-99
contained a small
dip between 260 and 290 nm that
appears to be an artifact. Using these standards with their man-
ufacturer-provided reﬂectivity values leads to
tivity for wavelengths below 300 nm (WS-1-LS), or a small peak
between 260 and 290 nm (SRS-99). Both reﬂection standards
were “certiﬁed reﬂection standards”, although only the SRS-99
was provided with a “reﬂection calibration certiﬁcate” (and cal-
ibrated against a NIST traceable standard within the last year).
Other groups have measured decreased reﬂectivity in Spectralon
samples over time , including samples that have not been
exposed to high illumination levels. In fact, Spectralon samples
stored in darkness have shown to degrade over time, and this is
the most likely cause for the results presented in Fig. 13.
We have measured the reﬂection coefﬁcient for several
commonly used reﬂectors. The reﬂectors were also screened
for ﬂuorescence and changing angular distribution with
wavelength. The highest reﬂectivity for short wavelengths
was measured for the PTFE based reﬂectors, with
diffuse reﬂector having the highest reﬂectivity over
the greatest wavelength range. PTFE based reﬂectors were the
JANECEK: REFLECTIVITY SPECTRA FOR COMMONLY USED REFLECTORS 497
only examined reﬂectors that had reﬂectivity for wave-
lengths below 300 nm, but all PTFE ﬁlms exhibited decreasing
reﬂectivity with increasing wavelength due to increased trans-
mission for longer wavelengths. To achieve
the PTFE ﬁlms have to be at least 0.5 mm thick. If a thinner
diffuse reﬂector is needed, nitrocellulose is a good alternative.
Several of the reﬂectors have sharp declines in reﬂectivity
below a cut-off wavelength, including
(420 nm), ESR ﬁlm
(395 nm), nitrocellulose (330 nm), Lumirror
(325 nm), and
(325 nm). Lumirror
, and ESR ﬁlm ex-
hibited strong ﬂuorescence, and Lumirror
exhibited changing angular distributions with wavelength.
The mechanical construction of the setups presented in
this paper was done by David Wilson. Dr. Jacob C. Jonsson,
Johan Borglin, and Ilya Zorikhin-Nilsson assisted in the ﬂuo-
rescence and reﬂection measurements. Dr. William W. Moses
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