Method for determination of the absorption coefficient in films based on photoluminophore suspension for white LEDs
Developed in this work is the method for measuring the absorption coefficient in optically non-homogeneous media (films prepared from photoluminophore suspension). Using this new method, the authors have measured the absorption spectrum of the above films with inorganic photoluminophore FLY-7. At th...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
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nasplib_isofts_kiev_ua-123456789-1218182025-02-09T09:45:40Z Method for determination of the absorption coefficient in films based on photoluminophore suspension for white LEDs Khmil, D.N. Kamuz, A.M. Oleksenko, P.F. Kamuz, V.G. Aleksenko, N.G. Kamuz, O.A. Developed in this work is the method for measuring the absorption coefficient in optically non-homogeneous media (films prepared from photoluminophore suspension). Using this new method, the authors have measured the absorption spectrum of the above films with inorganic photoluminophore FLY-7. At the film absorption peak (near 448 nm) and photoluminophore concentration close to 20%, the absorption coefficient reaches 124 cm⁻¹. Ascertained have been the conditions that should be provided when using this method. 2015 Article Method for determination of the absorption coefficient in films based on photoluminophore suspension for white LEDs / D.N. Khmil, A.M. Kamuz, P.F. Oleksenko, V.G. Kamuz, N.G. Aleksenko, O.A. Kamuz // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2015. — Т. 18, № 2. — С. 215-219. — Бібліогр.: 8 назв. — англ. 1560-8034 DOI: 10.15407/spqeo18.02.215 PACS 85.60.Jb https://nasplib.isofts.kiev.ua/handle/123456789/121818 en Semiconductor Physics Quantum Electronics & Optoelectronics application/pdf Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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Developed in this work is the method for measuring the absorption coefficient in optically non-homogeneous media (films prepared from photoluminophore suspension). Using this new method, the authors have measured the absorption spectrum of the above films with inorganic photoluminophore FLY-7. At the film absorption peak (near 448 nm) and photoluminophore concentration close to 20%, the absorption coefficient reaches 124 cm⁻¹. Ascertained have been the conditions that should be provided when using this method. |
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Article |
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Khmil, D.N. Kamuz, A.M. Oleksenko, P.F. Kamuz, V.G. Aleksenko, N.G. Kamuz, O.A. |
| spellingShingle |
Khmil, D.N. Kamuz, A.M. Oleksenko, P.F. Kamuz, V.G. Aleksenko, N.G. Kamuz, O.A. Method for determination of the absorption coefficient in films based on photoluminophore suspension for white LEDs Semiconductor Physics Quantum Electronics & Optoelectronics |
| author_facet |
Khmil, D.N. Kamuz, A.M. Oleksenko, P.F. Kamuz, V.G. Aleksenko, N.G. Kamuz, O.A. |
| author_sort |
Khmil, D.N. |
| title |
Method for determination of the absorption coefficient in films based on photoluminophore suspension for white LEDs |
| title_short |
Method for determination of the absorption coefficient in films based on photoluminophore suspension for white LEDs |
| title_full |
Method for determination of the absorption coefficient in films based on photoluminophore suspension for white LEDs |
| title_fullStr |
Method for determination of the absorption coefficient in films based on photoluminophore suspension for white LEDs |
| title_full_unstemmed |
Method for determination of the absorption coefficient in films based on photoluminophore suspension for white LEDs |
| title_sort |
method for determination of the absorption coefficient in films based on photoluminophore suspension for white leds |
| publisher |
Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| publishDate |
2015 |
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https://nasplib.isofts.kiev.ua/handle/123456789/121818 |
| citation_txt |
Method for determination of the absorption coefficient in films based on photoluminophore suspension for white LEDs / D.N. Khmil, A.M. Kamuz, P.F. Oleksenko, V.G. Kamuz, N.G. Aleksenko, O.A. Kamuz // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2015. — Т. 18, № 2. — С. 215-219. — Бібліогр.: 8 назв. — англ. |
| series |
Semiconductor Physics Quantum Electronics & Optoelectronics |
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2025-11-25T12:26:13Z |
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2025-11-25T12:26:13Z |
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| fulltext |
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2015. V. 18, N 2. P. 215-219.
doi: 10.15407/spqeo18.02.215
© 2015, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
215
PACS 85.60.Jb
Method for determination of the absorption coefficient in films
based on photoluminophore suspension for white LEDs
D.N. Khmil, A.M. Kamuz, P.F. Oleksenko, V.G. Kamuz, N.G. Aleksenko, O.A. Kamuz
V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine,
41, prospect Nauky, 03680 Kyiv, Ukraine.
Phone: +38 (044) 525-6168; e-mail: deniskhmil@ukr.net
Abstract. Developed in this work is the method for measuring the absorption coefficient
in optically non-homogeneous media (films prepared from photoluminophore
suspension). Using this new method, the authors have measured the absorption spectrum
of the above films with inorganic photoluminophore FLY-7. At the film absorption peak
(near 448 nm) and photoluminophore concentration close to 20%, the absorption
coefficient reaches 124 cm
–1
. Ascertained have been the conditions that should be
provided when using this method.
Keywords: absorption coefficient, photoluminophore, light scattering, white LED.
Manuscript received 18.12.14; revised version received 25.03.15; accepted for
publication 27.05.15; published online 08.06.15.
1. Introduction
Nowadays, LED lighting becomes widely spread as one
of promising energy-saving technologies. In lab
conditions, these light sources reach the values of
efficiency up to 303 Lm/W for the correlated color
temperature 5150 K [1]. Due to this efficiency, it
becomes possible to change not only incandescent
filament lamps but the compact luminescent ones, too.
However, beside their high efficiency, they should
satisfy some other requirements: to have the correlated
color temperature within the range 2700 to 6500 K, high
index of color transfer and be resistant to degradation.
These parameters depend predominantly on the
layer of photoluminophore suspension, i.e., on
photoluminophore properties. In relation with it,
development and synthesis of new highly-efficient
photoluminophores last, which means necessity to
satisfy specific requirements to measurements of their
optical and colorimetric parameters. Among optical
parameters, the absorption coefficient plays a main role,
because it defines the quantum yield and, respectively,
the efficiency of white LEDs.
Up to date, the only several methods for
determining the absorptive of photoluminophore
suspension are known. One of the most used methods is
the measurement of excitation spectrum (i.e.,
dependence of the luminescence intensity corresponding
to the wavelength of the luminescent peak on the
excitation wavelength). As it was shown in the works
[5, 6], the luminescence intensity at the wavelength λmax
is in proportion to the absorption coefficient only in
specific conditions:
low extinction of light that passes through the
studied sample (it means using thin luminescent
samples);
absence of saturation in matter that radiates (i.e.,
one should use low light fluxes for excitation);
the same light intensity that serves for excitation
within the whole studied range of wavelengths;
independency of the luminescence yield of the light
wavelength that provides excitation over the whole
range.
In practice, these conditions are never fulfilled. As
a result, this method is capable to provide only
qualitative data. Especially often, the first condition is
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2015. V. 18, N 2. P. 215-219.
doi: 10.15407/spqeo18.02.215
© 2015, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
216
not fulfilled, since there always used are infinitely thick
samples but not the thin ones. In this case, if the
conditions 2 and 3 are fulfilled, the brightness of
luminescence that arises under action of beams with a
definite wavelength is in proportion to the luminescence
yield but not to the absorption coefficient value [2, 3].
Recently, there appear the works where the
methods of modeling of light propagation in a layer of
photoluminophore suspension are used. Proposed in the
works [7, 8] have been complex methods for calculating
the absorption coefficient value only at the wavelength
alone (455 nm [8]).
The aim of this work was in development of a new
method for express determining the dependence of the
absorption coefficient on the wavelength of optically
inhomogeneous films prepared from photoluminophore
suspension.
2. Theory
As it is known from classical optics, the absorption
coefficient for a plane-parallel light beam is determined
via changes of the light intensity caused by absorption in
optically homogeneous medium (Bouguer-Lambert law).
It is the so-called natural absorption coefficient. But in
optically inhomogeneous medium, the light intensity is
changed not only by absorption but due to light
scattering by optical inhomogeneities (to be more exact,
due to transformation of the scattering indicatrix) [3-6].
In the theory of multiple scattering by W. Hartel
[5, 6] that is based on the Mie theory, it was shown how
the light plane-parallel beam changes its scattering
indicatrix up to the spherical shape when propagating in
optically inhomogeneous medium.
Let us consider in more details the process of
transformation of the scattering indicatrix shape for the
case of blue LED light (456 nm) passage through a
composite film prepared from photoluminophore
suspension, when using the optical setup shown in Fig. 1.
Let us assume that a quasi-plane wave is incident
onto the film, and its scattering indicatrix at the input to
optically inhomogeneous medium has the shape of a
narrow peak (Fig. 2a, curve 1), and the space angular
aperture Ω of the radiation source is equal to that of the
detector in a spectroradiometer (for example, for the
device HAAS-2000 Ω is close to 0.02391 steradian). We
shall not take into account the fine structure of the
indicatrix describing the light scattering by microparticles.
At first, let us consider the case when
microparticles do not absorb light but only scatter it. Let
the light intensity in the plane 1 is equal to I1 (Fig. 2b).
When passing from the plane 1 to the plane 2, light
undergoes multiple scattering. As a result, the scattering
indicatrix will be expanded (Fig. 2a, curve 2), and the
light intensity will be decreased along the direction of
the coordinate axis L down to the value I2 wa (Fig. 2c,
curve 1). The subscript „wa‟ means “without
absorption”, i.e., microparticles of powder-like photo-
luminophore do not absorb light but only scatter it.
Fig. 1. One of the possible setups for studying the absorption
spectra: 1 – detector, 2 – lenses, 3 – studied sample, 4 – light
source (LED, 456 nm).
On its path from the plane 2 to the plane 3, the
shape of the scattering indicatrix continues to change,
and in the plane 3 it becomes practically spherical
(Fig. 2a, curve 3), while the light intensity along the
direction of the L axis decreases down to the value I3 wa
(Fig. 2c, curve 1).
In the plane 4, the indicatrix of light scattering
acquires the spherical shape, and the light intensity I4 wa
possesses equal values in an arbitrary direction within
the space angle 4π (Fig. 2a, curve 4, Fig. 2c, curve 1).
The range of the film (between the planes 1 and 4),
where transformation of the shape inherent to the
indicatrix of light scattering takes place, we shall call the
“range of transformation”, and its thickness we shall
designate as ltr. It should be noted that this parameter
depends on composition of photoluminophore
suspension, concentration of photoluminophore powder
in the film, its thickness, and the refractive indexes of
powder and substance that is used as a binder.
Fig. 2. Transformation of the shape inherent to the light
indicatrix within the range 1–4 (a), schematic view of the film
prepared from photoluminophore suspension (b), and the
dependences of the light intensity on the film thickness L for
microparticles without light absorption (1) and those absorbing
light (2) (c).
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2015. V. 18, N 2. P. 215-219.
doi: 10.15407/spqeo18.02.215
© 2015, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
217
As in the planes 5, 6 and so on, the indicatrixes of
light scattering also will have the spherical shape, and
particles of powder-like photoluminophore do not absorb
light, the light intensity along the L axis in these planes
should have some constant value, i.e., I5 wa = I6 wa = I7 wa
= I4 wa (Fig. 2b, horizontal part of the curve 1). Thus, due
to transformation of the scattering indicatrix, the light
intensity along the L-axis decrease in transformation
region even under the absence of light absorption by
particles of powder-like photoluminophore (Fig. 2c, the
part 1–4 of the curve 1).
In what follows, we shall assume that the relative
elementary change of the light intensity
)(
)(
lI
ldI
along the
L-axis in every elementary layer dl in the film does not
depend on the light intensity and is in proportion to the
thickness of the elementary layer, that is
dl
lI
ldI
tr
)(
)(
, (2)
where tr is the coefficient of proportionality or
coefficient of transformation of the scattering indicatrix
shape.
The total change in the light intensity (along the L-
axis) in the transformation region can be found after
integrating the left and right parts from I0 to Il and the
film thickness 0 up to l (0 < l < l4):
dl
lI
ldI
lI
I
l
4
0 0
tr
)(
)(
. (3)
As a result, one can obtain
l
lI
ldI
trln
)(
)(
ln
(4)
or
l
e IlI
tr
0)( (5)
where I0 is the intensity of light beam at the input to the
film, l – thickness of the substance layer through which
light passes.
Changes in the shape of scattering indicatrix take
place only within the transformation region (from l = 0
up to l = l4).
The parameter of transformation region, which
defines the velocity of changes in the light intensity, is
the transformation coefficient for the shape of scattering
indicatrix tr .
When photoluminophore micro particles absorb
light, the light intensity in the transformation region
(Fig. 3c, part of the film 1–4, curve 2) decreases both
due to scattering indicatrix transformation and
absorption, while on the part 4–7 only due to absorption
(Fig. 3c, curve 2). At the planes 2, 3, 4, 5, 6 and 7, the
light intensity will have the following values I2a, I3a, I4a,
I5a, I6a and I7a, respectively (index „a‟ means that
photoluminophore microparticles absorb light).
Fig. 3. Dependence of the transmitted radiation intensity on the
thickness of the film prepared from composite mixture. The
light wavelength is 456 nm.
As it is well known from classical optics, to
determine the absorption coefficient of optically
homogeneous media, one usually uses the Bouguer-
Lambert law
l
eIlI
0)( , (6)
where – absorption coefficient.
So, for the parts 1–4 and 4–7, with account of the
transformation coefficient for the scattering indicatrix,
the Bouguer-Lambert formulae acquire the following
appearance:
l
eIlI
)(
0
tr)(
(from l = 0 up to l < l4) (7)
and
lll
eeIelIlI
4tr )(
04 )()(
(from l = l4 up to l = l7), (8)
where I(l4) is the light intensity after passing through the
layer with the thickness l4, I(l) – light intensity in the
part 1–4 and 4–7, respectively.
Using the same law, expressions for the light
intensity at the planes 5 and 7 that are out of the
transformation range can be written, respectively, as
follows:
)()(
0
)(
05
454tr5tr)(
llll
eeIeIlI
(9)
and
.)(
)()(
0
)(
07
474tr7tr llll
eeIeIlI
(10)
Dividing Exp. (9) by (10), one can obtain
.
)(
)(
)(
)(
)(
)()(
0
)()(
0
7
5
75
47
45
47
45
474tr
454tr
ll
ll
ll
ll
ll
lll
lll
е
ee
ee
e
e
eeI
eeI
lI
lI
(11)
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2015. V. 18, N 2. P. 215-219.
doi: 10.15407/spqeo18.02.215
© 2015, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
218
Let us take the logarithm of Exp. (11) and write the
expression for the absorption coefficient of the
suspension film :
)(
)(
ln
57
7
5
ll
lI
lI
. (12)
It is seen from Exp. (12) that, to find the value of
absorption coefficient inherent to the suspension film,
it is necessary to determine only the light intensities
I(l5) and I(l7) at the outputs of films with the
thicknesses l5 and l7, respectively. It means that one
should use two composite films with the thicknesses
higher than the transformation region. Therefore, this
method for measuring the absorption coefficient
inherent to the suspension film we called “the method
of two thicknesses”. It should be noted that our method
differs from the classical method of two thicknesses
that is often used when determining the absorption
coefficient of optically homogeneous medium. If using
our method, it is necessary that every thickness should
exceed the thickness of scattering indicatrix
transformation in this film. But it implies that first one
should determine the thickness of indicatrix
transformation range.
3. Experiment and results
The developed method needs knowledge of the thickness
inherent to the region of indicatrix transformation in the
composite film. It is obvious that its value depends on
the concentration of photoluminophore microcrystals in
binder, sizes of these microcrystals, refractive indexes of
photoluminophore microparticles and binder.
The simplest way to determine the thickness of
transformation range is to plot the dependence the
radiation intensity on the thickness of composite film
(Fig. 3) after measurements using the optical scheme
shown in Fig. 1. To determine the thickness of
transformation range with the method of two
thicknesses, we used the 205- and 515-nm thick films.
As seen from Fig. 3, the thickness of transformation
region is approximately 350 μm. Thereof, for the method
to be applicable, it is expedient to use films with the
thickness higher than 350 μm.
To determine the absorption coefficient of
composite films prepared from photoluminophore
suspension, we made these films with various
thicknesses (205, 265, 315, 365, 420, 470, and 515 μm).
Composition of the films consisted of: epoxy resin,
photoluminophore FLY-7 (20%) and thixotropic
dopants. As an excitation source, we used a blue LED
with the emission peak at the wavelength 456 nm. These
investigations were performed using the scheme shown
in Fig. 1.
Fig. 4 shows absorption spectra obtained using the
method of two thicknesses (8–10) and classical
Bouguer-Lambert law (curves 1–7, without account of
tr value). Having analyzed the obtained data, one can
draw the conclusion that, when using the Bouguer-
Lambert law deduced for a parallel light beam and
optically homogeneous medium, the obtained data are
unreliable. So, when using the sample with the thickness
205 nm (Fig. 4), the curve 1 mainly describes a decrease
in the light intensity caused by transformation of the
scattering indicatrix. When the sample thickness is
increased, the influence of this transformation is
gradually decreased, while contribution of
photoluminophore absorption is increased. It results in
appearance of the absorption peak at the wavelength
448 nm (in Fig. 4, the curves correspond to the following
sample thicknesses: 2 – 265, 3 – 315, 4 – 365, 5 – 420,
6 – 470, and 7 – 515 μm).
When applying the method of two thicknesses
(sample thicknesses being chosen to exceed the
thickness of transformation range), we obtained the
absorption spectra (Fig. 4, curves 8–10). To plot the
curve 8, we used the samples with the thicknesses 365
and 415 μm, curve 9 – 365 and 470 μm, and curve 10 –
365 and 515 μm.
The accuracy of this method, like to that inherent
to the Bouguer-Lambert formula, is defined by the
accuracy of sample thickness measurements as well as
by homogeneity of photoluminophore distribution,
and by the influence of sample luminescence. The
latter increases the absorption coefficient value with
increasing the thickness. These reasons enable to
explain insignificant differences between the curves 8
to 10 in Fig. 4. To enhance the accuracy of the
method, one should use more samples with different
thicknesses (higher than the transformation one) as
well as apply the excitation source with the spectral
half width lower than the Stokes shift inherent to this
photoluminophore.
Fig. 4. Dependence of absorption coefficients on the light
wavelength after determination by using the Bouguer-Lambert
law (1–7) or the method of two thicknesses (8–10).
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2015. V. 18, N 2. P. 215-219.
doi: 10.15407/spqeo18.02.215
© 2015, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
219
Conclusions
The results of this work have demonstrated that the
shape of scattering indicatrix in optically
inhomogeneous medium is changed from narrow
directed up to the spherical one. It has been shown
that, for low thicknesses of composite films and
low concentrations of microparticles, the change in
emission intensity is mainly realized via the change
in the scattering indicatrix. Only when the film
thickness exceeds that for the range scattering
indicatrix transformation, the change in the
emission intensity is exclusively conditioned by
absorption.
Performed in this work has been investigation of
changes in the emission intensity in optically
inhomogeneous medium (film prepared from
photoluminophore suspension) in dependence of
the film thickness.
Having taken into account the change in the
scattering indicatrix, we offer the new method of
two thicknesses for determination of the absorption
coefficient dependence of composite films on the
light wavelength and consider the conditions that
should be kept when using it.
It has been shown experimentally that, when using
the classical Bouguer-Lambert law for
determination the spectral dependence of the
absorption coefficient, it is necessary to take into
account transformation of light scattering
indicatrix.
The developed method has been demonstrated
using inorganic photoluminophore FLY-7 with the
mean size of microcrystals 5 μm. Near the
absorption peak of the film (448 nm), its absorption
coefficient reaches the value 124 cm
–1
, if the
photoluminophore concentration is close to 20%.
References
1. http://www.cree.com/News-and-Events/Cree-
News/Press-Releases/2014/March/300LPW-LED-
barrier
2. D.N. Khmil, A.M. Kamuz, P.F. Oleksenko,
V.G. Kamuz, N.G. Aleksenko, O.A. Kamuz,
Luminophors in white LEDs: advantages and
deficiencies // Optoelektronika i poluprovod-
nikovaya tekhnika, 47, p. 5-23 (2012), in Russian.
3. V.L. Lyovshin, Photoluminescence of Liquids and
Solids. Gostekhizdat, Moscow–Leningrad, 1951,
p. 39-44 (in Russian).
4. A. Isimaru, Propagation and Scattering of Waves
in Randomly-Inhomogeneous Media. Vol. 2.
Moscow, Mir, 1981, p. 20-26 (in Russian).
5. T.H. James, Theory of Photographic Process.
Leningrad, Khimiya, 1980, p.565-570 (in Russian).
6. H.H. Theissing, Macrodistribution of light scattered
by dispersions of spherical dielectric particles //
JOSA, 40, p. 232-242 (1950).
7. D.-Y. Kang, E. Wu, and D.-M. Wang, Modeling
white light-emitting diodes with phosphor layers //
Appl. Phys. Lett. 89, 231102 (2006).
8. Z. Liu, S. Liu, K. Wang, and X. Luo, Measurement
and numerical studies of optical properties of
YAG:Ce phosphor for white light-emitting diode
packaging // Appl. Opt. 49, p. 247-257 (2010).
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