ETC FEDSM2000

Proceedings of ASME FEDSM’00
ASME 2000 Fluids Engineering Division Summer Meeting
June 11-15, 2000, Boston, Massachusetts
FEDSM2000-11043
DUAL EMISSION LASER INDUCED FLUORESCENCE TECHNIQUE (DELIF) FOR
OIL FILM THICKNESS AND TEMPERATURE MEASUREMENT
Carlos H. Hidrovo and Douglas P. Hart
Massachusetts Institute of Technology
Cambridge, MA 02139
ABSTRACT
This paper presents the development and implementation
of a Dual Emission Laser Induced Fluorescence (DELIF)
technique for the measurement of film thickness and
temperature of tribological flows. The technique is based on a
ratiometric principle that allows normalization of the
fluorescence emission of one dye against the fluorescence
emission of a second dye, eliminating undesirable effects of
illumination intensity fluctuations in both space and time.
Although oil film thickness and temperature measurements are
based on the same two-dye ratiometric principle, the required
spectral dye characteristics and optical conditions differ
significantly. The effects of emission reabsorption and optical
thickness are discussed for each technique. Finally, calibrations
of the system for both techniques are presented along with their
use in measuring the oil film thickness and two-dimensional
temperature profile on the lubricating film of a rotating shaft
seal.
INTRODUCTION
Laser Induced Fluorescence (LIF) is based on the use of a
light source to excite a fluorescence substance (fluorophore or
fluorescent dye) that subsequently emits light. The fluorescence
substance is used as a tracer to determine characteristics of
interest.
LIF has gained popularity as a general purpose
visualization tool for numerous 1-D, 2-D, and 3-D applications.
It, however, has seen limited use as a quantitative tool. The
reason for this stems primarily from the difficulty in separating
variations in excitation illumination and vignetting effects from
tracer emission. Presented herein is a two-dye ratiometric
technique that allows measurement of temperature and film
thickness while minimizing variations in excitation illumination
and non-uniformities in optical imaging.
Fluorescence is the result of a three-stage process that
occurs in fluorophores or fluorescent dyes (Haugland, R. P.,
1999). The three processes are (Fig. 1):
1: Excitation
A photon of energy hvEX is supplied by an external source
such as an incandescent lamp or a laser and absorbed by the
fluorophore, creating an excited electronic singlet state (S1’).
2: Excited-State Lifetime
The excited state exists for a finite time (typically 1–10 x
10-9 seconds). During this time, the fluorophore undergoes
conformational changes and is also subject to a multitude of
possible interactions with its molecular environment. These
processes have two important consequences. First, the energy of
S1' is partially dissipated, yielding a relaxed singlet excited state
(S1) from which fluorescence emission originates. Second, not
all the molecules initially excited by absorption (Stage 1) return
to the ground state (S0) by fluorescence emission. Other
processes such as collisional quenching, fluorescence energy
transfer and intersystem crossing may also depopulate S1. The
fluorescence quantum yield, which is the ratio of the number of
fluorescence photons emitted (Stage 3) to the number of
photons absorbed (Stage 1), is a measure of the relative extent
to which these processes occur.
3: Fluorescence Emission
A photon of energy hvEM is emitted, returning the
fluorophore to its ground state S0. Due to energy dissipation
during the excited-state lifetime, the energy of this photon is
lower, and therefore of longer wavelength, than the excitation
photon hvEX. The difference in energy or wavelength
represented by (hvEX–hvEM) is called the Stokes shift. The
Stokes shift is fundamental to the sensitivity of fluorescence
techniques because it allows emission photons to be detected
against a low background, isolated from excitation photons.
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Ie
If
If,1
If,1’
If,2
Io
t
T
x
y
∆V
ε(λ)
Figure 1: Fluorescence principle (Haugland, R. P., 1999)
From this description it is apparent that LIF can be used to
measure any scalar that affects the fluorescence of the dye.
Fluorescence is a function of the dye characteristics, the dye
concentration, the exciting light intensity, and the scalar being
measured. Once a particular dye and concentration are
selected, the fluorescence dependence on these factors is
constant. The problem lies in the irregularity of the illumination
light intensity when a laser is used. Most laser beams are not
uniform. They fluctuate in intensity in space and time. Pulsed
Nd:YAG lasers are particularly prone to exhibit this behavior.
Use of pulsed Nd:YAG lasers is desirable though, because of
their short pulse duration (consequently short fluorescence
emission), which allows for nearly instantaneous measurements
of the desired scalar.
In order to correlate two-dimensional fluorescence intensity
to the scalar of interest, spatial variations in illumination
intensity must be determined. This can be accomplished by
using a ratiometric technique where the fluorescence intensity
containing the desired scalar information is divided by the laser
intensity eliminating the fluorescence dependency on excitation
intensity. One way to achieve this is by using two fluorescent
dyes. This technique is known as Dual Emission Laser Induced
Fluorescence (DELIF) (Coppeta, J., Rogers, C., 1998, Coppeta,
J., et. al., 1997, Sakakibara, J., Adrian, R. J., 1999).
NOMENCLATURE
A
area of one pixel
C
dye molar concentration, effective two-dye molar
concentration
C1
dye 1 molar concentration
C2
dye 2 molar concentration
dIf
differential fluorescence intensity
dIf,1
dye 1 differential fluorescence intensity, without
reabsorption
dIf,1’
dye 1 differential fluorescence intensity, with
reabsorption
dx
differential length in x-direction
F
fluorescence power
ε1(λ)
ε2(λ)
Φ
Φ1
Φ2
η1(λ)
η2(λ)
λlaser
λfilter1
λfilter2
τ
exciting light intensity
total fluorescence intensity
dye 1 total fluorescence intensity, without reabsorption
dye 1 total fluorescence intensity, with reabsorption
dye 2 total fluorescence intensity
exciting light intensity at x=0
film thickness
temperature
coordinate perpendicular to plane of observation
coordinate parallel to plane of observation
volume element
molar absorption (extinction) coefficient at a given
wavelength (absorption spectrum); effective two-dye
molar absorption (extinction) coefficient
dye 1 molar absorption (extinction) coefficient at a
given wavelength (absorption spectrum)
dye 2 molar absorption (extinction) coefficient at a
given wavelength (absorption spectrum)
quantum efficiency
dye 1 quantum efficiency
dye 2 quantum efficiency
dye 1 relative emission at a given wavelength
(emission spectrum)
dye 2 relative emission at a given wavelength
(emission spectrum)
laser wavelength
narrow band filter 1 wavelength
narrow band filter 2 wavelength
time
LIF BASICS
Optically Thin versus Optically Thick
Consider a rectangular differential volume of fluid mixed
with a fluorescent dye with cross-sectional area A and length ∆x
irradiated by light (normal to the area A) with uniform intensity
Ie (see figure 2). The total fluorescence, F, emitted by this
differential volume is given by:
F =I e laser
)C
V
(1)
Figure 2: Fluorescence of fluid element
2
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From equation (1), it is evident that fluorescence intensity
is dependent on:
(1) the amount of exciting light available to produce
molecular transitions to higher, excited levels,
(2) molar absorpsivity, which determines how much of the
incident light per molecule produces actual molecular
transitions,
(3) dye concentration, which is a measure of the number
of molecules present,
(4) quantum efficiency, which is the ratio of the energy
emitted by the energy absorbed, and is a measure of
how much of the energy stored in the higher electronic
states is emitted as fluorescence light, when the
molecules return to their ground state, and,
(5) the volume of the element, which is the control volume
over which excitation and fluorescence takes place.
Dividing equation (1) by the area A, the fluorescence
intensity normal to the area A is obtained,
I f =I e laser
)C
x
( 2)
If the area A is assumed to be the projected area of a single
pixel, it is apparent that pixel intensity is proportional to the
excitation intensity, dye characteristics, concentration, and
thickness of the fluid element. For very thin film thickness, this
representation is accurate. If the excitation intensity is known,
dye characteristics, and concentration are constants, the fluid
film thickness can be directly inferred from the fluorescence. A
more accurate representation of the fluorescence phenomena
can be obtained by from Lambert’s Law of Absorption (Poll, G.,
et. al., 1992), which takes into account the absorption of the
exciting light by the finite fluid through which it travels;
[
I e(x)= I o exp − laser
)C x
]
[
I f (t)= ∫ 0 dI f = ∫ 0 I o exp − t
t
laser
]
)C x laser
)C dx
( 6)
such that
I f (t)= I o
{1 −exp[−
laser
)C t
]}
(7 )
For small values of t (thin films), equation (7) can be
approximated as:
I f (t)≈ I o laser
)C t
(8)
This is identical to equation (2) and is the basis for the
concepts of optically thin and optically thick systems. The
fluorescence dependence on film thickness is linear for optically
thin systems, while it is exponential for optically thick systems.
What is considered a thin or thick film thickness depends on the
product ε(λlaser)C.
fluid film
differential
element
excitation
emission
camera
dx
Y
X
x
t
(3)
Figure 3: Fluorescence through a thick fluid film
Consider the differential element shown in figure 3 within a
region of finite film thickness. The fluorescence intensity
collected by the CCD from this fluid element is
dI f = I e laser
)C dx
( 4) .
Thus, from equation (4):
[
dI f = I o exp − laser
]
)C x laser
)C dx
(5)
For a given fluid thickness, t, the total intensity collected
by the CCD is
Reabsorption
Emission reabsorption is often encountered in fluorescence
techniques and is generally problematic. Fluorescent dyes have
different absorption spectrums and emission spectrums (Fig. 4).
When the emission spectrum of one dye overlaps the absorption
spectrum of another or with its own absorption spectrum,
reabsorption of the dye fluorescence occurs (Fig. 5). This has
two effects: (1) it increases the fluorescence emission of the
second dye as, in addition to the external light source excitation,
it is being excited by the fluorescence of the first dye. More
importantly, (2) the fluorescence emission of the first dye is
reduced since it is being reabsorbed by the second dye. In LIF,
the external illumination intensity is generally much greater
than dye fluorescence. Consequently, the increase in
fluorescence emission due to excitation by the fluorescence of
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molar absorpsivity (ε)
fluorescent emission (η)
one dye by another can be neglected. This is not the case for
the reduction in fluorescence of a dye due to reabsorption by a
second dye since this reduction can be substantial in
comparison with the total emission of the dye when there is no
reabsorption. From the differential element of figure 3, it is
apparent that the differential fluorescence emission produced by
any single element must travel back through the medium before
reaching the CCD.
wavelength
wavelength
Figure 4: Emission and absorption spectrums
[
dI f,1’= I o exp − I f,1’(t,
If there is reabsorption of the differential element
fluorescence, Lambert’s law must be applied to the differential
fluorescence emission in order to compute the actual
fluorescence collected by the CCD. Thus, assuming the
situation represented in figure 5 occurs,
(
1
dI f,1’=dI f,1 exp[− 2( )C 2 x ]
laser
)C1
laser
( )
1 1
(11)
filter1
)=
[
)exp − 2(
I o 1(
laser
laser
laser
laser
filter1
)C1
1
)C + 2(
)C + 2(
)C x
]
(
)C1
1
]
)C 2 x dx (12)
(
filter1
filter1
)C 2
1
filter1
laser
1
]
)
)C 2 t })
(13)
For reabsorption to play a significant role on the
fluorescence, the system must be optically thick and ε2(λfilter1)C2
>≈ O[ε(λlaser)C].
Figure 5: Reabsorption schematic
]
[
filter1
[
)C x
(
1
) = ∫0 I o exp − t
filter1
× (1−exp{− laser
]
Equation (5) has been modified in equation (9) to reflect
the fact that the fluorescence emission occurs over a wide range
of wavelengths that constitute the emission spectrum. In the
same way equations (10) and (11) portray a reabsorption that
occurs over a wide range of wavelengths. If the emission
spectrum of dye 1 and the absorption spectrum of dye 2 are
known, equation (11) can be integrated over varying film
thickness and wavelengths in order to compute the total
intensity collected by the CCD. If a very narrow interference
filter is used to filter all wavelengths except for the one of
interest, equation (11) can be simplified by removing the
dependence of the differential intensity on the emission and
absorption spectrums. Thus, the total intensity collected on the
CCD can be calculated as:
× 1(
[
)C x
× exp[− 2( )C 2 x] dx d
I f,1’(t,
dI f,1 = I o exp − laser
)C 1
( ) dx d
(9)
1 1
DELIF, THE RATIOMETRIC APPROACH
Principle
In the previous analysis, based on figure 3, and in equations
(1) through (13), the non-uniformity of the exciting light
intensity over the plane of observation and in time is not taken
into consideration. In reality, illumination intensity is a function
of both position and time;
I o = I o(y, (14)
(10)
Therefore,
I f = I f (t, y, 4
(15)
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Consequently, film thickness cannot be inferred from
fluorescence intensity unless illumination intensity at a
particular location and time is known. The ratio of the
illumination intensity and the fluorescence intensity, however, is
independent of spatial and temporal variations in excitation
light intensity.
If
Io
R(t,
×
≡ R = R(t, (16)
Obtaining illumination intensity is not trivial. A twodimensional instantaneous illumination map, however, can be
inferred from the fluorescence of a second dye. This is the
principle behind DELIF:
(1) the fluorescence of dye 1 in a two-dye system contains
the desired information (film thickness, temperature,
which will be discussed later), along with exciting light
intensity information.
(2) the fluorescence of dye 2 also contains the exciting
light intensity information but behaves differently than
dye 1 to the scalar of interest.
(3) By rationing the fluorescence of dye 1 with the
fluorescence of dye 2, the excitation light information
is canceled out, giving a ratio that contains only the
desired scalar information.
(
[
filter1
,
filter 2
)=
I f,1 ’
I f,2
[
=
laser
)C (1−exp{− laser
)C + 2(
filter1
1
(
laser
)C1
1
(
laser
)C 2
2
2
laser
]
)C + 2(
[
(
1
2
filter1
)C 2 (1−exp{− (
filter1
)
filter 2
]
)
)C 2 t })
laser
]
)C t })
(19) .
is obtained.
By taking the ratio of the emission of the two dyes, the
excitation light intensity dependence is cancelled leaving a ratio
that is only dependent on film thickness. As film thickness
information is contained in the reabsorption of the fluorescence
of dye 1 by dye 2, the system must be optically thick, in order
for the reabsorption to be substantial and measurable (Fig. 6).
Oil Film Thickness
Oil film thickness measurements are achieved using an
optically thick system that takes advantage of reabsorption. The
film thickness information is contained in the reabsorption of
the fluorescence of dye 1 by dye 2. The excitation light
intensity information is contained both in the fluorescence of
dye 1 and dye 2. If two narrow-band interference filters are
used to capture the two distinctive fluorescence emissions, an
emission intensity defined by
I f,1’(t,
filter1 , y, =
[
× (1−exp{− I f,2(t,
filter 2
, y, =
I o(y, 1(
laser
laser
laser
filter 1
[
× (1−exp{− laser
]
filter1
)
)C 2
]
)C 2 t}) (17)
laser
)C t })
1
filter1
laser )C 2
(
1
)C + 2(
)C + 2(
I o(y, 2(
)C1
2
2(
filter 2 )
)C
Figure 6: Film thickness ratio
Temperature
It is possible to use LIF as a temperature indicator when
there is a dependence of either the molar absorption (extinction)
or quantum efficiency coefficients on temperature.
= 7
(18)
5
(20)
Copyright © 2000 by ASME
and/or
= 7
(21)
The problem of separating the temperature information
from the exciting light intensity contained in the fluorescence
still exists. This is further complicated by the film thickness
information that is also imbedded in the fluorescence. The
same two-dye fluorescence ratiometric approach used to
separate the film thickness information from the exciting light
intensity information can be used for temperature measurement.
However, the optical conditions for proper temperature
measurement are quite different from that of film thickness
measurement. Reabsorption of one dye fluorescence by the
other must be avoided as it adds film thickness information to
the fluorescence making it difficult to separate the temperature
information contained in the fluorescence. In addition, the
system must be optically thin. There are two reasons for this:
(1) even if there is reabsorption (it is hard to control whether a
system will have reabsorption or not, and in most practical
situations reabsorption is present) an optically thin system will
ensure that the reabsorption effects are minimal as the
fluorescence is approximately linear with film thickness. More
importantly, (2) it is easier to deal with temperature variations
in the direction of observation (i.e., x direction in figure 3). Let
us explore the last point in more detail. The goal in using
fluorescence for temperature measurement is to obtain a twodimensional map of temperature, that is, temperature variations
in the plane of observation. It, however, is very likely that the
temperature field also varies in the direction of observation. If
this is the case and, if in particular, equation (20) holds, one can
rewrite equation (6) as
[
I f (t,T)=∫ 0 dI f = ∫ 0 I o exp − t
t
× C dx
laser
]
,T)C x laser
the direction of observation on the fluorescence are not as
substantial and a more accurate two-dimensional map of the
temperature can be obtained. In the limit of optically thin
systems, the fluorescence will correlate to the temperature at the
boundary of the film, that is at location x = 0 for figure 3.
For optically thin systems with no reabsorption, one can
use the ratiometric approach to obtain a fluorescence ratio that
will correlate to temperature. Using equation (8) to calculate
the fluorescence of optically thin systems, one has:
R(T,
filter1
,
filter 2
)=
I f,1
I f,2
=
(
1
2
laser
(
,T)C1
laser
1
)C 2
2
1
(
filter1
(
filter 2
2
)
)
(23)
and/or
R(T,
filter1
,
filter 2
)=
I f,1
I f,2
=
(
1
2
laser
(
)C1
laser
)C 2
(T) 1(
1
2
2
(
filter 1
filter 2
)
)
(24)
The dependence of fluorescence on excitation light
intensity and film thickness cancels when the ratio of the two
fluorescences is used. By using this ratiometric approach on
optically thin systems, temperature variations in the direction of
observation are averaged over the film thickness and the
fluorescence ratio can be correlated to an average temperature
in the direction of observation (see figure 7).
,T)
(22)
However, since T = T(x), equation (22) cannot be
integrated unless the temperature field as a function of x is
known. This implies that, in order to correlate fluorescence to
temperature, an a priori knowledge of the temperature field in
the direction of observation is needed defeating the purpose of
the technique. Thus, the two-dimensional temperature map
cannot be easily inferred from the fluorescence for optically
thick films if the fluorescence temperature dependence is
contained in the molar absorption (or extinction) coefficient. In
general, it is difficult to correlate fluorescence with temperature
if there are temperature variations in the direction of
observation. However, if the temperature dependence is
contained in the quantum efficiency coefficient and/or the
system is optically thin, the effects of temperature variation in
Figure 7: Temperature ratio
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EXPERIMENTAL AND RESULTS
A two-12-bit-camera system was implemented in order to
simultaneously capture the two fluorescence intensities (see
figure 8). Dichroic mirrors and interference filters were used to
separate the laser intensity from the fluorescence and the
particular fluorescence emissions from each other. For oil film
thickness measurements, the oil was mixed with Pyrromethene
567 and Pyrromethene 650, and for temperature measurements
the combination of Pyrromethene 567 and Rhodamine 640 was
implemented. Two optical flats were placed on top of each
other with a 110 mm thick shim placed between them at one of
the ends. This produces a linearly increasing oil film thickness.
incoming laser
fluorescence 1 (567 nm)
fluorescence 2 (650 nm)
Film Thickness
Relative Intensity
3000
camera 1
sample
2000
1000
0
10
35
60
85
110
Thickness (m icrom eters)
camera 2
600 nm
dichroic
570 nm
dichroic
Figure 8: Experimental setup schematic
Pyr 567 (570 Filter)
Pyr 650 (620 Filter)
Ratio
Figure 9: DELIF for oil film thickness measurement
Figure 9 shows the two simultaneous fluorescence pictures
(Pyrromethene 567 and Pyrromethene 650) of the oil film
contained between the optical flats and their ratio. It becomes
apparent how the ratiometric technique eliminates the
fluorescence intensity variations due to the irregular laser
intensity profile. Figure 10 depicts the change in fluorescence
intensity with temperature for both Pyrromethene 567 and
Rhodamine 640, and their ratio. In addition, it can be seen in
Figure 11 how the ratiometric technique eliminates the laser
intensity and film thickness variation, leaving only temperature
information.
Fluorescence vs. Temperature
250
Relative
Intensity
200
150
100
50
0
20
70
120
170
Tem perature (C)
Rhod 640 (620 Filter)
Pyr 567 (570 Filter)
Ratio
Figure 10: Fluorescence versus temperature
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REFERENCES
Coppeta, J., Rogers, C., Philipossian, A., Kaufman, F. B.,
1997, “Characterizing Slurry Flow During CMP Using Laser
Induced Fluorescence”, Second International Chemical
Mechanical Polish Planarization for ULSI Multilevel
Interconnection Conference, Santa Clara, CA, February 1997.
Coppeta, J., Rogers, C., 1998, “Dual Emission Laser
Induced Fluorescence for Direct Planar Scalar Behavior
Measurements”, Experiments in Fluids, Volume 25, Issue 1, pp.
1-15.
Haugland, R. P., 1999, “Handbook of Fluorescent Probes
and Research Chemical”, Seventh Edition, Molecular Probes.
Poll, G., Gabelli, A., Binnington, P. G., Qu, J., 1992,
“Dynamic Mapping of Rotary Lip Seal Lubricant Films by
Fluorescent Image Processing”, Proceedings, 13th International
Conference on Fluid Sealing, B. S. Nau, ed., BHRA.
Sakakibara, J., Adrian, R. J., 1999, “Whole Field
Measurement of Temperature in Water Using Two-Color Laser
Induced Fluorescence”, Experiments in Fluids, Volume 26,
Issue 1/2, pp. 7-15.
Tem perature
30000
Relative
Intensity
25000
20000
15000
10000
5000
0
10
12
14
16
18
20
Thickness (m icrom eters)
Pyr 567
Rhod 640
Ratio
Figure 11: DELIF for temperature measurement
SUMMARY AND CONCLUSIONS
The bases for a two-dye Dual Emission Laser Induced
Fluorescence (DELIF) technique for film thickness and
temperature measurement were presented along with the basic
equations relating these scalar measurements to dye
characteristics and illumination intensity. Shown is that the
non-linearity resulting from emission reabsorption, while
detrimental to measurement of temperature, can be used to
accurately quantify film thickness. Experimental measurements
using Pyrromethene 567, Pyrromethene 650, and Rhodamine
640 demonstrate the feasibility of this technique at accurately
eliminating variations in illumination intensity to extract scalar
information.
8
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