Thermometry using thermographic phosphors
1. Introduction
In recent years a new technique has been developed for remote measurements of surface
temperature, but also with a potential for spray and even gasflow diagnostics. It has mainly
been used in scientific and industrial applications of surface thermometry to complicated
geometries, e.g., rotor engines, turbine engines, and also in medicine. Other quantities such as
heat flux through a surface have been investigated, because of its high importance to science
and engineering community.
During the last years, as the applications of thermographic phosphors have expanded, some
attempts have also been made in combustion environment. A useful review article [1] could
be a good assistance as an introduction to the subject of phosphor thermometry.
Thermometry based on the use of thermographic phosphors utilises the physical properties of
the phosphor particles for assessing temperature. The phosphor particles used in thermometry
are usually inorganic materials having the form of a white-brown powder some 1 - 10 µm in
diameter. Such a phosphor consists of a host material and a doping agent from which the light
is emitted. A large number of different phosphors are produced today. These cover a wide
range of temperatures, from cryogenic temperatures up to 1700 C or higher, making them
suitable for many different applications. Each phosphor that is selected is highly sensitive
within a specific range of temperatures, exhibiting an accuracy in the order of 1-5 C. Once
deposited on the surface of interest and excited by a suitable wavelength, mainly UV light, the
phosphor particles emit an intense luminescent light. This emission is called phosphorescence
or fluorescence, terms often used interchangeably, although fluorescence usually refers to
emissions having a duration of 10-10 -10-7 s and phosphorescence to their having a duration in
the order of 10-7-1 s. After excitation of the thermographic phosphor, the subsequent emission
is imaged onto a detector. The temperature can then be deduced from the spectral or temporal
properties of the recorded signal. This technique provides a high quantum yield, twodimensional measurements and remote thermometry, as well as a high degree of accuracy. In
contrast to pyrometry, it is not influenced by the emissivity of the material, background
reflection from the surroundings or light absorption by optical windows or by surrounding
gases. These advantages have allowed thermographic phosphors to be used in a wide variety
of applications and in harsh environments.
Thermographic phosphors (TP) have normally been used for hard non-combustible materials
such as steel or concrete. For those surfaces the phosphor can be applied to the surface and
only a very thin layer is required ensuring that the phosphor layer does not influence the flow
and heat transfer from and to the surface. In this Chapter its application for combustion
diagnostics are described.
2. Background
Thermographic Phosphors which have temperature dependant emission are mostly inorganic
and made of some ceramic material. The thermographic phosphor is composed by a carrier
material which is doped with some activator material. The activator is often a rare earth metal.
The doping concentration is typically about one percent, which is a small enough
concentration for the activator atoms to be isolated from each other by the host matrix. The
host material is mostly transparent to radiation, i.e. it is mostly the activators that absorb and
emit radiation. Most phosphors used for thermometry are exited by laser radiation. The energy
is absorbed by the rare earth metal in an electron excitation. The electron is then nonradiatively relaxed to a meta-stable energy level, i.e. a level from where no transitions are
allowed.
1
The emission spectra of any material will change with temperature since the Boltzmann
distribution affects vibrational levels in ground- and excited states. Thermographic phosphors
have a strong temperature dependency due to several reasons. When reaching a certain
temperature, the excited electrons in the activator atoms will populate high enough energy
levels to transfer its energy to the surrounding host material. This energy level is called the
Charge Transfer State (CTS) of the host material and is described in the energy level diagram
in Figure 1.
The Charge Transfer State is located at very low energies (<40*103 cm-1) for e.g. Eu3+ and
Yb3+. Energy transferred to the CTS will relax non-radiatively to its lowest energy state. In
the case of Eu3+ the CTS will then feed the D5 levels from where relaxation will occur in form
of radiation. At high temperatures fast de-excitation via the CTS to the lowest CTS energy
state will be more probable and thus shorten the lifetime of the phosphorescence. Measuring
lifetime is consequently one way to determine the temperature of the phosphor.
A high temperature will also increase vibrations in the host material crystal lattice. This will
broaden the linewidths of the emitting transitions in the phosphorescent process. Thermal
expansion in the host lattice will also induce a frequency shift in the phosphorescence spectra,
which accordingly can be used for temperature determination.
Energy /
103 cm-1
40
Host CTS
3
30
5
D
2
1
0
20
Eu3+-levels
10
7
F
6
5
4
3
2
1
0
0
Figure 1. The charge transfer state (CTS) as it affects the de-excitation in the La2O2S:Eu
phosphor. Excited electrons at high 5D-levels in the Eu3+-ion can relax via the La2O2S CTS to
the Eu3+ 7F ground state level.
There are, as mentioned above, a number of physical phenomena that influence the spectral
shape of phosphorescence from thermographic phosphors depending on temperature. The
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changes in the phosphorescence spectra will take form as changes in phosphorescence
intensity, changes in phosphorescence lifetime and/or line shifts in the phosphorescence
spectra. The two most common changes used for thermometry are lifetime changes and
changes in intensity at specific wavelengths.
3. Temperature measurement approaches
3.1 Lifetime method
As described above most thermographic phosphors have a lifetime which is temperature
dependent. According to Eq. 1 the intensity will decay exponentially according to
I =I o e
−
t
τ
(1)
where I0 is the initial emission intensity, t is time and τ is the lifetime of the phosphorescence.
τ is the amount of time to which the intensity has decreased to 1/e of the initial emission I0. In
order to cover a large temperature interval several different phosphors may be used. Some of
these and their corresponding lifetimes are shown in Fig. 2a. An example is shown in Fig 2b
where the temporal decays are measured for three different temperatures. The
phosphorescence lifetime will decrease with temperature depending on the phosphor material.
Temperature can thus be calibrated and determined by calculating the phosphorescence
lifetime from the measured intensity decay. This is normally done by fitting the intensity
decay to the theoretical model (eq. 1), using a non-linear fitting procedure. The error in
temperature from such a measurement can ideally be less than 1%.The lifetime method is
usually used for point temperature measurements using a Photo-Multiplier Tube (PMT) for
detection.
Figure 2a. Temperature sensitivity of different phosphor materials.
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Figure 2b.Phosphorescence
decays measured at different
temperatures. The lifetime
becomes shorter when the
temperature increases.
3.2 Intensity ratio method
The intensity of certain peaks in the phosphorescence spectrum is often temperature
dependant and can thus be calibrated to temperature. Simultaneous measurements of two
different wavelengths allow an instantaneous temperature measurement. The phosphorescence
spectra of the thermographic phosphor YAG:Dy at different temperatures is shown in Figure
3.
Figure 3 - Phosphorescence spectra as a function of temperature. Comparison of the peak at
455 nm with the one at 493 nm is used for determination of temperature.
4
The non-radiative deexcitation rate is strongly dependent on temperature resulting in a
decrease of emission efficiency and shortening of the emission decay time at high
temperature, although not at the same rate for the different emitted wavelengths. The
advantage of this technique is that the ratio between the two apparent peak intensities of the
phosphor is only dependent on temperature. Following a UV excitation the emission signal,
Sj, can be represented by:
Sj = CjTeNj(T)τj I
(2)
Where Cj is the detection efficiency for the jth transition Te is the camera exposure time,
Nj(T) is the temperature dependent quantum efficiency for the jth transition, τj is the optical
filter transmission and I is the intensity of the laser beam. By rationing the signal for the two
different transitions, the resulting value is only dependent on temperature as:
S j +1
Sj
=K
N j +1(T)
N j (T)
(3)
Where K is a constant
4. Calibration procedures
In order to correlate the phosphorescence to temperature, a series of calibration measurements
must be performed under well controlled conditions. Figure 4 shows the set-up of a
calibration measurement using a laser for excitation, fibres for guiding light, filters for
increasing the signal to background ratio, and detectors for acquiring the phosphorescence
emission.
Figure 4 Calibration set-up using a spectrometer, ICCD, laser, PMT, thermocouple and the
test cell.
During the calibration procedure several phosphors have been calibrated and both temperature
measuring approaches were used and sometimes compared. In the experiments low laser
intensity, 50 µJ, was enough to obtain phosphorescence. A power dependence test on the
intensity of the emission did not show any noticeable variation of the phosphor lifetime to the
laser power between 50 µJ and l mJ.
5
The phosphor was excited by the third or fourth harmonic of a Nd: YAG laser at 355 and 266
nm, respectively, with a pulse duration of about 7 ns and a repetition rate of 10 Hz. The laser
light was focused into a fiber with a diameter of 0.7 mm and then transmitted to the phosphor
in the test cell at the other end of the fiber. The subsequent emission was collected by two
other fibers, which individually supplied a spectrograph connected to an intensified charged
coupled, device (ICCD), and a photomultiplier tube (PMT), for spectrally and temporally
resolved measurements, respectively.
The test measurements were done inside a cell used for the calibration. A rod of the same
material as the cell (inox) was coated at one end with a mixture (50: 50)% of the phosphor
and a ceramic cement (cerastil-c) to a thickness of approximately 200 µm.
The rod was inserted into the cylindrical chamber of the cell, which had one small input port
for the laser beam and two output ports for the emission. The radiation was guided by quartz
fibres (Ø=0.7mm). A calibrated thermocouple with an accuracy of ±1 K was used to measure
the temperature close to the phosphor inside the cell. The temperature of the wall of the
cylinder could be changed between room temperature and 1000 K using heating wires.
An example on the output from these calibration experiments using the wavelength ratio
approach is shown in Figure 5 giving the calibration curve using the YAG:Dy phosphor when
the ratio between the peaks at 455 nm and 493 nm seen in Figure 33 was evaluated for
different temperatures. As can be seen the calibration curve is very smooth and is then used
for evaluating the experiments taken place at different applications.
Figure 5 - Calibration curve for YAG:Dy using the intensity ratio method. The ratio of the
peaks at 455 and 493 nm as a function of temperature.
As described above an alternative, or a complement, to the intensity ratio technique is to
measure the life time of the phosphorescence decay. Using the same set-up as above it was
also possible to measure the decay as a function of temperature. In Figure 6 are shown two
calibration curves using different phosphors. As can be seen it is important to use the one with
highest sensitivity in the temperature region of interest. .
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Figure. 6. Calibration curves for two thermographic phosphors
Figure 7 shows the resulting lifetime and intensity ratio calibration curves for one selected
phosphor. In this case a phosphor to be used for measurements of temperatures on burning
materials, in the temperature range between 300 and 600 C. The precision of temperature
determination in the case of the ratio technique was found to be 5-10°C while the lifetime
technique showed a higher precision of 1-5 °C. This is generally the case that the lifetime
approach has better precision than the intensity ratio approach. One reason for this is that a
lifetime is measured in several points whereas the intensity approach is based on a simple
ratio.
Fig. 7 Calibration of temperature against the lifetime and spectral intensity ratio of the
phosphorescence. The left axis shows the lifetime versus temperature on a logarithmic scale
for two different calibrations days. The right axis presents the relative intensity ratio of the
peaks of the spectrally resolved emission against temperature.
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5. Two dimensional surface temperature measurements.
So far the discussion has been limited to a principal discussion of the two temperature
measuring approaches from a point of view that point measurements are to be performed. In
practice it is, however, as pointed out in the course at several occasions, very important to
perform two-dimensional measurement. Clearly the approach for intensity ratio is straight
forward. In this case the measurement object is illuminated by a laser beam and the object can
then be imaged by lenses on two different CCD cameras where the key wavelengths of
interest are spectrally isolated by interference filters. The two-dimensional temperature data
can then be calculated by dividing the two images and comparing each pixel to the calibration
curve, with an error of about 5%. An alternative and a cheaper solution allowing one detector
to be used is to use a special designed stereoscope shown in Figure 8.
Figure 8. An optical stereoscope producing two images in different colours
After excitation, the incoming phosphorescence emission passes through separate filters
before encountering a 45 degrees mirrors and a prism. The result is two identical images on
the CCD ship of the same object. The reflection curve of the mirror surface shows a constant
reflection in the wavelengths range of the incoming phosphorescence. Interference filters are
placed in front of each entrance of the stereoscope in order to detect phosphorescence signals
at these wavelengths. For each measurement, the intensity images corresponding to each
wavelength are warped using a grid image, then they are subtracted from background and
digitally divided by a reference image as;.
R=
I phos1 − B1
Ref1
⋅
Ref 2
I phos 2 − B2
(4)
Where R, is a matrix holding the ratio values of all the pixels. Iphos1 is the detected image
through filter 1, and Iphos2 is the one detected through filter 2. B is the background
corresponding to each image. Ref stands for the reference image. The reference image is
recorded to compensate from a possible non uniformity of pixel sensitivity in the CCD
camera. The reference image is recorded using a uniform light source imaged through the
stereoscope using the same running conditions (CCD gain and exposures time etc). The
resulting temperature ratio image is finally inferred from a previously recorded calibration
curve.
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With this stereoscope it is thus possible to produce two images, at different colours, imaged
onto one CCD detector thus eliminating the need for two CCD when using the spectral ratio
approach. (The warping procedure as referred to above assures so that the two images on the
CCD get a perfect match on a pixel-by pixel basis. This is not always easy without this
process since the fibre bundle leading the light from the image intensifier to the CCD detector
to a certain extent may be twisted).
Still, as indicated above it is also of interest to use the lifetime approach since that often have
a higher precision. This can also be done in 2D by using the fast framing camera described in
a previous Lecture. The phosphor was excited by the same source of laser light 266 nm as in
the calibration measurements. The phosphorescence images were obtained by eight
consecutively gated CCD detectors enable pixel by pixel-lifetime evaluation of the
phosphorescence by interpolating an exponential-decay curve to the counts of the
corresponding pixel positions of the sequential CCD images. The temperature at each pixel
position was evaluated using a calibration procedure of temperature against lifetime (see Fig.
6).
The 2D-temperature imaging technique was tested on a low-density fiber board in a small
flame spread scenario. The phosphor was applied to a surface of 4x3cm by pressing the
phosphor powder onto the sample so that a thin coating was obtained. The board was placed
vertically and heated at the lower part by burning alcohol. The use of alcohol as ignition fuel
was convenient since it generated less soot that might interfere with the measurement. The
phosphorescence from the surface of the combustible board was recorded with the framing
camera. The detectors inside the camera were sequentially gated, as illustrated in Figure 9a
with an individual exposure time of 100 µs so that immediately as one detector was
deactivated the following detector was activated. This was done to capture as much
phosphorescence as possible at high temperature under an observation time of 800 µs. The
acquired images were first subtracted from the background and divided by a reference image
obtained from the previous camera calibration. Figure 9b shows the experimental set-up used
to measure the 2D temperatures during the flame spread experiments.
Fig. 9a The lifetime was extracted by fitting a simple exponential function to the detected
intensities at each pixel position. Then temperature information was deduced from the
lifetime. 9b. The experimental set-up for the measurement of flame spread.
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Figure 10 shows the surface temperature at the higher part of the board during different
moments of an upward spreading flame. Figures 10a and 10b give the temperature soon after
(4 seconds and 18 seconds, respectively) the time of ignition of the alcohol before any strong
temperature changes were observed in the upper region. Later, 32 seconds after ignition, the
flames have reached the investigated region and regions of hot temperature are seen in Fig.
10c. The flame spread increased faster until it reached maximum temperature as shown in Fig.
10d, corresponding to 48 seconds after ignition. A maximum temperature of 673-773 K and
strong gradients of temperature are observed in the figure. Thirty seconds after, Fig. 10e, the
flame started to decrease in intensity and local flame extinction started to appear some tens of
seconds later as shown in Fig. 10f and 10g. Finally flames were only observed at the upper
edges of the board and in Fig. 10h flames do not appear any longer. Just some local areas
continued to react, most probably by pyrolysis. In the lower right figure, contour plots of
temperatures (from Fig. 10g) are presented with a temperature resolution of 5 K so that
gradients can be more clearly seen. The results from these 2-D measurements are in
agreement with previous one-point measurements performed on the same type of fiberboards,
where the highest flames were observed 42 seconds after ignition and a surface temperature of
710 K was found. The temperature measurements showed the best accuracy and precision at
high temperatures (673-773 K). At lower temperatures the properties were less good (or more
critical) mainly because the total time window used for the decay measurements was not wide
enough. The total exposure time used was 800 µs whereas the lifetime was 2-3 ms at low
temperatures.
Fig. 10 Temperature during flame-spread: Two-dimensional surface temperature of a lowdensity fiber board in a flame-spread scenario. Time shown below each image is time after
pool ignition
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6. Temperature measurements of decomposing materials
Biocomposites include a wide range of products for different applications, from construction
or insulation panels to plastic products based on polymers. The composition boards, including
particle boards and fibre boards, especially medium-density fibreboards (MDF) belong to the
most common materials for construction, furniture, and interior decoration. The application of
these materials, which are relatively new, is constantly growing in the field of furniture
manufacturing.
The wide range of use of these construction materials in our surrounding is motivated for
economical reasons, however, questions rise about safety aspects especially in the case of a
flame spread hazard. Much research is going on today to study the materials physical
properties, their decomposition in a low oxygen concentration environment such as flame
spread scenario. Measurements of particle surface temperature and mass loss during pyrolysis
could provide modellers with precise information on the kinetics properties during
decomposition, e.g. the activation energy and Arrhenius factor.
Surface temperature constitutes an important parameter in models that determine the heat
flow, in or out of the solid, and the material ignition temperature. Since surface temperature is
at the boundary between the gas and the solid phase, it is a critical parameter for fire
modellers. Many attempts have been made to characterise and measure this parameter as
accurate as possible. In Figure 11 is shown an experimental set-up used for analyses of
surface measurements during pyrolysis.
Fig. 7 A reactor filled with nitrogen gas was used to study the pyrolysis of wood particles.
After the excitation with 266 nm, the phosphorescence emission, from the wood particles, was
collected by a spectrograph and a photomultiplier.
The surface temperature of individual wood particles was measured inside a high
temperature reactor using thermographic phosphors which allowed remote, instantaneous
measurements. An ultraviolet laser beam was used to excite the phosphor particles which
were deposited on the investigated materials. The emission from the thin coating of phosphor
powder was spectrally resolved.
The material chosen was birch particles; the pyrolysis of birch particles was thus investigated
at a reactor temperature of 733 K. The reactor was stabilized at a constant temperature before
the particle was introduced. Then the surface temperature of the particle was measured.
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In Figure 12, results from these measurements are presented.
Temperature( K )
723
673
623
573
0
100
Time(seconds)
200
Fig. 12 Using the ratio technique, the surface temperature during pyrolysis was measured.
The bump seen in the figure was taught to come from the drying phase (water evaporation).
After 100 seconds, the surface temperature was seen to converge to the reactor temperature.
The measured surface temperature increased to reach a temperature slightly lower than 733 K.
It was noticed at a time duration between 50-100 seconds after the introduction of the particle
in the reactor, an evaporation phase of the water contained inside the particles occurred. This
evaporation phase decrease the surface temperature and thus could alter the pyrolysis rate.
From these tests, interest increased to investigate other materials in a wet and dry phase. In a
recent work some decomposing materials such as low-density fibre board, medium board,
particle board and polymethylmethacrylate (PMMA) were studied along with their mass loss
rate.
7. Internal combustion engine valve temperature measurements
Thermographic phosphors thermometry have also been used to measure engine valves and
transparent piston temperatures in two dimensions as well point wise of a running, optically
accessible, gasoline direct injection engine. The engine, fuelled with isooctane, was operated
in continuous and skip-fire mode at 1200 and 2000 rpm. A calibration of the phosphorescence
lifetime and spectral properties against temperature allowed temperature measurements
between 25 and 600°C. Results from the measurements show the potential of the technique
for two-dimensional mapping of engine walls, valves and piston temperatures inside the
cylinder. The work presented was performed in an optical direct-injected stratified-charge
(DISC) engine. The engine is based on a AVL 528 engine with a Volvo 4-valve gasoline
cylinder head. A schematic experimental set-up is presented in Figure 13 showing laser beam
alignment and signal collection, as used in the spectral method.
For the two-dimensional measurements, a CCD-camera was used together with an imaging
stereoscope. As described above the imaging stereoscope allows for two color operation via
image doubling. The stereoscope was mounted through an adapter on the objective lens of the
ICCD camera.
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Valves
Quartz Piston
Spherical lens
Stereoscope
+ Filters
ICCD
266 nm
Mirror
Figure. 13 The experimental setup for the case of the spectral method, showing the optically
accessible engine (to the left) and the CCD-camera with the stereoscope and filters (to the
right).
Images (single-shot) of the temperature of the four valves are presented in Figure 114. Each
image was recorded at a certain time after start of the engine. As can be seen, the temperature
of the exhaust valves started to increase earlier than the temperature of the intake valves. This
is due to the burned gases heating the exhaust valves while leaving the combustion chamber.
On the contrary, the intake valves were cooled by newly introduced air. From the figures, one
could notice that the temperature started usually to rise from the edges of the valves to the
center. After 120 s, high heat conduction from the exhaust to the intake valves could be seen.
In several tests achieved on the valves, the average temperature is stable, but the temperature
distribution was observed to be usually higher in the surrounding edge of the valves.
After 150 seconds, some islands of low temperatures appeared on the exhaust valves. This
could be related to soot deposits on the coated surface or on the piston. These deposits could
decrease the strength of the transmitted signal and thus increase the errors of the processed
temperature images.
Deg. C
500
400
10 s
50 s
300
70 s
80 s
150 s
180 s
200
100
100 s
120 s
Figure 14 Temperature images of the valves at different times after start of the engine. Intake
valves are seen in the upper part and exhaust valves in the lower part. The laser was fired 30
CAD BTDC. The engine was run at 2000 rpm
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8. Droplets and spray measurements.
Sprays are crucial in a number of different industrial areas today, such as combustion and ink
jet printing. In particular the development of internal combustion engines to comply with
tightening emission legislation has led to an increased interest in sprays. To be more efficient
in this work, modelling plays an important role. However, the models for in-cylinder spray
break up, atomization, and vaporization are still not satisfactorily describing/predicting
several situations. To further develop the models, diagnostic tools are needed, e.g. for
studying droplet velocity, droplet size distribution, and droplet temperature. Laser-induced
phosphorescence from thermographic phosphors, seeded to distillate water and iso-octane, has
been shown to measure temperatures of single falling droplets.
The phosphors were excited by the fourth and third harmonics of a Nd:YAG laser, the
subsequent emission was evaluated by spectral and temporal investigations of the
thermographic phosphors Mg4FGeO6:Mn and La2O2S: Eu, respectively. The spectral and the
temporal methods allowed temperature measurements of free falling droplets up to 433 K.
Results from both methods are presented with an estimated accuracy of better than 1%.
In order to demonstrate this technique, the method was applied to mono-dispersed droplets. A
quantity of phosphors around 1 % (by weight) was added to the investigated liquid. The
experimental set-up is shown in Figure 15. A He-Ne laser was used to monitor the passage of
the falling droplets. Whenever a droplet crossed the continuous beam a trigger signal was sent
to the Nd:YAG laser. A UV beam with a diameter of 10 mm was then sent towards the
droplet at the correct spatial position. The laser beam was not focused on the droplet in order
to cover the entire droplet with light
Fig. 15 Experimental setup: A He-Ne laser was
used to trigger the
acquisition
of
the
phosphorescence light due
to the interaction of the
UV laser light with the
droplet.
The
phosphorescence signal
was stored in a detector
for subsequent processing.
For one point temperature measurements the subsequent emission from the droplet was
detected by a photomultiplier detector or a spectrograph for the temporal and the spectral
measurements, respectively. The temperature of the liquid could be regulated using heating
wires around the liquid container. Also 2D temperature measurements could be made and in
Figure 16 is shown a single-shot temperature measurement of a droplet.
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Figure 16. Two dimensional
temperature measurement using
phosphorescence from
thermographic phosphor particles
seeded to the liquid.
9. Conclusions
Thermographic phosphors are successfully used for temperature measurements of reactive,
non-reactive surfaces and in droplets. Special interests towards the exploitation of the
technique for temperature measurement of internal combustion engines components, gas
phase and the liquid phase e.g. droplets, and sprays are driving the investigation further.
Reference
1.
S.W. Allison, G.T. Gillies, Remote thermometry with thermographic phosphors:
Instrumentation and applications, Review of Scientific Instruments 68 (7), 2615-2650
(1997).
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