Fluorescent microthermal imaging-theory and methodology for achieving high thermal resolution images Page: 3 of 10
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bright fluorescent line at 612 nm which is used for FMI. Figure 2 shows the emission
spectrum for crystalline EuTTA at 25 *C.
For thermal imaging applications, we need to know how the emission spectra changes
with temperature. Figure 3 shows the measured absolute quantum yield versus temperature
and figure 4 shows the decay time of the fluorescent yield versus temperature. Both of these
plots were generated for EuTTA in an ether:iso- pentane:ethanol (5:5:2) solution. For the
FMI application, a curve will need to be generated for each compound mixture that is used.
These data have been included to illustrate the temperature dependence of EuTTA.
The temperature-dependent quantum efficiency (Q(T)) of the EuTTA compound in
figure 3 can be fit to an equation of the form,
Q(T)= A + Be- . (1)
Monitoring changes in the quantum yield provides a simple way of imaging the temperature
of an object.
The standard FMI technique for EuTTA is to incorporate the chelate into a PMMA
(polymethyl- methacrylate) matrix. A typical base-mixture solution consists of 1.2 wt%
EuTTA, 1.8 wt% PMMA, and 97 wt% MEK (methylethylketone). The MEK is a very high
vapor pressure solvent that evaporates rapidly leaving the EuTTA/PMMA mixture on the
sample [1]. Typically this mixture is spun on the sample and allowed to cure in an oven at
125 *C for about 30 minutes. Ideally, the film should be only several optical absorption
lengths thick. At an excitation wavelength of 365 nm, a 300 nm film is approximately 3.5
optical absorption lengths thick. The concept is to have the film thick enough that most of the
UV light is absorbed, but thin enough that the thermal profile of the sample surface is not
distorted. The film should be as uniform as possible, but great pains to achieve perfect
uniformity of film composition and thickness are not necessary. The advantage of using
EuTTA/PMMA-based mixture is that it can easily removed once the thermal analysis is
completed. Rinsing the sample in acetone will dissolve the film in several minutes.
Now that we have an understanding of the fluorescent film properties which allow us
to use this technique to generate thermal images, we need to cover how that information is
converted to temperature data. The light intensity at a given point, (x,y), on the image can be
represented by
S(x, y) = I(x, y) ri(x, y) -r(x, y)- Q(T(x, y)) (2)
where I(x,y) is the illumination intensity, I(x,y) is the optical collection efficiency, r(x,y) is
the sample reflectivity, and Q(T(x,y)) is the quantum efficiency. Assuming that there are no
variations in I(x,y), g(x,y) and r(x,y) during the measurements, dividing an image taken with
the sample without bias, i.e. a cold image, by one under bias, i.e. a hot image gives the ratio
of quantum efficiencies between the cold and hot images
S R(x, y)= SH (x, y) _ I(x, y) - h(x, y) - r(x, y) - Q(TH (x, y)) Q(TH (x, y)) (3)
The expression of the quantum efficiency ratio can be greatly simplified at the expense
of a slight loss in accuracy for temperature determination if we ignore the leading constant in
the expression of Q(T) which makes the natural logarithm of the quantum efficiency ratio
proportional to the change in temperature. The proportionality constant, a, is determined
experimentally for a given film composition and can be used to calculate a relative
temperature change at a given pixel location.
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Barton, D. L. & Tangyunyong, P. Fluorescent microthermal imaging-theory and methodology for achieving high thermal resolution images, article, September 1, 1995; Albuquerque, New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc627709/m1/3/: accessed April 20, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.