LED-induced fluorescence diagnostics for turbine and combustion engine thermometry Page: 2 of 8
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deposited via electron-beam or ion-beam sputtering. A typical system will consist of an excitation light source, an optical
system for delivering the excitation to the surface of interest, an optical means for directing the resultant fluorescence to a
detector, and a data analysis system.
This paper contains a discussion with examples of issues and considerations with regard to utilizing blue LEDs as light
sources for fluorescence-based thermometry of turbine and combustion engines. The last section discusses phase
modulation techniques that are enabled with continuous emitting light emitting diodes.
The first consideration is the expected temperature range. Phosphors that are made from refractory materials such as garnets
and oxides are usually best for temperatures above about 700 C. A wide variety of other luminescent materials may be used
for the lower ranges. Choice of phosphor may further be delimited by the second consideration, whether or not the surface
of interest is moving. For surfaces moving at high speeds, a fluorescing area may move into and out of the field-of-view of
the collection optics. This is especially crucial for imaging applications of moving surfaces. In such cases, motion of a few
tens of microns may blur the image. The received signal will exhibit a time dependence governed by this motion in addition
to the exponential time dependence. For a phosphor with a sufficiently short decay time, this is not a problem.
Non-contact applications of phosphor thermometry formerly required laser sources. However, recent tests with a blue LED
peaked near 450 nm showed that it is possible to excite a wide variety of these materials with simple setups such as the one
below in Figure 1. There, an LED pumps a 1 mm diameter optical fiber which illuminates a sintered Y2O3:Eu sample seen
in the foreground. In the upper left hand corner, a photomultiplier, with bandpass filter, detects fluorescence. The LED is a
gallium nitride type from Ledtronics, part number BP280CWB1K-3.6Vf-050T, rated at 1000 mcd luminous intensity. All
efforts in this lab involve inorganic phosphors which consist of a host material and an activator atom, usually either a rare-
earth or transition metal. Excitable phosphor hosts include yttrium garnets, yttrium oxide, oxysulfides, vanadates, and
yttrium phosphates with activators of Eu, Tb, Dy, Pr, Sm, and Ce. To date the only materials not excitable by a blue LED
have been phosphors with emission bands below 450 nm and absorption well below 380 nm.
Phtomultiplier Fibereplic for [elIkering
Tube ieotcfiCe er
-b - *Excitation Iiaht fror LEE)
jrndc Sr"w t b w t 4
Filter . 4[,
o U * (' * * hr * 0 s.
i' *t w * 4" ~s * C! F M ti S
Figure 1. Sintered Y203:Eu sample illuminated by LED-pumped fiberoptic.
2.0 Long Decay Time
A very important host matrix material is Y3A15O12, commonly known by the acronym YAG which stands for yttrium
aluminum garnet. The phosphor YAG:Cr is a prime example of a material that luminesces efficiently when excited by a blue
LED. Figure 2 shows the excitation spectrum of YAG:Cr in relation to the emission spectrum of the blue LED used in the
tests described here. The sample was prepared in our organization by Matsubara.3 The LED emission peak falls between the
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Allison, S. W. LED-induced fluorescence diagnostics for turbine and combustion engine thermometry, article, August 17, 2001; Tennessee. (digital.library.unt.edu/ark:/67531/metadc715666/m1/2/: accessed January 18, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.