Editing Particle image velocimetry (section)

===Thermographic PIV===
Thermographic PIV is based on the use of thermographic phosphors as seeding particles. The use of these thermographic phosphors permits simultaneous measurement of velocity and temperature in a flow.

Thermographic phosphors consist of ceramic host materials doped with rare-earth or transition metal ions, which exhibit phosphorescence when they are illuminated with UV-light. The decay time and the spectra of this phosphorescence are temperature sensitive and offer two different methods to measure temperature. The decay time method consists on the fitting of the phosphorescence decay to an exponential function and is normally used in point measurements, although it has been demonstrated in surface measurements. The intensity ratio between two different spectral lines of the phosphorescence emission, tracked using spectral filters, is also temperature-dependent and can be employed for surface measurements.

The micrometre-sized phosphor particles used in thermographic PIV are seeded into the flow as a tracer and, after illumination with a thin laser light sheet, the temperature of the particles can be measured from the phosphorescence, normally using an intensity ratio technique. It is important that the particles are of small size so that not only they follow the flow satisfactorily but also they rapidly assume its temperature. For a diameter of 2&nbsp;μm, the thermal slip between particle and gas is as small as the velocity slip.

Illumination of the phosphor is achieved using UV light. Most thermographic phosphors absorb light in a broad band in the UV and therefore can be excited using a YAG:Nd laser. Theoretically, the same light can be used both for PIV and temperature measurements, but this would mean that UV-sensitive cameras are needed. In practice, two different beams originated in separate lasers are overlapped. While one of the beams is used for velocity measurements, the other is used to measure the temperature.

The use of thermographic phosphors offers some advantageous features including ability to survive in reactive and high temperature environments, chemical stability and insensitivity of their phosphorescence emission to pressure and gas composition. In addition, thermographic phosphors emit light at different wavelengths, allowing spectral discrimination against excitation light and background.

Thermographic PIV has been demonstrated for time averaged
<ref>
{{cite journal
 |last1=Omrane |first1=A. |last2=Petersson |first2=P. |last3=Aldén |first3=M. |last4=Linne |first4=M.A.
 |year=2008
 |title=Simultaneous 2D flow velocity and gas temperature measurements using thermographic phosphors
 |journal=[[Applied Physics B: Lasers and Optics]]
 |volume=92 |issue=1 |pages=99–102 |doi=10.1007/s00340-008-3051-1
|bibcode = 2008ApPhB..92...99O |s2cid=121374427 }}</ref>
and single shot
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{{cite journal
 |last1=Fond |first1=B. |last2=Abram |first2=C. |last3=Heyes |first3=A.L. |last4=Kempf |first4=A.M. |last5=Beyrau |first5=F.
 |year=2012
 |title=Simultaneous temperature, mixture fraction and velocity imaging in turbulent flows using thermographic phosphor tracer particles
 |journal=[[Optics Express]]
 |volume=20 |issue=20 |pages=22118–22133 |doi=10.1364/oe.20.022118
|pmid=23037361 |bibcode = 2012OExpr..2022118F |doi-access=free }}</ref>
measurements. Recently, also time-resolved high speed (3&nbsp;kHz) measurements
<ref>
{{cite journal
 |last1=Abram |first1=C. |last2=Fond |first2=B. |last3=Heyes |first3=A.L. |last4=Beyrau |first4=F.
 |year=2013
 |title=High-speed planar thermometry and velocimetry using thermographic phosphor particles
 |journal=[[Applied Physics B: Lasers and Optics]]
 |volume=111 |issue=2 |pages=155–160 |doi=10.1007/s00340-013-5411-8
|bibcode = 2013ApPhB.111..155A |doi-access=free }}</ref>
have been successfully performed.