Planar Laser-Induced Fluorescence
Introduction

Features

Planar laser-induced fluorescence (planar-LIF) is an
optical measuring technique used to measure instantwhole-field concentration or temperature maps in liquid
flows.
Applications can be found in process engineering (e.g.
mixing in stirring vessels, heating and cooling systems),

biomedical engineering (e.g. transport of drugs inbiological flows such as in model veins) and fluid
dynamics research (e.g. turbulent mixing and heattransfer modelling, indoor climate etc.)

Detection oflight intensity• Non-intrusive technology• Quantitative, precise and accurate measurements ofconcentration or temperature fields• In combination with velocity measurements, transport
properties (e.g. Reynolds flux, turbulent diffusion
coefficients and other parameters) are made experimentally available

Cameracalibration
DataI

Measurement
volumeYData
analysisC

Opticalfilter (red)

XZ

Light
sheett

PLIF
image

Laser
Cylindrical lens

OpticalfilterFlow with(green)
fluorescent dyeand particles∆X
PIV double
imagesExperimental set-up

The level of fluorescence is known to vary with theconcentration or the temperature and several otherexperimental parameters.
- With rhodamine 6G as marker and the concentration
as measured variable, S is defined as:
S = ƒoptic · AC · (EVc · Qλ ) · C where AC = e-ε L·C- With rhodamine B as marker (for constant concentration, Co) and the temperature as measured
variable, S is defined as:
S = ƒoptic · AT · (EVc · Co ) · Qλ (T)Where:- C and T are the dye concentration and thetemperature, respectively- E is the laser light intensity- Qλ is the quantum efficiency of the dye (at the laser
excitation wavelength λ)- foptic corresponds to optical factors- Vc is the sampling volume- AC and AT represent absorption phenomena integrated on the light path (L) in the fluid characterisedby absorption index (ε)

Signal of fluorescence, S (a.u.)360
340
320700
Fcritic

600

Signal saturation

Linear regime

500
400
300

Experimental
results

200
100

Ccritic

300
0280
260
240

0510

Experimental
results

15

2025

Concentration (µg+l-1)

Combining concentration or temperature with velocity
measurements enables mixing or heat transfer to bestudied at the scalar transport level. In this case, asecond camera picks up the laser light scattered bysmall particles through an optical narrow-band filter. The
signal is processed to measure velocities, as in standard
PIV. The two LIF and PIV receivers are brought toobserve the same locations by means of beam splitter
optics and geometrical calibration.

220
200

Example: Jet mixing in a cross-flow

Calibration curve(at E=E0 and C=C0)180160102030405060

708090Temperature (C°)

Typical single-pixel temperature and concentration
calibration curves

At low concentration levels, absorption phenomena are
negligible (AC, AT ≈ 1), which leads to a linear relationship between the signal (S) and (C,E,α) or (T,β) - with(α, β) two constants that characterise all experimentalparameters. In such conditions, concentration (ortemperature) is accurately measured as the amount oflight received by the detector. The calibration procedure
consists of determining α (or β) at every pixel of thecamera.

A laminar water-jet flow is established in a semienclosed mixing chamber with a water cross-flow loop. A
Nd:YAG green laser light sheet illuminates the centre
plane of the jet and excites the rhodamine 6G that isadded to the fluid. The LIF camera collects the brightorange light and the images are processed to give
instant concentration maps. Eddy structures issued fromthe jet are clearly identified on the instant concentrationmap (image 1). Statistical analysis of the data such as
average (image 2) and rms (image 3) of concentration
further provide meaningful information on the mixing
process.

Instant concentration map

1

Signal processing
Signal processing consists of converting the recorded
raw images of fluorescence to concentration or temperature maps, via the calibration map determinedpreviously. For liquid-liquid mixing, one of the fluids ismarked with the dye, whereas the other is fresh fluid.
When passing through the laser sheet, the dye is excitedand re-emits fluorescent light. The light intensity is thenprocessed as follows to give concentration:
C≅ SαEFor temperature measurements, the dye is first mixed
with the fluid and the fluorescence level re-emitted
varies with the local temperature. In this case, temperature is determined as follows:(T -Tref ) ≅ (S -Sref )β
where Sref is the reference fluorescence signal at T=Tref

Average concentration map

2

Cross-flow

Rms. of concentration

3

Jet

Example of instant and statistical maps of concentration
obtained with planar-LIF measurements

www.dantecdynamics.com

Calibration

380

Signal of fluorescence, S (a.u.)• A laser source (typically Nd:YAG or Argon-ion lasers)with the appropriate optics to form a thin sheet of light• A fluorescent dye that marks the fluid and that istraced during the measurements. This chemicalcompound absorbs the laser light energy and re-emits
light at a longer wavelength that can be detected by aphotodetector. Commonly used dyes for measurements in liquids are rhodamine 6G (for concentrationmeasurements), rhodamine B (for temperaturemeasurements) and fluorescein disodium (for concentration or temperature measurements)• A CCD camera equipped with a sharp cut-off ornarrow-band filter, so that only the fluorescent light isrecorded. This camera acts as an array of lightdetectors (pixels)

Synchronised planar-LIF and PIV
measurements

Linear fit (at E=E0)

800

The basic equipment needed to carry out planar-LIF
measurements is: