Laser Induced Fluorescence

Planar Laser Induced Fluorescence (PLIF) is a powerful diagnostic tool for the investigation of nonreacting as well as reacting gas and liquid flows. PLIF is a nonintrusive, instantaneous flow visualization technique with high spatial and temporal resolution and is applied to determine different flowfield variables in the plane of a laser light sheet: concentration (mole fraction), density, temperature and velocity fields can be derived from calibrated LIF images.

LIF is applicable to a large number of molecules and atoms for combustion, spray, and various fluid mechanical flow studies. The LIF detection of atomic species is also called Laser Excited Atomic Fluorescence (LEAF). Combustion species like flame radicals and most fuel species can be visualized directly using LIF. If the flow itself contains no LIF-active species (like N₂, CH₄ or water), seeding of the flow with fluorescing markers (tracers) are used for flowfield imaging (Tracer LIF).

The LIF emission is spread over many wavelengths (emission spectrum), with most of the emission red-shifted from the laser line. Due to this spectral shift of the LIF emission unwanted interferences from straylight or Mie scattering can be effectively suppressed.

LIF is a technique with high selectivity. It is possible to selectively address species to emit light even in combustion environments where hundreds of different species are present. For small, typically diatomic molecules single quantum states can be detected which allow to determine gas temperatures even under non-equilibrium conditions.

LIF imaging is particularly attractive due to the strength of the resonant absorption process compared with the non-resonant Rayleigh and Raman techniques. Due to this sensitivity LIF has the capability to detect flame radicals and other species at the ppm or even sub-ppm level. Sensitivity and selectivity are the two main advantages of the LIF technique.

Principle of LIF imaging
In a typical LIF experiment the flow is illuminated by a laser light sheet whose wavelength is tuned to excite a particular transition within the LIF-active molecule (atom). A fraction of the ground state molecules absorbs the incident light and is promoted to a higher (excited) electronic energy state. A fraction of these excited molecules emittes light (fluorescence), the rest which are not dissociated return to the ground state by transfering the excess energy through nonradiative decay processes like collisional quenching or intramolecular deactivation.

For LIF imaging the laser beam is spread into a sheet, passed through the fluid of interest, and the resulting fluorescence light from the light sheet is imaged through a filter onto a time-gated camera.

LaVision’s Multi-Parameter Laser Imaging concept combines LIF imaging in a unique way with Particle Image Velocimetry (PIV), Laser Induced Incandescence (LII) or Interferometric Mie Imaging (IMI).