In many combustion, propulsion, and chemical processing applications, the mixing of reactants is a crucial factor in the control of processing speed, power output, and waste/pollution reduction. PLIF is a method by which a chemical species of interest can be captured in a two-dimensional image.  A simple optical arrangement showing a source laser, sheet forming optics, the flow being interrogated, and a camera is shown in Fig. 1. Here, acetone is seeded into the jet and excited with the fourth harmonic of an Nd:YAG laser.

For reacting flows, it is possible to tune the laser to excite a radical species, such as the hydroxyl radical (OH). This can be used to track flame structure.  Scientists and collaborative partners with Spectral Energies, LLC have significant experience applying the PLIF technique in a number of applications. The example shown in Fig. 2 utilizes simultaneous NO and acetone PLIF for studies of gas-phase molecular-scale mixing.   
Fig. 1. Typical PLIF arrangement using a laser 
sheet to excite acetone within a seeded jet.
Fig. 2. Images of a mixing layer collected 
using simultaneous NO and acetone PLIF.