Planar laser-induced fluorescence (PLIF) is an optical diagnostic technique widely used in fluid and gas applications.

PLIF has proven to be a valuable tool for visualization of flows, as well as for quantitative measurements of field-wide scalars, such as concentration and temperature in liquids and concentration in gaseous flows. Applications can be found in process engineering, biomedical engineering, and fluid dynamics research. By adding a fluorescent tracer species (e.g., acetone) to a nonfluorescent fuel or gas, the gas phase mixture can be visualized and quantitative concentration measurements can be made using LIF.

A common application is fuel visualization in combustion engine development, where LIF allows for pre-combustion and combustion process diagnostics.