LaserInduced Fluorescence (LIF) MIT（激光诱导荧光(生活)麻省理工学院）.pdf

Laser-Induced Fluorescence (LIF) Fluorescent dyes (molecules) can absorb light at one frequency and subsequently re- emit (fluoresce) light at a different frequency. In experiments, the dyes are excited by laser light whose frequency closely matches the excitation frequency of the dye. For example, Fluorescein (maximum excitation at 490 nm) is best excited by an Argon- Ion, Blue-Green laser which predominantly emits wavelengths 488 (blue) and 514 (green) nm. Once excited, Fluoresceins maximum emission is at 520 nm. Because the fluoresced light is of a different frequency than the excitation light, the latter can be filtered out. Furthermore, only the dye that is exposed to the laser fluoresces, so specific planes within a flow field can be visualized. Blue Dye Fluorescent Dye Comparison of a planar visualization, accomplished with LIF, to a volumetric visualization accomplished with a non-fluorescing blue dye. From V. C. Patel T. A. Johnson /projects/low_reynolds/ To illuminate the specific plane, the laser beam is expanded into a sheet using either a cylindrical lens or a scanning mirror. The cylindrical lens produces a light sheet with a Gaussian profile of light intensity. The scanning mirror produces a light sheet with a uniform profile of light intensity. This difference is important, if quantitative estimates of dye concentration are desired, as described below. The intensity of light emitted from a dyed region of flow is proportional to the intensity of excitation energy and to the concentration of dye. If the excitation energy is locally uniform, then the emitted light intensity will be linearly related to the dye concentration. Then, with a simple calibration the emitted light intensity can be directly converted to dye concentration. Two conditions affect the spatial uniformity of the excitation intensity. First, as described above a light sheet const