The Beer-Lambert law relates the attenuation of a monochromatic light source to various flow parameters. Most notably, the attenuation is sensitive to the concentration of the absorbing species. Consequently, by measuring the attenuation of such a light source, it is possible to infer the concentration of an absorbing species.
Lasers produce light that is both spatially coherent (i.e., highly collimated) and temporally coherent (i.e., nearly monochromatic); this second feature makes lasers ideal for use with the Beer-Lambert law. In the case of an unknown concentration of some absorbing species, the absorption coefficient of that molecule can be calculated and used with the measured attenuation of a laser to infer the concentration of the absorbing species. Conversely, in the case of a known concentration of the absorbing species, the measured attenuation can be used to obtain information about the spectral characteristics of the absorbing species (e.g., line strengths, line shapes). Both types of measurements can be readily paired with shock-tube experiments, where the high temperatures/pressures behind the reflected shock wave can provide chemical kinetic and/or spectroscopic insights that are difficult to garner using other experimental apparatuses.
The laser diagnostics employed in our laboratory fall into two categories: fixed-wavelength diagnostics and scanned-wavelength diagnostics. Follow the below links for more information on these diagnostics.