It utilizes a narrow linewidth laser to produce Rayleigh-scattered light from the flow, and the collected light is filtered by a cell containing a molecular vapor, for example, iodine, before detection. Variations in the Rayleigh-Brillouin lineshape profile from the gas being measured cause variations in the detected light intensity that depend on the property or properties of interest.

A schematic of a FRS system is illustrated in Figure 1. The flow field is illuminated using a laser sheet from a frequency-doubled, injection-seeded Nd:YAG laser, resulting in Rayleigh-Brillouin scattering (RBS), which is passed through an iodine filter placed in front of the camera (see Figure 1). The filter is a glass cylindrical cell containing a small amount of iodine vapor with a known absorption profile in the frequency region of interest. When the laser wavelength is tuned to an iodine resonance, the iodine cell absorbs stray laser light scattered from the walls and windows, but the RBS light is thermally broadened due to the kinetic motion of the molecules, and thus a portion of it gets transmitted (see Figure 2). Thus, the signal measured by the camera represents the gas phase RBS, while stray laser light from surfaces gets blocked. Broadening of the RBS spectrum due to either thermal or collisional effects increases the signal, and can be used for determining temperature or pressure, respectively. Furthermore, bulk motion of the gas will cause a Doppler shift of the RBS spectrum, which results in a change in cell transmission that can be useful for measuring the velocity.