Resonance fluorescence imaging of CH in hydrocarbon flames using a polaroid filter to reject Rayleigh scattered light

Electronic imaging of laser induced fluorescence from a plane of laser light that intersects a reactive flow is becoming commonplace. Quite often, the fluorescence wavelength is longer than the laser excitation wavelength and hence the fluorescence is easily discriminated from the Rayleigh and Mie scattering, which is at the laser wavelength. In the case of resonance fluorescence, the fluorescence is sufficiently near the laser excitation wavelength that low fluorescent signals are obscured by Rayleigh and Mie scattering. However, recognizing that the fluorescence scattering is weakly polarized while the Rayleigh scattering light is strongly polarized suggests that a polaroid filter could improve the signal to noise by eliminating Rayleigh scattered light and passing half of the fluorescent scattered light. By rotating the polaroid filter, any amount of Rayleigh scattering and resonance fluorescence from CH as it occurs in the flame front of premixed methane flames.