Flow field imaging through sharp-edged atomic and molecular 'notch' filters

Sharp cut-off atomic and molecular notch filters simultaneously provide hig h spectral resolution and allow imaging by collecting light over a wide fie ld of view. Many important properties of flow fields can be observed by ima ,sing light elastically scattered from small particles, molecules or electr ons. In order to extract information about the flow field from elastic scat tering, the spectrum of the scattering must be resolved and the background scattering must be suppressed. Very high resolution, on the order of a few tens of megahertz, is usually required. The spectrum of the scattered light is broadened and shifted by the motion of the scatterers. For particles, w hich have relatively Little thermal or acoustic motion, the spectral shift is only a function of the velocity. For molecules, the scattering spectrum is a function of the temperature, velocity and pressure of the gas as well as its composition. For electrons, the spectrum is a function of the electr on temperature and electron number density in a plasma. In this paper, shar p edged notch filters made of rubidium, iodine or mercury vapour are used t o image shock wave and boundary layer structure by Rayleigh scattering from particles, to image gas pressure, velocity and temperature by molecular Ra yleigh scattering, and to measure electron temperature and electron number density by Thomson scattering. For molecular scattering, filter transmissio n is generally a function of velocity, temperature and pressure, but, under some circumstances, it is a function of only one or two variables, so a no tch filter can provide single-pulse images of a specific flow field paramet er.