Digital holography for instantaneous spray diagnostics on a plane

We present a measurement technique that is capable of simultaneously determining sizes and positions of multiple transparent droplets in a plane from scattered light features. The technique is largely independent of particle intensity and mutual obscuration. Reflected and refracted light from the droplets in a pulsed laser sheet is recorded holographically to yield the smallest possible probe volume and the largest possible number density. Larger droplets are best analyzed at the image plane; in this case, the droplets appear as two spots (glare points), whose separation is proportional to the droplet diameter. Smaller droplets are easier to analyze at an out-of-focus plane, where their images appear as fringe patterns whose spatial frequency is related to droplet size. Photographic techniques allow only one of these planes to be chosen and are therefore not suitable for multidisperse sprays. Optical holography allows to analyze arbitrary depths, but often suffers from low sensitivity and long analysis times. With digital holography, the spray images are captured digitally by a CCD camera and reconstructed numerically; as in optical holography, the particle reconstruction plane can be freely chosen a posteriori to optimize the measurement. We discuss the issues raised by the transition from holographic film to a CCD sensor as the recording medium, and demonstrate the capabilities of the digital technique.