Combustion Physics

Lund University

Ballistic imaging

David Sedarsky, Edouard Berrocal and Mark Linne

Contact person: Mark Linne

Ballistic imaging is a laser diagnostic which takes advantage of the properties of scattered light to discriminate photons carrying image information from the light which carries no image information.  This allows high-resolution imaging in optically dense environments where conventional imaging is not possible.

Ballistic imaging takes advantage of optical techniques that were originally developed in the medical field for certain types of transillumination tissue problems.  In these measurements, the region of interest is embedded in a medium where a large fraction of the input light is absorbed or scattered away (i.e., optically thick). Recording detailed, high quality images is very difficult in this situation because of the “scrambling” and attenuation effects of multiply-scattered light.  In essence, the optically dense space surrounding the region of interest acts as a diffuser, which significantly alters the light field detected in the image plane.

Optically dense situations are common in combustion and fluid dynamics measurements as well, along with new challenges.  These systems are dynamic, often turbulent, flows which may include multi-phase constituents in rapidly varying concentrations. In many such cases, conventional measurement techniques are intractable due to reduced signal-to-noise ratios, and irregularity in the signal field. Several forms of ballistic imaging have been adapted to deal with these issues and acquire instantaneous images of the liquid core in atomizing sprays; a measurement that is not possible with conventional imaging.

Figure 1. Results obtained using ballistic imaging in (a) a water jet in cross flow, and (b) a transient Diesel fuel spray.

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