Imaging in the optically dense regions of a spray: A review of developing techniques

Since the year 2000, a number of very different techniques to image spray formation in optically dense regions of atomizing sprays have appeared in the literature, been further developed, and applied to sprays. Three of them are transillumination techniques (meaning a beam is passed all the way through the spray and imaged on the other side, often called ‘line-of-sight’), one technique is internally illuminated (the signal originates inside the spray), and one is a planar laser imaging technique. Researchers intending to use these new results need to understand with clarity what exactly the measurements provide and how reliably they can provide them. This article intends, therefore, to bring some order to the discussion of techniques. It includes a description of each of the five techniques, a review of advantages and limitations for each of them, a comparison, and a discussion of future trends. Most of the techniques are certain to evolve and improve further, but this article can provide a snapshot in time and help create a context for understanding.