The photographic negatives obtain in the PIV process often contain a very clear image of the water surface where the laser sheet passed through. The conditions for this to occur and be readily identified are that the camera view of the surface should be unobscured and that there should be some seeding particles on the surface to scatter the light. The first condition can generally be met by careful positioning of the camera, and seeding is generally present on the surface, unless the current is being run. The most successful method found to extract the surface information from the negatives was to enlarge the image with a standard photographic enlarger onto a digitising tablet, and to manually identify points on the surface.
Manual processing is necessary given that the quality of the surface image can be quite variable.
In some portions, for example where the surface is starting to break, a judgment has to be made as to where the surface actually is, and these areas should be assessed with some caution. For each negative, a reference point is first located to relate this point on the digitising tablet to known physical coordinates. Next, two known reference points are identified and recorded in order to deduce any rotation between the digitising and physical planes. A sequence of points is now recorded along the water surface and, if appropriate, additional fluid interfaces are recorded. With a knowledge of the enlargement factor and the reference points, the physical coordinates of the surface are determined.