L-RAY Differometry
Differograms are in 256 shades of gray only. 0 is black and 255 is white. No complex shapes have to be analyzed such as butterflies and fringe patterns. The darker the area, the more movement of the surface. Voids/separations are typically seen with a defined edge. Surface movement from other causes are typically seen as a darkened area. Areas that are bright beyond the capacity of the system (blooming/no data) can be identified as any color using a computer setting.
L-RAY film Shearography
In Shearography, a separation is characterized by a double bull’s-eye fringe pattern. Where the two bull’s-eyes meet is the peek of the bulge. The density of the fringe pattern is proportional to the steepness of the slope from the peek. For separations of the same size, the one closer to the surface has a higher fringe density. This can be explained by assuming that deformation of rubber above a separation is similar to that of a plate subject to uniform pressure. Then the magnitude of the bulge is inversely proportional to the flexural rigidity of the plate, which is proportional to the cube of the thickness (depth).
All three of these processes gather information in the same way, with laser light reflected back off the surface of the tire. As the tire is stressed in a vacuum, the surface topography inside of the tire changes and the reflected light comes back at a slightly different angle. Images are captured before and after the vacuum cycle or in the case of Differometry before, during and after the vacuum cycle and this image information is processed giving the resultant image. The way this reflected light information is collected and processed is what makes these systems/methods different. In Shearography the reflected light must pass thru a shearing optical element, hence the name Shearography (shear, to cut or separate and graph to record). The shearing optical element separates the view producing a interferogram that displays a double or laterally displaced image. Both the L-RAY film Shearography and the digital Shearography use this method. In Differometry the reflected light information is captured directly by the camera and the process shows what is different/changed before, during and after the vacuum cycle, hence the name Differometry (differ, to be different and metry to measure) and produces a single scalable image. The commonality is the ability to sense change/movement in the tire surface. Change/movement during the vacuum cycle is typically caused by a void or separation between the ply's or layers of the tire expanding. It can also be caused by the relaxation from trauma (pushing on, spreading or lifting the tire) or by thermal changes induced by a external source, the vacuum cycle or by vibration from a external source.
