Investigation of visualization characteristics of the sharp focusing schlieren method

The schlieren method is most often used in the high speed wind tunnel testing as the flow visualization technique around the airfoil models etc. It is well known that the schlieren system is unable to obtain the three dimensional information occurring at various locations along the optical path. A large field focusing schlieren system was improved to visualize the three dimensional density gradients by Leonard M. Weinstein. His system uses a fresnel field lens to increase the brightness of image compared to the previous focusing schlieren system. This system is a useful diagnosis tool to investigate complex three-dimensional flow fields. This system also uses the diffuser lens or sheet to obtain a uniform lighting for the plane at the test section. However it is well known that the diffuser lens or sheet causes the loss of a light intensity. Also, the focusing schlieren system has the source grid to achieve multiple light sources that constructed with closely spaced thin lines (shading plane). So the strong extended light source is necessary, and the decrease in a light intensity for the source grid is a serious problem. Therefore, a countermeasure should be prepared to achieve the higher brightness system. In the present study, we would like to discuss especially the countermeasure of the loss of brightness by using the diffuser lens or sheet. In the present system, we adopt the rod lighting system (combination a general extended light source with a Kaleidoscope) as the higher brightness and uniform extended light source.