COLOR SCHLIEREN TECHNIQUE

The color schlieren technique (also called rainbow schlieren deflectometry) uses a white light source instead of a laser. Images formed are in color and hence a color CCD camera is used to record the schlieren patterns. The basic principle of a color schlieren technique is identical to that of the monochrome schlieren, in the sense that density gradients are mapped into light beam deflection. The knife edge enables the measurement of the angle of deflection in a monochrome technique. In color schlieren, the knife edge is replaced by a color filter. Here, a photographic film on which a color scale is printed is employed. The scale varies from red-to-green-to-blue (or the VIBGYOR scale). Light deflection shifts the light beam from one color zone to another. Changes in color in the image, thus, determine the angle of deflection and in turn, the density gradient field in the test section. Clearly, the color filter can be designed in such a way that appropriate resolution in measurement is possible.

The schematic diagram of the color schlieren deflectometry technique set up by the authors is shown in Figure 1(a). A twin halogen cold white light source (Optochem International, is connected to a pinhole with a fiber optic cable for obtaining a point light source. The pinhole is placed at the focus of an achromatic lens of 500 mm focal length and 65 mm diameter. A collimated light beam of size 65 mm is thus generated. It passes through the test cell and falls on another achromatic lens of 750 mm focal length and 100 mm diameter. This lens decollimates the beam and focuses it onto the color (rainbow) filter. A 3-color CCD camera (Basler, model A201bc), pixels spatial resolution connected to a personal computer through an 8-bit A/D frame grabber card is used to record the color images formed on the plane of the filter.