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1949
SCHLIEREN PHOTOGRAPHY
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studies. One must be careful to differentiate between laminar boundary layers and the halos of Fig. 6. This can usually be done by taking a schlieren photograph of the wind tunnel with all optical adjustments identical to those used under test conditions but with no flow in the tunnel. It should be possible by comparing this tare photograph with subsequent test photographs, to differentiate between a halo and a boundary layer.
There is no sharp line of demarcation between the images of the regions 1, 2, and 3 of Fig. 5, so that an actual size determination from
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Fig. 7 — Image contrast and density of Fig. 6 for disturbances of various geometrical size.
the schlieren picture is most difficult. The light intensity for the case where a = 1 falls to half its maximum value at z — 1.5a; for a = 0.01 it falls to half maximum at z = 5a while for a = 0.0001 half maximum is reached at z = 20a. For a source of finite size these values may be different, however, the general tendency will still exist.
It is interesting to note that the solution of (17) for the above cases, i.e., w = yi = 0, using point-source illumination with a knifeedge at the optic axis indicates that the focal length of the optics used has no effect on the quality of the image. However, while the light distribution and contrast will not change, the shorter the focal length for a given aperture, i.e., the smaller the /ratio, the more light will be available for a photograph and the higher will be the density in the image.
A more complete analysis of the effect of the several variables on the quality of a schlieren picture can be obtained by examining the