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1949 SCHLIEREN PHOTOGRAPHY 543
2. Focal Length
(a) A long focal length will give rise to a larger displacement of the disturbance diffraction pattern from the optical axis, thus enabling the use of a wide light source.
(b) A long focal length will give rise to a large Airy disk thus enabling the use of large light sources.
(c) A long focal length will give rise to small astigmatic errors.
(d) A short focal length will give the system greater light-gathering ability.
3. Light-Source Size
(a) A large light source will give high density and low contrast.
(b) A small light source will give high contrast and low density.
(c) The optimum-size light source is one whose geometrical image in the focal plane of the objective is equal to the width of the Airy disk of the objective.
(d) The maximum-size light source is one whose half width is equal to the distance of the central maxima of the diffraction pattern of the disturbance under study, to the optic axis in the focal plane of the objective.
4. Knife-Edge Position
(a) A small amount of cutoff will produce small contrast with little change in density.
(b) A large amount of cutoff will produce high contrast with a subsequent decrease in density.
(c) In general the optimum knife-edge position appears to be at the optic axis.
The effects of aperture size and focal length can be combined in the effective / ratio of the system. From optical considerations a system of large / ratio should be favored even though such a system is relatively inefficient in its utilization of the available light. Experience indicates that an //10 system is most satisfactory with presently available light sources. Since it is simpler to make large /-ratio mirrors than small /-ratio mirrors of the same quality, the economic factor also favors a large /-ratio system.
Actual light-source size can best be determined by experiment. The general trend of image contrast and density as a function of source size has been given earlier in this paper. The contrasts arrived at