Optics of the Integrating Schlieren Instrument

An analysis is presented of the optics encountered in the integrating schleiren arrangement as first described by Resler and Scheibe. Expressions have been developed for the signal which will be received on the phototube, both for the instrumentation using an inclined knife edge and for that with a vertical knife edge. Attention is given to the conditions which must be satisfied to guarantee that the response of the phototube shall be proportional to the difference in refractive index at the beginning and the end of the test section. A simple method is described for calibrating the density scale; the method involves displacing the knife edge over a known distance and measuring the resulting signal. The influence of certain diffraction phenomena on the measurements is briefly discussed, and it is concluded that Fresnel diffraction occurring in the edges of the light beam is one of the factors determining the resolving power. Another factor important for the resolving power is the divergence of the incident light. In comparing the various possible positions of the knife edge, it is concluded that the inclined knife edge, besides allowing operation over a wide range of density gradients, has the advantage of giving a linearity of response which suffers little from diffraction effects. The advantage of the vertical knife edge is the very high sensitivity which can be obtained.