It is common practice in schlieren optical systems to use a phase plate diaphragm incorporating a fine wire. The optical theory of such diaphragms is, however, complex, and their behavior over a range of refraction gradients has never been critically evaluated. It is shown that the older knife edge diaphragm, the optics of which are relatively simple, can be used in conjunction with modern double‐sector ultracentrifuge cells, thus providing a reference standard against which the behavior of a phase plate diaphragm can be checked over a range of conditions. Results are presented showing near ideal behavior for an MSE phase plate.