Photoelectric Flow Birefringence Instrument of High Sensitivity

Abstract

We have modified a conventional concentric‐cylinder flow birefringence instrument (built by the Rao Instrument Company) by adding a photocell and a rotating disk that scans the annulus with a moving spot of light. In this way the optical intensity pattern formed in the image of the annulus by the combination of polarizer, analyzer, and quarter‐wave plate is converted into an electrical wave which is then analyzed by an electronic device. Full advantage can be taken of the increased sensitivity to birefringence caused by the presence of the quarter‐wave plate, since the presence of a bright background, so disturbing to the eye, is not important to the photocell. In effect, the system is converted to a linear dependence on the magnitude of the birefringence, instead of the quadratic one found in the conventional method of observation. The sensitivity is in fact increased by about a factor of ten over the same instrument used in the conventional way, and at the same time the measurements are made independent of the visual judgment of the experimenter. The linearity of the system also has considerable advantages when extraneous sources of birefringence are present, as is commonly the case.