In the rainbow schlieren apparatus, a continuously graded rainbow filter is placed in the back focal plane of the decollimating lens. Refractive-index gradients in the test section thus appear as gradations in huerather than irradiance. Asimple system is described wherein a conventional color CCD array and video digitizer are used to quantify accurately the color attributes of the resulting image, and hence the associated ray deflections. The present system provides a sensitivity comparable with that of conventional interferometry, while being simpler to implement and less sensitive to mechanical misalignment.