Optical considerations and limitations of the Schlieren method

Abstract

The elementary principles of the schlieren method are first described, with reference to an ideal basic system. Various developments from this basic system are then considered with particular reference to their advantages and disadvantages from the optical point of view. The experimental procedure in setting up the system is also covered from the same aspect. The general optical theory of the schlieren method is worked out, firstly in terms of the deflection of a ray which passes through a medium of varying refractive index, secondly in terms of the change in illumination caused by this deflection, which is calculated from diffraction theory. Some typical examples are worked out in the latter case. It is concluded that the schlieren method may be used qualitatively at extremely high sensitivities with satisfactory results, but is not suitable for quantitative work where small pressure or density changes are involved. A secondary conclusion is that the twin-mirror system is in general the best for overall ease of interpretation of results, though local considerations may modify this choice.