纹影照相1864

测量对象

纹影法测量的是从光源发出的光线在通过不均匀折射率场时，受扰动的光线对于未扰动光线的偏转角。

测量方法

纹影法测量偏转角时，由于偏转角太小，无法直接测量。此时需要用一块屏幕放在离光源足够远的地方，离开测试段的光线曲率将发生了很大变化。纹影仪是用刀口去切割光源像。因此设置相应的光路布置，在光源焦点处放置一个刀口去切割光源像，由刀口放的位置与屏幕上的光强分布，即亮、暗程度来判断偏转角的大小，再由偏转角来决定不均匀折射率场的折射率梯度。

Schlieren photography is a process for photographing optical inhomogeneities in transparent media that are not necessarily visible to the human eye, typically used to study fluid flow, like the flow of air around objects in aeronautical engineering. The conventional schlieren imaging system was developed in 1864 by the German physicist August Toepler to detect schlieren (striae, streaks) in optical glass.

The process works by imaging the deflections of light rays that are refracted by a moving fluid, allowing normally unobservable changes in a fluid's refractive index to be seen. Because changes to flow rate directly affect the refractive index of a fluid, one can therefore photograph a fluid's flow rate (as well as other changes to density, temperature, and pressure) by viewing changes to its refractive index.

Using the schlieren photography process, other unobservable fluid changes can also be seen, such as convection currents, and the standing waves used in acoustic levitation.

Classical optical system

The classical implementation of an optical schlieren system uses light from a single collimated source shining on, or from behind, a target object. Variations in refractive index caused by density gradients in the fluid distort the collimated light beam. This distortion creates a spatial variation in the intensity of the light, which can be visualised directly with a shadowgraph system.

Classical schlieren imaging systems appear in two configurations, using either one or two mirrors. In each case, a transparent object is illuminated with collimated or nearly-collimated light. Rays that are not deflected by the object proceed to their focal point, where they are blocked by a knife edge. Rays that are deflected by the object, have a chance of passing the knife edge without being blocked.

Focusing schlieren optical system

In the mid 20th century, R. A. Burton developed an alternative form of schlieren photography, which is now usually called focusing schlieren or lens-and-grid schlieren, based on a suggestion by Hubert Schardin. Focusing schlieren systems generally retain the characteristic knife edge to produce contrast, but instead of using collimated light and a single knife edge, they use an illumination pattern of repeated edges with a focusing imaging system.

The basic idea is that the illumination pattern is imaged onto a geometrically congruent cutoff pattern (essentially a multiplicity of knife edges) with focusing optics, while density gradients lying between the illumination pattern and the cutoff pattern are imaged, typically by a camera system.

Background-oriented techniques

Background-oriented schlieren technique (BOS) has initially been developed at the German Aerospace Center (DLR).

It relies on measuring or visualizing shifts in focused images. In these techniques, the background and the schlieren object (the distortion to be visualized) are both in focus and the distortion is detected because it moves part of the background image relative to its original position. Because of this focus requirement, they tend to be used for large-scale applications where both the schlieren object and the background are distant (typically beyond the hyperfocal distance of the optical system). Since these systems require no additional optics aside from a camera, they are often the simplest to construct but they are usually not as sensitive as other types of schlieren systems, with the sensitivity being limited by the camera resolution.

Variations and applications

Variations on the optical schlieren method include the replacement of the knife-edge by a coloured target, resulting in rainbow schlieren which can assist in visualising the flow. Different edge configurations such as concentric rings can also give sensitivity to variable gradient directions, and programmable digital edge generation has been demonstrated as well using digital displays and modulators. The adaptive optics pyramid wavefront sensor is a modified form of schlieren (having two perpendicular knife edges formed by the vertices of a refracting square pyramid).

Complete schlieren optical systems can be built from components, or bought as commercially available instruments. Details of theory and operation are given in Settles' 2001 book.