Schlieren optics come in many forms, from very primitive apparatus that have been used since the 1800’s, to more advanced systems that are used in today’s aerospace industry. There are even different variations that are set up slightly differently to conventional Schlieren optics. Regardless of the set-up, they all work by propagating light through a series of optical components, where the air flow around an object can be imaged through changes in the refractivity of the air flow. In this article, we look at the different types of Schlieren optics that can be set up.
Basic Set Up
This is the apparatus that has been used since the 1800’s and is therefore more primitive in its design than modern day Schlieren optics. These basic systems use a combination of lenses and parallel beams of light. Often, three lenses are placed in a line. The light passes through a slit, followed by being focused with the first lens. The (parallel) light beam becomes refracted in the subject test area (refracted by the object), where it then passes through a second lens and on to the edge of a knife (or other sharp metal object). The hitting of the light on the knife edge causes the light to be cut-off and the image is projected on to a screen by a third lens. Systems are still created nowadays with this basic set up, and in these systems, the screen can be replaced with a photographic sensor if desired.
Advanced Set Up
More advanced systems still often use traditional light sources, although some newer systems now use light-emitting diodes (LEDs). Even though they use more advanced components, the basic principles are the same.
More advanced systems still focus the light through a slit, but instead of a series of lenses, the light is focused on to a mirror, or a series of mirrors (depending on the whether the test area is directly in front of the focused light beam, or not). Once the light has reflected of the mirror(s), it enters (and passes through) the test area and on to a metallic edge, which acts a light blocker. More advanced systems can still use a knife edge, but it can also be a thin wire or the edge of a razor blade. Positioned behind the light blocker is a camera, which is orientated to face towards the mirror.
Advanced systems still measure the refraction of air within the test area, much like the basic systems do. However, more advanced systems recognize the change in refractive index because of the known positions of the camera and light source, as well as the amount of light passing over the blocker—i.e. when there is a refraction of the air in the test area, the direction of light will change and more (or less) light will travel past the light blocker and this change is then imaged by the camera as ‘streaks’. This set up allows the size of the refractive index change to be easily determined, as the degree of refraction is proportional to the amount of light passing the light blocker.
Setting Up the Different Variations
As well as the conventional set ups, the principles of Schlieren optics, and their components, can be adapted for other hybrid techniques, with the main two variations being shadowgraphy and Schlieren interferometry. The principles of both variations are similar, in that they image changes in the refractive index of transparent media and visualize flow density, but they produce different types of images because of how they are set up.
The set up of a shadowgrpahy system is very similar to that of a Schlieren system and a Schlieren system can easily be modified into a shadowgraphy instrument. Instead of measuring the refractive index of air (or other transparent media) as streaks, shadowgraphy projects the image of the air flow as a shadow on a projection screen. In these set ups, the light source used is a natural light source, and this is projected through the test area and on to a projection screen. A camera is then used to take an image of the shadows on the projection screen. Overall, it is a much simpler set up than many advanced Schlieren systems and its complexity is more in line with basic Schlieren systems.
Schlieren interferometry is closer to other interferometry methods than it is of conventional Schlieren optics, but it does use some of the principles of Schlieren optic set ups. Schlieren interferometry is seen as a way of introducing a quantifiable measurement into what is otherwise a visual analysis. Schlieren interferometry is set up like a basic Schlieren system, with the main differences being that only 2 lenses are needed, and the knife edge is replaced with a prism to introduce interference/diffraction into the refraction pattern. These patterns can then be analyzed.
There are also a few further variations of this method depending on what type of prism is used. A Wollaston-Prism shearing interferometer uses the same basic set up, but uses a Wollaston polarizer sandwiched between crossed polarizers, and this produces fringe interferometric images that are very close to the images produced by Schlieren optics. The next variation is Color Schlieren interferometry, which produces a color Schlieren-like image when the knife edge is replaced with a color filter. Other techniques, such as particle-image velocimetry, can then be used for the quantitative measurements.