A Simple and Inexpensive Schlieren Optical System Using a Fresnel Lens | PetaPixel

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Schlieren imaging is a fairly standard optical technique for visualizing heat, sound, or pressure differences in air. Technically, a schlieren system is able to see clear disturbances in air due to slight differences in the speed of light in the air. The technique can be sensitive enough to see the heat rising off a human hand at room temperature.

High-quality schlieren systems typically use large optical mirrors that are expensive and beyond the budget of most optics classes. Here I will present the simplest design for a system that any student can build, align, and experiment with.<br>This simple design uses a plastic Fresnel lens. A Fresnel lens is a flat lens that is most commonly found in overhead projectors. Historical Fresnel lenses were used for light gathering applications in which an image was not dependent on the quality of the lens. The first Fresnel lens was designed for a lighthouse.<br>In recent years, the Fresnel optics have improved so it was time to see if a Fresnel lens could be used in a schlieren system. The real interest in a Fresnel lens is that the lens has a large clear aperture — this means that relatively large objects can be tested in this area. High-quality lenses with a 250 mm clear aperture are very expensive.

The ideal lens for a simple schlieren system is between /4 and /8. The -number of the lens is simply the ratio of the focal length of the lens divided by the clear aperture of the lens. Most Fresnel lenses have a very low f-number and are not desirable for schlieren systems. Modern manufacturers offer /4 lenses.<br>To test out some of these lenses I ordered one from a Chinese manufacturer that was selling surplus optics on a popular on-line auction. I was unable to find a US source of a similar optic.- (if you find one please drop a note in the comments below). My price for a 250mm diameter lens with a focal length of 1,000 mm was only $20. This /4 Fresnel lens would be perfect to test in a simple schlieren system.

This particular design is called Toepler’s single-field-lens schlieren arrangement and is considered a basic schlieren set up to build. The equipment needed is: 1 is the light source, 2 is the Fresnel lens, 3 is a heat source (hot soldering iron works well), 4 is a knife edge, 5 is the camera and lens.<br>The black rays in the diagram above represent rays from the light source that do not interact with any heat source and are blocked by the knife edge. The red ray represents rays of light that have been bent by the heat source and are seen by the camera. The knife edge does not have to be a sharp edge – a sheet of black paper works well and is probably safer for students to use in a dark room. The camera lens for my test was a 300 mm Canon lens and was focused on the hot object. For the images for this article, I used a candle. In this setup, the hot object is on the same side of the lens as the camera, so the lens can easily be focused on the hot object.<br>The light source is a simple white LED run from a 5 VDC lab power supply. To keep the LED from burning out, I used a 330-ohm resistor in series to limit the current.<br>The larger the interaction distance of the heat source in the schlieren system, the larger the sensitivity of the system. This location is a good compromise between image quality and sensitivity. A student should test the best location of the heat source in the system for themselves. Moving the heat source to different parts of the system and testing the results is a great way to familiarize yourself with the sensitivity of the system<br>The Parts<br>The White light 5 mm LED connected to a 5 VDC supply. Note the 330ohm resister used in series to limit the current. To align the LED and the lens, make sure the LED is the exact height above the table as the center of the lens. The fresnel lens is mounted to a ring cut into plywood. The clamps hold the lens in place and keep it flat. Above the orange clamp is the glowing white light LED. The 5mm white LED is focused on a white sheet of paper. Here the image is about 12 mm in diameter. The two star flairs seem to be due to manufacturing defects in the lens. These could not be eliminated by changing the alignment of the LED to the lens. Not a very good image from this lens.

The red area shows approximately how much of the image needs to be covered by the knife edge to observe the schlieren effect. Here the cardboard used as the knife edge is shown. Most optical setups will use an open razor blade as the knife edge. The paper is a bit safer for students working in a darkened room. The lens is a 300mm canon lens. The live view mode is a great help in placing the cardboard knife edge in just the right position. The Fresnel lens is shown in the setup....