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Schlieren Imaging

Visualizing Airflow, Heat Gradients, and Acoustic Fields with Light

In Short

Schlieren imaging is a powerful optical technique that reveals refractive index gradients in transparent media — such as air — by making otherwise invisible phenomena visible. Temperature differences, gas flows, shock waves, and even sound fields can be observed in real time. Our setup demonstrates this principle using a point light source, a precision spherical mirror, and a knife-edge aperture in front of a camera. Subtle changes in the air — from warm breath to standing acoustic waves — appear as striking patterns of light and shadow.

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Details and Theory

The Schlieren method relies on the fact that light is refracted when it passes through regions of varying density — and thus varying refractive index — in a transparent medium. These variations are caused by changes in temperature, pressure, or composition.

In our installation, a small LED serves as a point light source. Its rays are collimated and reflected by a concave spherical mirror with a 75 cm focal length and 203 mm diameter. At the focal point, a custom 3D-printed knife-edge aperture is positioned directly in front of the camera lens. This aperture selectively blocks rays that have been slightly deflected by refractive index gradients. As a result, the camera captures contrast variations that correspond to invisible physical processes in the air.

Because this technique is sensitive to extremely small deviations in light paths, it allows us to directly observe phenomena such as:

  • Convection from a human hand or candle flame

  • Thermal plumes and gas leaks

  • The breath of a person or animal

  • Shock waves or standing acoustic waves

Our Schlieren system can also be combined with some of our other experiments. For example, placing the Acoustic Levitator within the Schlieren field allows us to visualize the standing acoustic wave itself — the very structure that traps particles in mid-air becomes visible as a pattern of alternating high- and low-density zones.

Similarly, placing the Tesla Coil in the field of view makes it possible to visualize the heating and ionization of air caused by electrical discharges. The hot plasma filaments disturb the air’s refractive index, generating vivid contrast patterns as the arcs dance and dissipate.