Background Oriented Schlieren Method (BOS)

A method to measure density gradients

The Background Oriented Schlieren Method is a very simple technique to visualize density gradients, that base on the deviation of light rays due to refractive index changes.

The technique can be applied for all kinds of refractive indices. Even very weak density gradients cause refractive indices, which can be detected safely. E.g. the convection of warm air over a human hand in a normal temperatured room can be visualized as well as it is possible to determine the tenth of a percent of a Helium plume in air.

Draft of a typical BOS-Set-up consisting of a background and a camera (displayed by a lens and an object plane). The increasing density is displaying the to be investigated density change.

Refractive index gradients are present over a burning candle or a hot street in summer time. Looking through this hot and unsteady plume, the background points seems to shake. In fact the inhomogeneous heated and moving air cause a deviation of the light rays, which are reflected by the background, falling through this area into the observer’s eye. Because the background itself is absolutely not moving, the moving of the background points can only come from a different deviation of the light rays into the air.

A non-uniform temperature distribution in the air or a mixture of air and another gas cause locally different densities. The optical property of the gas – the refractive index – is locally changed with the density, too.

Helium concentration field over a nozzle in air. The lines are iso-concentration lines of He-lium.
zum Bild Helium concentration field over a nozzle in air. The lines are iso-concentration lines of He-lium.

The apparent background point movement is measured by the Background Oriented Schlieren Method. First a reference picture is taken, without the refractive index gradient to be investigated between the background and the camera. The points of the background will be imaged by a cone of light rays into the black lines of the draft. The measurement picture is taken with the refractive index gradients (e.g. plume of a burning candle) between the background and the camera. The background points will be imaged along the red dashed line in the draft. They will appear on another position of the image compared to the reference image. The displacement of the background points between the reference and the measurement images can be derived exactly by means of the cross correlation if the background is an irregular point pattern with a distinct contrast. Result of the cross correlation is a displacement vector field.

Knowing the place of deviation, the deviation angle of the light ray can be derived. The deviation angle is directly linked to the optical property of the gas – the refractive index, if the wave length to the light is known.

If furthermore the refractive index into the observed volume of the reference is known, local density values can be derived in the investigated volume by means of the Background Oriented Schlieren Method. One example is the helium concentration over a nozzle.

Dr.rer.nat. Reinhard Geisler
German Aerospace Center

Institute of Aerodynamics and Flow Technology
, Experimental Methods
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