Background Oriented Schlieren Method (BOS)

• Measurement of three-dimensional density distributions in gases

The main components of the BOS setup is a background with a randomly distributed dot pattern, a camera to capture the background and a computer to store and process the images. During a measurement, the dot pattern without the measurement volume is first captured as a reference image.

Then the actual measurement image is taken with the measurement object. A point on the background is imaged with a certain distance on the photo sensor if a density field is present due to the refraction of light (see Figure 1). The displacement of the points is determined with suitable cross-correlation algorithms. From the geometry of the setup and the displacement of the point, the deflection angle in the center of the measured object and subsequently the gradient of the refractive index can be inferred. By taking simultaneous images from different directions, a tomographic algorithm (filtered back projection) can be used to determine the refractive index in three dimensions and, via the Gladstone-Dale relationship, the density of the gas (see Figure 3). If the pressure is measured on top, the temperature can be determined via the ideal gas equation.