Background Oriented Schlieren Method (BOS)

The only way to determine local density information from optical measurements is to use inverse methods from a series of measurements with different viewing directions. With classical methods, this leads to a high complexity for setup and execution. In contrast, the background schlieren method offers the advantage of a simple experimental setup using comparatively inexpensive components.

Field of Application

  • Measurement of three-dimensional density distributions in gases

Description

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.

Picture - Measuring principle of BOS Picture - Measuring principle of BOS Picture - Measuring principle of BOS
Measuring principle of BOS
Picture - BOS setup during a measurement at the Acoustic Wind Tunnel of the TFD Picture - BOS setup during a measurement at the Acoustic Wind Tunnel of the TFD Picture - BOS setup during a measurement at the Acoustic Wind Tunnel of the TFD
BOS setup during a measurement at the Acoustic Wind Tunnel of the TFD
Picture - Point shift between a reference and measurement image (top) and the density determined from it (bottom). Picture - Point shift between a reference and measurement image (top) and the density determined from it (bottom). Picture - Point shift between a reference and measurement image (top) and the density determined from it (bottom).
Point shift between a reference and measurement image (top) and the density determined from it (bottom).

Contact

Dipl.-Ing. Ole Wil Willers
Management
Chief Engineer
Address
An der Universität 1
30823 Garbsen
Building
Room
312
Dipl.-Ing. Ole Wil Willers
Management
Chief Engineer
Address
An der Universität 1
30823 Garbsen
Building
Room
312