of the 3D gradient field tomographically reconstructed from 16 cameras. Far right: volume rendering of the final refractive index field after

Figure 1: Measurements of the refractive index field caused by rising hot air from a gas burner. Far left: magnitude of refraction for each pixel in one view. Center left: vector field of 2D displacements. Center: distortion measurements from one view are used as an environment matte to distort the background, data from another view is used to cast a caustic (shadowgraph). Center right: maximum intensity projection volume rendering of a 3D refractive index field recovered with tomography from 8 views. Far right: volume rendering of the 3D reconstruction. We present a technique for 2D imaging and 3D tomographic reconstruction of time-varying, inhomogeneous refractive index fields. Our method can be used to perform three-dimensional reconstruction of phenomena such as gas plumes or liquid mixing. We can also use the 2D imaging results of such time-varying phenomena to render environment mattes and caustics. To achieve these results, we improve a recent fluid imaging technique called Background Oriented Schlieren imaging, and develop a novel theory for tomographic reconstructions from Schlieren images based on first principles of optics. We demonstrate our approach with two different measurement setups, and discuss example applications such as measuring the heat and density distribution in gas flows. 1

We present a method that allows for reconstructing non-stationary, time-varying gas flows around moving objects. Our work extends the background oriented Schlieren tomography (3D-BOS) acquisition technique to capture gas flows also in the presence of occluding objects. An algorithm is presented that exploits the unique properties of BOS background patterns to robustly segment occluding objects. Numerical issues in the refractive index field reconstruction are addressed and successfully solved by the new method. 1

A transparent medium of inhomogenous refractive index will cause light rays to bend as they pass through it, resulting in a visible distortion of the background. We present a simple 2D imaging method for measuring this distortion and then show how it can be used to visualise gas and liquid flows. Improvements to the existing Background Oriented Schlieren method for acquiring projected density gradients are made by placing a multi-scale wavelet noise pattern behind the flow and measuring distortions using a more reliable optical flow algorithm. Dynamic environment mattes can also be acquired, allowing us to render the flow into novel scenes. Capturing from multiple viewpoints then allows us to tomographically reconstruct 3D models of the temperature distribution of the fluid. iii Contents Abstract......................................