Volumetric lighting information is commonly stored as regular or irregular clouds of point samples,called light probes. This presentation describes a new method for light probe interpolation based on a tetrahedralization of space. Light probes may be placed anywhere, with varying density to capture important high-frequency changes in lighting. Our method calculates a Delaunay tetrahedralization of the point set, which is then extrapolated outside the bounds of the point set by creating 'open cells' along the boundary. Interpolation is performed using tetrahedral barycentric coordinates, which combines smooth interpolation with good user control and low memory footprint. Objects may cache their current tetrahedron and update it using a temporally coherent walk of tetrahedron connectivity. The algorithm is fast, performing 2000 queries in <1ms on iPad2. The presentation will include formulas and code example taken from Unity3.5, but the techniques are general purpose and readily applicable to any game engine on any platform.