Optical analysis of processes inside an Otto engine provides useful information on how emissions can be reduced and performance of an engine can be increased. However, acquisition of optical data in a cylinder is highly complicated. So far endoscopeshave been used to study the interior of a combustion chamber, with only two-dimensional images. Lacking depth information does not allow for a reliable determination of the position of a flame front. Identifi cation of single flame sources is even more difficult. The 3D-SIS system of the Institute of Reprocicating Engines (IFKM) allows for the three-dimensional representation of recordings from the combustion chamber also when several smaller fl ames occur. The conventional measurement system with an endoscope and a camera is replaced by three UV-capable endoscopes and three photomultiplier cameras. They can record 200,000 images per second. The two-dimensional images of the three cameras only show the intensity distribution of the flame light, as before. To obtain three-dimensional CAD representations, the IFKM scientists have developed a voxelbased reprojection method (voxels are volumetric pixels). For this, a precise calibration of the three cameras and a CAD reproduction of the real combustion chamber are required. In the CAD reproduction, every voxel is pre-defined by a coordinate value. In this "pre-assembled" combustion chamber, every possible flame can be reconstructed by linking virtual voxel and real pixel values. Every pixel of a two-dimensional camera image is assigned to a certain voxel group. If a flame is recorded, the brightness value of the pixel exceeds a threshold. The process associates this brightness value with the brightness value of the voxel group and checks the plausibility by comparison with the values of the other two camera images. All three camera images of a discrete point of time are transformed to the CAD representation. The result is a close-to-reality reproduction of the flames.