A computer-implemented method for determining the relative position between a first camera and a second camera, wherein the first camera is a thermal camera and the second camera emits light onto a hybrid phantom and analyzes the reflected light, thus generating a 3D point cloud representing points on the surface of the hybrid phantom, wherein the hybrid phantom has a carrier part which carries a plurality of marker patches, at least a part of the carrier part has one of a first property and a second property, the marker patches are made of solid matter and have the other one of the first property and the second property, wherein the first property is being reflective for the light emitted by the second camera, thus reflecting light emitted by the second camera back to the second camera, and wherein the second property is reflecting less of the incident light emitted from the second camera than the first property, and there is a thermal insulation between the marker patches and the carrier part, the method comprising the steps of: - generating a temperature difference between the marker patches and the carrier part of the hybrid phantom, - capturing a thermal image of the hybrid phantom using the first camera, - identifying an edge of each of the plurality of marker patches of the hybrid phantom in the thermal image, - identifying the locations of the plurality of marker patches from the identified edges, thus generating thermal marker locations, - capturing a 3D point cloud of the phantom using the second camera, - identifying the locations of the plurality of marker patches from the 3D point cloud, thus generating 3D marker locations, and - determining the relative position between the first camera and the second camera from the thermal marker locations and the 3D marker locations, wherein identifying the locations of the plurality of marker patches from the 3D point cloud, thus generating 3D marker locations, involves the steps of: - calculating phantom position data describing the position of the hybrid phantom relative to the second camera from the 3D point cloud, - obtaining marker data representing the locations of the marker patches on the hybrid phantom and - calculating the location of each marker patch from the phantom position data and the marker data. - generating a temperature difference between the marker patches and the carrier part of the hybrid phantom, - capturing a thermal image of the hybrid phantom using the first camera, - identifying an edge of each of the plurality of marker patches of the hybrid phantom in the thermal image, - identifying the locations of the plurality of marker patches from the identified edges, thus generating thermal marker locations, - capturing a 3D point cloud of the phantom using the second camera, - identifying the locations of the plurality of marker patches from the 3D point cloud, thus generating 3D marker locations, and - determining the relative position between the first camera and the second camera from the thermal marker locations and the 3D marker locations, wherein identifying the locations of the plurality of marker patches from the 3D point cloud, thus generating 3D marker locations, involves the steps of: - calculating phantom position data describing the position of the hybrid phantom relative to the second camera from the 3D point cloud, - obtaining marker data representing the locations of the marker patches on the hybrid phantom and - calculating the location of each marker patch from the phantom position data and the marker data.