Combining new, low-cost thermal infrared and time-of-flight range sensors provides new opportunities for person detection. We show how to calibrate a system combining both sensors. Our calibration approach differs from the traditional stereo camera calibration in two fundamental ways. A first distinction is that the spectral sensitivity of the two sensors differs significantly. In fact, there is no sensitivity range overlap at all. A second distinction is that their resolution is typically very low, which requires special attention. We assume a situation in which the sensors' relative position is known, but their orientation is unknown. In addition, some of the typical measurement errors are discussed, and methods to compensate for them are proposed. We show how the fused data can be used to increase accuracy and robustness in a variety of applications, and we outline our future work.