Abstract: In a tympanic thermometer probe cover including a film, an additional, substantially non-stretchable structure is mechanically coupled to the film in the area where the probe contacts the film. This additional structure is designed so that it does not occlude infrared radiation from reaching the probe while maintaining the portion of the film over the probe distal end in a prestretched, wrinkle-free condition and preventing that portion of the film from stretching further. The structure may comprise a concentric registration ring that is adhered to the film. The registration ring includes a central open hole portion through which infrared radiation may pass without occlusion. The ring self-adheres to the film, maintaining the film in a prestretched, wrinkle-free condition. When the probe cover is applied to the probe, the probe contacts the ring. Since the ring is substantially non-stretchable, the ring prevents the film from further stretching over the probe.