An apparatus for measuring body core temperature includes a light guide. The light guide can be coupled to an earpiece, or it can be a standalone device. The apparatus also includes a sensor positioned at one end of the light guide, and a processor coupled to the sensor. The sensor is operable to sense infrared radiation from an infrared source at the opposite end of the light guide. The processor is operable to determine a temperature of the infrared source at the opposite end of the light guide via a transfer function that correlates a measurement of the infrared radiation observed by the sensor and an effect of radiation of the light guide.