An infrared thermopile sensor includes a silicon cover having an infrared lens, an infrared sensing chip having duo-thermopile sensing elements, and a microcontroller chip calculating a temperature of an object. The components are in a stacked 3D package to decrease the size of the infrared thermopile sensor. The infrared sensing chip and the microcontroller chip have metal layers to shield the thermal radiation. The conversion from wrist temperature to body core temperature uses detected ambient temperature and fixed humidity or imported humidity level to calculate the body core temperature based on experimental data and curve fitting. The skin temperature compensation can be set differently for different sex gender, different standard deviation of wrist temperature and external relative humidity reading.