The SI-111 (previously known as the IRR-P) is a precision infrared radiometer made by Apogee Instruments that makes continuous measurements in field conditions. The SI-111 is composed of a thermopile, which measures surface temperature, and a thermistor, which measures sensor body temperature. The two temperature probes are housed in a rugged aluminum body that contains a germanium window. Both the thermopile and the thermistor output a millivolt signal. Our dataloggers measure the millivolt signals and apply the Stefan-Boltzman equation, which corrects for the effect of sensor body temperature on the target temperature.