Abstract: A new approach to the calibration and measurement of extended sources was introduced in the infrared integrating radiometer BIIR. Major factors influencing the precision of the BIIR were measured and analysed. Based on the Planck principle, it is deduced that the BIIR output voltage is the product of a constant and a monotonic increasing function of radiation luminance from the extended sources. The increasing function can be calculated, thus one standard measurement can determine the constant and complete the calibration. The result of measurement of the standard extended source showed that the relative error of the instrument is less than 5%.