Numerical estimation of effective emissivities of low-temperature blackbody cavities with apertures

NMIJ has established the facilities and calibration techniques for the infrared radiation thermometry in the low temperature range from -30/spl deg/C to 100/spl deg/C. In the actual calibration processes of the industrial standard blackbodies, we use aperture plates in front of the blackbody cavities to reduce the size-of-source effect of the infrared radiation thermometer used as a comparator. However, the aperture plate introduces a change in an apparent emissivity of the cavities, and this contributes to the uncertainty of the calibration in the comparison method. The effect of the aperture plates for the cavities has been deduced quantitatively based on the calculation with the Monte Carlo method. The results show that the temperature-controlled black surface facing to the cavities does not affect the apparent emissivity of the cavities significantly among the uncertainty budgets of the calibration.