Improved agreement in aperture area measurement between radiometric and physical probing methods at NMIA

摘要

At NMIA, a set of knife-edged apertures 4-25 mm in diameter are used to convert from the base standards (W of power in a collimated laser beam) to units of irradiance (Wm~(-2)) or radiance (Wm~(-2)sr~(-1)). They are also used to realise the units of the candela, lux and lumen. In previous work, agreement between optical and mechanical measurements of the aperture areas was at a level of only 0.5percent, somewhat larger than the estimated uncertainty in either method. Uncertainties in aperture area are a dominant factor in the uncertainty of the above units, as well as heat-flux and thermodynamic temperature, for which an uncertainty of 0.01percent is required. This paper presents measurements on a new set of precision ground apertures with more stringent machining tolerances (replacing an older set of turned apertures). Measurements of the area ratio determined using an integrating sphere and lamp source at the 0.01percent level are compared to mechanical measurements obtained using a CMM and roundness machine (again at the 0.01percent level). Significant improvement in the agreement is found over the previous measurements.