Minimising the uncertainty in T -T90 above the silver point

Above the silver point (~1235 K), the difference between thermodynamic temperature, T, and ITS-90 temperature, T90, can be expressed as an analytic function in terms of the thermodynamic and ITS-90 temperatures of a single point anywhere above the silver point. The uncertainty in this function across the whole radiation thermometry range of ITS-90 depends only on the uncertainty of T -T90 at this one point, and scales with T 2 . This paper develops a criterion for minimising the uncertainty in this T -T90 function. To satisfy this criterion, two radiation thermometry methods are compared for their ability to measure the thermodynamic temperatures of the silver, gold, and copper points-a downwards extrapolation method at short wavelengths using high-temperature fixed points and an upwards extrapolation method from fixed points in the SPRT range of ITS-90 using a longer-wavelength radiation thermometer. It is concluded that the latter method may achieve the lowest uncertainty.