[old_tv_nut]

The standard incandescent bulbs I mentioned were calibrated with respect to standard bulbs maintained by the NBS, and originally traced back to the definitive standard consisting of a black-body cavity in a piece of platinum at a specified temperature

Today, of course, instruments use stable solid state silicon photodetectors. I believe (but don't quote me, have to research it) the base standard now consists of lasers whose output can be measured accurately thermally and then transferred to a standard photodiode and from there to secondary standards.