CIE 198

This report is a recommendation for the determination of measurement uncertainties associ-ated with the values of selected quantities in photometry. The report explains the steps from the initial procedures for a measurement of input quantities to a final statement of the output values with associated expanded uncertainties. The main part summarises the fundamental definitions for the evaluation of standard, combined and expanded uncertainties following the rules in the internationally agreed "Guide to the Expression of Uncertainty in Measurements" (GUM) and its supplement 1. In conjunction with this report supplements, with a wide selec-tion of examples, provide guidance on how to model measurement procedures in a measurement equation, combine uncertainties in a budget and convert to the presentation of expand-ed uncertainty.