This report is a second supplement to CIE 198:2011 Determination of Measurement Uncertainties in Photometry for the determination of measurement uncertainties associated with the values of selected quantities in photometry. It deals with spectral measurements and combinations of measured spectral distributions, in particular with the effect of correlations between the measured spectral values that arise from the measurement and calibration processes.

Guidelines are given for identifying sources of correlation in spectral measurements. Calibration is treated as a transfer from a reference to the object of interest. Various processes of interpolation and correction may be applied to measured spectral values before they are combined. Examples of treating correlations introduced by such processes are included. Uncertainties in a number of important radiometric and photometric quantities determined from spectral measurements are provided, including the use of Monte-Carlo methods.

Photometer examples covered are photometric response, photometer V() mismatch index and spectral mismatch factor. Dominant wavelength, correlated colour temperature and blue-light hazard measurements are covered, with particular emphasis on LED sources.