CIE Division 1 entrusted TC 1-74 to investigate the possibility to develop a smooth curve version of the D illuminants. This article investigates the possibility to create such smoothed curves with minimal colorimetric error. Six different smoothing algorithms were investigated; the best method, using a locally weighted regression and smoothing algorithm, enabled a smooth function, where the worst colour difference for a 100 000 sample set was less than E-ab(*) = 0.5, comparing calculations using the CIE standard daylight spectrum and the smoothed function. Thus, the smoothed function can be recommended for redefining the daylight spectra for colorimetry without loosing continuity to results using the current calculation tables and for designing daylight simulator spectra. (c) 2012 Wiley Periodicals, Inc. Col Res Appl, 38, 316-321, 2012