Reevaluation of daylight spectral distributions

The spectral power distribution of colorimetric daylight illuminants was determined based on measurements' done some 40 years ago. The data were averaged and standardized at every full 10 nm. Later, to follow colorimetric practice, these data have been interpolated (linearly) to every 5 nm, but the factors to calculate the values have nor been changed. We show in this article that this is not correct and leads to some discrepancies, especially if data with even smaller step size are required We suggest that when correcting this discrepancy one should at the same time go from the linear interpolation technique, a rather crude method for a spectral distribution with many minor minima and maxima in the spectrum, to a nonlinear one. We show that a third-order spline interpolation can provide smooth functions resembling the original data to a high degree. (C) 2000 John Wiley & Sons, Inc.