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This document specifies methods of calculating the tristimulus values of colour stimuli for which the spectral distributions are provided. These colour stimuli can be produced by self-luminous light sources or by reflecting or transmitting objects. This document requires that the colour stimulus function be tabulated at measurement intervals of 5 nm or less in a wavelength range of at least 380 nm to 780 nm. Extrapolation methods are suggested for cases where the measured wavelength range is less than 380 nm to 780 nm. The standard method is defined as summation at 1 nm intervals over the wavelength range from 360 nm to 830 nm. Alternative abridged methods are defined for larger intervals (up to 5 nm) and shorter ranges (down to 380 nm to 780 nm). The alternative methods are to be used only when appropriate and when the user has reviewed the impact on the final results. This document can be used in conjunction with the CIE 1931 standard colorimetric observer or the CIE 1964 standard colorimetric observer.