A method of calculating tristimulus values from spectral data which reduces the error introduced by variations among different instruments used for color measurement is described. Uncorrected, this error can be up to two CIELAB color difference units. Use of precise definitions for the tristimulus values permits the calculation of an instrument function specific to each type of equipment. Using such functions, the errors between instruments may be reduced to the region of 0.1 CIELAB color difference units for spectrophotometers with a bandwidth of less than 20nm. The method was tested on a series of four colored glass filters and a didymium glass filter, the latter selected because its complex transmittance spectrum simulates a worst-case sample color.