Measurements have been made of the spectral response of the filter-cell combinations of a commercial colorimeter. Regression analysis was used to determine which linear combination of filters and cells gives the best least squares to fit to the CIE colour matching functions. Calculations and measurements show that good accuracy is attainable even in the measurement of the chromaticities of discharge lamps. The method, which is a mathematical analogue of the Dresler (1933) filter principle, can be applied to improving the accuracy of existing colorimeters and to the design of high accuracy filter colorimeters, photometers and filter radiometers.