The computation of tristimulus values is a basic step for obtaining various colourimetric quantities. Firstly, the definition of CIE tristimulus values and their numerical computation methods are reviewed. Secondly, three types of ASTM weighting tables used in industrial applications and Venable optimum weighting tables are briefly described and discussed. Finally, based on the work of Venable, a criterion different from that of Venable's in form for determining the optimum weighting factors to minimize the influence of errors in measurement of spectral reflectance is proposed, and a new and simple method for computing the optimum weighting table is given. The new method is simpler and easier to program and thus is possible to be used as a standard method, as compared with other existing weighting tables. Numerical simulation results show that the tristimulus values computed by using the method proposed are closest to the CIE definition compared with those computed by using the three types of ASTM weighting tables. Therefore, it is expected that the new method for computing weighting tables will find wide industrial applications.