result the modulation envelopes produced by the system 10 would exhibit u201c ringing u201d that affects the accuracy of vsi measurements we found that the power of the narrowband source 14 and that of the white light source 12 can be judiciously calibrated to approximate true white light in order to compensate for the change in reflectivity of different samples and for the different amounts of light collected by different objectives the total intensity is adjusted but the intensity ratio between the two sources 12 14 is kept the same accordingly an effective way to calibrate the illumination system is to produce a series of correlograms and corresponding modulation envelopes at increasing narrowband to white power ratios and to find the ratio that produces an envelope that best fits a gaussian curve according to some merit function for example the integral of the difference between the actual modulation envelope and a corresponding gaussian curve is calculated over the coherence length for different power ratios and the ratio corresponding to the minimum residual between the modulation envelope and the corresponding gaussian curve is used for white light measurements fig8 13 illustrate the mechanics of this approach fig8 is the modulation envelope 30 corresponding to a correlogram 32 produced by a green to white power ratio of 0 1 in a system using a white led a green led and a dichroic filter.