1. An optical measuring system comprising a master optical instrument comprising means to make optical measurements on test samples at different wavelengths throughout a spectrum, a field optical instrument comprising means to make optical measurements on test samples throughout said spectrum, a computer connected to said field instrument to receive measurement values made by field instrument, said computer comprising means to carry out arithmetic operations on the measurement values received from said field instrument in response to a given test sample to convert said values to a set of calibrated values which are substantially identical to the measurement values that would be obtained from said given test sample by said master instrument, said optical instruments being indexed to make optical measurements in response to different wavelengths at index points incrementally spaced throughout the spectrum, said computer including means to define index locations on said field instrument corresponding to the index points on said master instrument on a one to one basis, said field instrument producing an output for each of said index locations in response to the same wavelength that the master instrument responds to at the corresponding index point, said computer including means storing a plurality of correction coefficients corresponding on a one to one basis to said index locations, said computer further including means to correct the photometric value determined for each of said index locations in accordance with the correction coefficients stored in said computer for the corresponding index location.