The wide range of optical thin film applications utilizing gradient index coatings has prompted the development of advanced optical control techniques. These include ellipsometric and photometric instruments capable of in-situ measurement of optical performance as the optical structure is being deposited. This paper discusses design sensitivity analysis and instrument configuration for development of a control strategy. The ability to measure optical thickness, refractive index and mechanical thickness is a function of several instrument parameters including wavelength, number of wavelengths, angle of incidence, and complexity of measurement surface. The most critical control data in the fabrication of a particular rugate design, and the instrument parameters and techniques employed and how they affect the control strategy is presented in this discussion.