The period centers on systematic measurement and modeling of optical constants for thin films, with emphasis on refractive index and extinction coefficient as functions of wavelength, and on using interference in multilayer stacks to determine thickness and propagation characteristics. Experimental protocols were standardized to connect deposition conditions with optical performance, enabling design of coatings and energy-efficient windows and advancing early optoelectronic devices. This era unified experimental technique with theoretical insight to support practical thin-film technologies. Historical Significance: The suite of foundational works established essential data sets and methods that shaped later research. Optical Properties of Thin Solid Films (1956) introduced systematic optical constant extraction; Optical Constants of the Noble Metals (1972) provided reliable n and k for copper, silver, and gold; the 1976 figure of merit for transparent conductors introduced a predictive framework; the 1983 determination of the thickness and optical constants of amorphous silicon delivered rigorous index and thickness measurements; and the 1986 Evaporated Sn-doped In2O3 films review linked deposition control to transparency and conductivity. Collectively, these breakthroughs created a durable paradigm for thin-film optics, enabling predictive design of coatings, transparent electrodes, and energy-efficient devices.