The 1964–1970 interval centered on absolute phase sensing and path-length control as the core drivers of precision optical metrology, with interferometric methods, wavefront analysis, and material constant measurements becoming standard practice. The advent of folded optical delay lines and holographic wavefront reconstruction expanded capabilities for long-path metrology, nondestructive testing, and optical buffering in high-precision contexts. Across the era, systematic calibration and reproducible measurement of optical constants underpinned both coatings characterization and infrared metrology applications.