Overview
ISO 17411:2022 - Optics and photonics: Test method for homogeneity of optical glasses by laser interferometry specifies laser-interferometric procedures to measure the homogeneity of the refractive index in optical glass. The standard defines test methods, measurement apparatus, preparation and operation steps, calculation and reporting to support grading systems in ISO 10110-18 and ISO 12123. It is the second edition (2022) and expands measurement options to include PHom, FT-PSI and SCI methods in addition to transmission and oil-on-plate approaches.
Key Topics and Technical Requirements
- Measurement principle: Homogeneity is obtained from the peak-to-valley (PV) of the transmitted wavefront (W) measured by a laser interferometer; PV of refractive-index variation is derived from W.
- Accepted methods:
- Transmission method - preferred for polished, plane-parallel parts; sensitive to surface deformation.
- PHom method - recommended for very high homogeneity (refractive index differences ≤ 2×10^-6); provides high trueness and precision.
- Oil-on-plate method - uses index-matching liquid between test part and a reference plate; easier preparation but requires matched liquids per glass type.
- FT-PSI and SCI methods - Fourier Transform Phase Shifting Interferometry and Spectrally Controlled Interferometry allow observation of linear-change components in refractive index.
- Apparatus requirements: laser interferometer, interferogram analysis device, thermostatic chamber (ambient 20–25 °C; spatial/time fluctuation