Overview
ISO 17328:2021 - Optics and photonics - provides a standardized test method for measuring the relative refractive index to air of infrared optical materials in the spectral range 0.78 µm to 25 µm. The standard specifies a minimum deviation method using a polished prism specimen, defines required apparatus and procedures, and sets out reporting and uncertainty practices. It excludes measurements for birefringent materials and methods for determining the complex refractive index.
Key topics and technical requirements
- Measurement principle: Minimum deviation method - determine the apex angle (α) and the angle of minimum deviation (δ) and calculate the relative refractive index via the standard formula in the document.
- Spectral range: 0.78 µm to 25 µm; wavelengths must sample adequately for dispersion curve fitting and derivation of index at arbitrary wavelengths.
- Specimen: Wedged/prism shape with polished entrance and exit faces; apex angle guidance provided (α = 2 arctan(1/n_rel) as guidance).
- Surface accuracy: Entrance and exit faces assessed with an interferometer; recommended surface flatness error is 150 nm P‑V or less over the clear aperture.
- Apparatus (Annex A examples): collimated monochromatic light source (monochromator or band-pass filters), collimating unit, goniometer (rotating stage and arm), focusing unit, detector unit, and temperature/environment control.
- Environmental control: Measure specimen and ambient temperature, pressure and humidity; method assumes stable ambient temperature for measurement.
- Error analysis: Annex B provides analysis of angular and measurement errors and uncertainty reporting requirements.
- Test report: Must include specimen identification (per ISO 11382), date/place, measurer, ambient conditions, apex angle, specimen temperature, surface accuracy, wavelengths and bandwidths, angles of minimum deviation, and calculated relative refractive indices - all with stated uncertainties.
Applications and users
ISO 17328:2021 is intended for:
- Manufacturers and quality control of infrared optics and components (e.g., lenses, windows, prisms).
- Optical material suppliers characterizing infrared optical materials (Ge, ZnS, ZnSe, chalcogenides, etc.).
- Metrology and research laboratories performing refractive index measurements and dispersion characterization.
- Optical designers and system engineers needing accurate refractive index data for modeling IR imaging, sensing, and photonics systems.
Keywords: ISO 17328:2021, refractive index, infrared optical materials, minimum deviation method, goniometer, prism, spectral range 0.78–25 µm, surface accuracy, test report.
Related standards
- ISO 11382:2010 - Characterization of optical materials used in the infrared spectral range (referenced for specimen naming and classification).
- ISO 12123:2018 - Definition of refractive index (referenced for terminology).