ISO 21395-1:2020

Overview

ISO 21395-1:2020 - "Optics and photonics - Test method for refractive index of optical glasses - Part 1: Minimum deviation method" specifies a high‑accuracy laboratory method to measure the refractive index of optical glasses. The standard covers measurements with an accuracy of 1 × 10−5 across the spectral range 365 nm to 2 400 nm. It defines the minimum‑deviation prism technique, required instrumentation, specimen prism geometry and environmental conditions, and provides informative annexes on dispersion, correction factors and alternative measurement procedures.

Key topics and requirements

• Measurement principle: determine apex angle (α) and angle of minimum deviation (δmin) and compute the relative refractive index:
  • n_rel = sin((α + δmin)/2) / sin(α/2)
• Accuracy: target uncertainty within 1 × 10−5; account for temperature, pressure and humidity.
• Spectral range & light sources: standard wavelengths from 365 nm to 2 400 nm; acceptable sources include mercury, hydrogen, helium, cadmium lamps, He–Ne and Nd:YAG lasers (ISO 7944 referenced).
• Goniometer performance: angular reading capability of ±1 arc second; prism alignment typically within ±3 arc sec (autocollimator).
• Specimen prism geometry:
  • edge length: 15–40 mm; thickness: 10–30 mm
  • useful apex angle typically 35°–80°
  • surface flatness: better than 1/4 λ over 80% aperture (measured at 546 nm or 632.8 nm)
• Environmental controls:
  • temperature 20 °C–25 °C (allow ~24 h acclimatization)
  • relative humidity 30%–70%, ±10% during measurement
  • atmospheric pressure 86–106 kPa, ±0.5 kPa during measurement
• Annexes: Annex A (dispersion metrics - Abbe number, partial dispersion), Annex B (dispersion formulae), Annex C (corrections for T/P/humidity), Annex D/E (alternative measurement methods).

Applications

• Precise refractive index measurement for optical glass characterization, quality control and certification.
• Input data for lens design, optical modelling and simulation (dispersion curves, Abbe numbers).
• Supplier–manufacturer communication of glass catalog data and tolerance verification.
• R&D in materials, coatings and high‑precision optics manufacturing.

Who should use this standard

• Optical glass manufacturers and suppliers
• Metrology and testing laboratories
• Optical designers and lens manufacturers
• Quality assurance teams in photonics and imaging industries
• Research institutions working on optical materials

Related standards

• ISO 12123:2018 (refractive index of air dependence on temperature/pressure)
• ISO 17328 (error relations for refractive index measurements)
• ISO 7944 (definitions for common laser sources)

This ISO 21395-1:2020 standard is essential when you need traceable, repeatable, high‑precision refractive index measurements of optical glasses for design, verification and quality control in optics and photonics.