ISO 6760-1:2024

Overview

ISO 6760-1:2024 - "Optics and photonics - Test method for temperature coefficient of refractive index of optical glasses - Part 1: Minimum deviation method" defines a standardized laboratory method to measure how the refractive index of optical glass changes with temperature. The standard uses the well-established minimum deviation method (prism-based) to determine the temperature coefficient of refractive index (Δn/ΔT) over a specified temperature and wavelength range, with high metrological accuracy for optical materials and components.

Key topics and technical requirements

• Measurement principle
  • Refractive index is obtained from prism minimum-deviation measurements at two or more temperatures; Δn/ΔT is calculated from the change in refractive index divided by the temperature change.
• Intended ranges and accuracy
  • Temperature range: –40 °C to +80 °C
  • Wavelength range: 365 nm to 1 014 nm
  • Target measurement accuracy: 1 × 10−6 K−1
• Apparatus and environmental controls
  • Use of a goniometer, collimated light source and detector per ISO 21395-1.
  • Specimen prism placed inside a thermal chamber capable of controlled temperature changes and uniformity.
  • Chamber requirements include:
    • Temperature distribution within ±1.0 K during ramping
    • Thermometer accuracy ±0.2 K
    • Ability to achieve vacuum < 10 Pa to make the refractive-index-of-air negligible and avoid condensation
    • Optical windows: quartz parallel plates, wedge ≤ 5 arc sec, flatness λ/10
• Measurement steps
  • Measure apex angle of prism and minimum deviation angle at each temperature (with correction from apparent air measurement to vacuum to obtain absolute refractive index).
  • Calculate absolute and relative refractive-index temperature coefficients and report per the document’s test-report requirements.
• Annexes and alternatives
  • Includes formulas for air refractive index, methods to compute relative refractive index at arbitrary pressure/humidity, a half‑prism alternative (Annex C), interpolation methods and derivations.

Practical applications and users

ISO 6760-1 is relevant for:

• Optical glass manufacturers validating thermo‑optical properties
• Metrology and calibration laboratories performing certified Δn/ΔT measurements
• Optical designers and systems engineers (cameras, microscopes, telescopes, lithography optics) assessing temperature sensitivity of optical assemblies
• R&D and quality assurance teams selecting glass types for stable performance across temperature ranges

Practical benefits include consistent, comparable Δn/ΔT data for lens design, thermal compensation analysis, material selection and contractual test reporting.

Related standards

• ISO 21395-1:2020 - Minimum deviation method for refractive index of optical glasses (measurement procedures and apparatus)
• ISO 21395-2 - V‑block refractometer method (other refractive-index methods referenced in the series)
• ISO 9802 - Definitions related to temperature coefficient terminology

Keywords: ISO 6760-1, temperature coefficient of refractive index, refractive index temperature dependence, optical glass, minimum deviation method, thermal chamber, optical metrology.