ISO 6760-2:2024

Overview

ISO 6760-2:2024 - "Optics and photonics - Test method for temperature coefficient of refractive index of optical glasses - Part 2: Interferometric method" specifies an interferometric test method to determine how the refractive index of optical glass changes with temperature. Using interferometry (typically a Fizeau setup), the method measures changes in optical path length as temperature is varied continuously. The standard targets measurements across the wavelength range 365 nm to 1 014 nm with an intended accuracy of 1 × 10‑6 K‑1 for the temperature coefficient of refractive index.

Key topics and requirements

• Measurement principle: Count cycles of light/dark fringe changes in an interferometer to quantify optical path length change and derive the temperature coefficient of refractive index.
• Intended ranges: Wavelengths 365–1 014 nm; arbitrary temperature range (as defined by test setup).
• Accuracy and resolution: Intended accuracy within 1 × 10‑6 K‑1; interferometer fringe-count resolution of 0.1 cycle (1/10 cycle) or better.
• Thermal chamber: Must control specimen temperature (±0.5 K stability) and include a thermometer accurate to ±0.2 K; options to use dry air at 0% RH or vacuum (residual pressure < 10 Pa) to avoid condensation.
• Flat plates and specimen mounting: Transmission and reference flats (quartz or low-expansion ceramics) with wedge ≈ 6 arc min (~0.1°) and interference-surface flatness ≤ λ/2; specimen is sandwiched between flats for combined optical-path and linear-expansion measurement.
• Apparatus components: Coherent monochromatic light sources (lasers recommended), detectors for optical-path and expansion measurement, beam splitters, temperature sensors, barometer/vacuum gauge.
• Calculations and reporting: Annexes provide formulas for absolute/relative temperature coefficients, correction for air refractive index, specimen expansion, and reporting requirements.

Applications and who uses it

• Optical glass manufacturers for material characterization and quality control.
• Lens and system designers who must account for refractive index changes with temperature in high-resolution optics (cameras, microscopes, telescopes).
• Metrology and test laboratories validating optical materials and publishing certified data.
• R&D groups developing low-thermal-drift glasses or compensating optical designs.

Using ISO 6760-2 helps ensure reproducible, high-accuracy data for thermal behavior of refractive index, essential for performance prediction and environmental tolerance in precision optical systems.

Related standards

• ISO 21395 series (refractive index measurement: minimum deviation and V‑block methods)
• ISO 9802 (terminology including temperature coefficient definitions)
• ISO 7944 (examples of light sources for interferometry)

Keywords: ISO 6760-2, temperature coefficient of refractive index, optical glass, interferometric method, interferometry, Fizeau interferometer, refractive index temperature dependence, thermal chamber, metrology.