Overview
ISO 19740:2018 defines a standardized test method for homogeneity of infrared optical materials. It specifies the principle, apparatus, environmental conditions, sample types, measurement procedures and data processing required to determine refractive-index homogeneity for materials that are opaque in visible light and transmit beyond 0.78 µm - for example infrared optical glass, infrared crystals and infrared ceramics. The standard covers both wedge-shaped and plane-parallel samples and provides methods to calculate PV and standard-deviation homogeneity metrics.
Key topics and technical requirements
- Measurement principle
- Two main methods: four-step method (wedge-shaped samples) and two-step method (plane-parallel samples). Both use infrared interferometry to derive refractive-index error distribution.
- Apparatus and optics
- Infrared interferometer, reference flat, standard mirror, precision stage, image sensor and data-processing system arranged on a vibration-isolated platform.
- Example light sources within the document: diode lasers (~1.55 µm), He–Ne (3.39 µm), CO lasers (10.6 µm) and quantum cascade lasers (QCLs). Beam aperture should cover the sample or use sub-aperture stitching.
- Reference flat and standard mirror surface deviation: ≤ 0.05 λ (λ = 0.633 µm). Apertures generally larger than the sample.
- Environmental and measurement conditions
- Temperature around 22 °C ± 5 °C with stability better than ±0.2 °C (see Annex B).
- Relative humidity ≤ 70 %.
- Vibration isolation and controlled airflow required.
- Sample preparation
- Wedge-shaped and plane-parallel geometries, with polished surfaces per procedures in Clause 7.
- Thickness measurement equipment with uncertainty ≤ 0.01 mm.
- Data processing and results
- Calculation of PV value (Δn_PV) and standard deviation (Δn_STD) of refractive index distribution, measurement uncertainty and expanded uncertainty (Clauses 9–11).
- Reporting
- Required test report contents are specified (Clause 12 and Annex D).
Applications and users
ISO 19740 is practical for:
- Manufacturers of IR lenses and windows performing quality control and acceptance testing.
- Optical materials suppliers assessing homogeneity of infrared glass, crystals and ceramics.
- Metrology and inspection laboratories validating refractive-index uniformity.
- R&D teams and optical engineers designing high-performance IR systems (thermal imaging, spectroscopy, aerospace, defense) where refractive-index variation affects wavefront quality.
Typical users: optical engineers, materials scientists, QA managers, and laboratory metrologists.
Related standards
- ISO 10110-7 - Preparation of drawings for optical elements - Surface imperfections
- ISO 10110-8 - Preparation of drawings for optical elements - Surface texture; roughness and waviness
Keywords: ISO 19740, homogeneity, infrared optical materials, infrared interferometer, refractive index, PV value, standard deviation, test method, infrared glass, infrared crystals, infrared ceramics.
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