Overview
ISO 19742:2018 specifies an optical test method to measure bubbles and inclusions in infrared (IR) optical materials. The standard applies to IR optical glass, crystals and ceramics that are opaque to visible light and transmit beyond 0.78 µm. It defines the required apparatus, test conditions, sample preparation, measurement procedure and data processing steps used to quantify internal defects and report a material imperfection value.
Key Topics
- Apparatus and setup: A diffuse infrared light source with a uniform diffuser, an infrared imaging system (lens + image sensor) and a computer-based data collection, processing and display system. The imaging system must provide sufficient depth of focus and spatial resolution (recommended object spatial resolution not more than 0.1 mm).
- Test conditions: Environmental temperature is controlled between 15 °C and 35 °C (±2 °C) and relative humidity should not exceed 80 %.
- Sample requirements:
- Typical maximum thickness recommended is 12 mm (normally 10 mm); scanning methods permit thicker samples.
- Polished surfaces: surface roughness Rq per ISO 10110-8 shall be less than 0.012 µm.
- Surface scratches/digs larger than the minimum bubble/inclusion size must be accounted for or subtracted from results.
- Measurement principle: Illuminate the sample with diffuse IR light, image bubbles and inclusions, measure their dimensions, calculate cross-sectional areas and sum areas per unit sample volume to derive a material imperfection value.
- Data processing rules:
- Symmetric defects: measure maximum length and compute cross-sectional area.
- Asymmetric defects: measure maximum length and width and use their product as area.
- Ignore bubbles and inclusions smaller than 0.1 mm maximum length.
- Compute total defect area, sample volume and the material imperfection value (total cross-sectional area per defined sample length/volume).
Applications
This test method supports practical quality assurance and acceptance testing for manufacturers and laboratories working with infrared optical materials and components. Typical applications include:
- Incoming inspection of IR optical glass, crystals and ceramics.
- Process control during material production and polishing.
- Final quality verification for IR lenses and assemblies used in thermal imaging, spectroscopy and other photonics systems.
Benefits include standardized defect quantification, improved traceability of inspection results, and clearer reporting for procurement and compliance.
Related Standards
- ISO 10110-8 - Surface texture; referenced for surface roughness (Rq) requirements.
- Other standards and military/industry references cited in the ISO document (e.g., JOGIS, MIL specifications) provide historical context and alternative methods but ISO 19742:2018 is the current international method for bubbles and inclusions in IR materials.
For laboratory reports, include instrument and method details, test wavelength, environmental conditions, sample dimensions, section and volume data, total defect area, material imperfection value, tester, reviewer and date as specified in ISO 19742:2018.
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