ISO 15368:2021

Overview

ISO 15368:2021 - Optics and photonics - Measurement of reflectance of plane surfaces and transmittance of plane parallel elements - defines rules for making spectral measurements of regular reflectance and regular transmittance of plane optical components using spectrophotometers. The standard applies to coated or uncoated optical elements without optical power (e.g., windows, filters, beamsplitters). It covers both dispersive (monochromator) and Fourier‑transform spectrophotometers and explicitly excludes diffuse reflectance/transmittance measurements.

Key topics and technical requirements

• Measurement scope: Spectral regular reflectance (ρr) and regular transmittance (τr) only - not diffuse components.
• Test samples: Preparation, storage and handling consistent with manufacturer instructions; specified angle of incidence, wavelength and polarization.
• Measuring apparatus: Requirements for dispersive and Fourier‑transform spectrophotometers including source, spectral unit, sample compartment and detector. Typical instrument layouts are shown and described.
• Test conditions: Selection and documentation of optical source stability, beam divergence and diameter, wavelength/wavenumber calibration, spectral resolution, stepping interval, incident angle and polarization (p/s).
• Test procedures: Direct and relative measurement methods for regular reflectance; procedures for regular and internal transmittance of plane‑parallel plates.
• Error sources and uncertainty: Identification and mitigation of common errors (wavelength/wavenumber uncertainty, source fluctuation, detector linearity, stray light, inter‑reflections, sample parallelism, misalignment) and guidance on documenting measurement uncertainty.
• Reporting: Required content of test reports to ensure reproducibility and traceability.
• Annexes: Informative guidance on spectrophotometer types and a refractive index table for synthetic fused silica.

Applications

ISO 15368:2021 is used to generate reproducible spectral data for:

• Quality control of optical components (windows, substrates, coated plates).
• Verification of anti‑reflection and coating performance.
• Supplier declarations and acceptance testing for aerospace, semiconductor lithography, laser optics and scientific instrumentation.
• R&D and material characterization where plane surfaces or plane‑parallel elements are evaluated.
  Using this standard helps ensure consistent reflectance measurement and transmittance measurement across labs and instruments.

Who should use it

• Optical manufacturers and coating suppliers
• Calibration and test laboratories (metrology labs)
• Optical designers and researchers requiring traceable spectral data
• Quality assurance and procurement teams specifying optical performance

Related standards

Normative references include: ISO 9211‑1, ISO 10110‑8, ISO 80000‑7, ISO/IEC Guide 98‑3 (GUM) and IEC 60050‑845. These provide vocabulary, drawing conventions, units, uncertainty guidance and related terminology.