ISO 25387:2026

Overview

ISO 25387:2026 provides a standardized procedure for determining the point resolution-specifically, the Scherzer resolution-of high-resolution transmission electron microscopes (HREM) used in microbeam analytical electron microscopy. This international standard outlines how to assess the microscopic performance of HREM through precise measurement of point resolution and real spherical aberration coefficients of the objective lens. The methodology relies on analyzing dark rings in the fast Fourier transform (FFT) pattern of HREM images from amorphous thin films, offering reliable and consistent results across different transmission electron microscopes (TEMs).

ISO 25387:2026 is applicable to HREMs equipped with various electron gun types-including cold field emission gun (CFEG), Schottky emission gun (SEG), thermal field emission gun (TFEG), and thermionic emission gun (TEG)-where three or more dark rings can be distinctly observed in the FFT pattern. Notably, this standard does not cover Cs-corrected TEM, information limits, lattice resolution, or STEM resolution.

Key Topics

• Scherzer Resolution: Defines point resolution for HREM, determined by the first zero in the phase contrast transfer function (PCTF) under the Scherzer focus condition.
• Measurement of Spherical Aberration: Specifies how to accurately assess the real spherical aberration coefficient-critical for evaluating TEM performance-by observing the diameters of dark rings in FFT patterns.
• FFT Pattern Analysis: Utilizes FFT diffractograms of amorphous thin film images, requiring at least three observable dark rings near the Scherzer focus.
• Applicable Electron Guns: Addresses CFEG, SEG, TFEG, and TEG types used across modern HREMs.
• Exclusions: Does not treat information limit, lattice resolution, STEM resolution, or Cs-corrected TEM.
• Impact of Envelope Functions: Discusses how spatial and temporal coherency affect the appearance of diffractograms, but not the actual measured point resolution.

Applications

Implementing ISO 25387:2026 ensures reliable, repeatable, and internationally comparable determination of HREM point resolution. Its practical applications include:

• Quality Control in Materials Science: Enables laboratories to verify HREM performance by regularly measuring and documenting point resolution, helping maintain consistent high-resolution imaging standards.
• Instrument Benchmarking: Provides a universal methodology for comparing instrument capabilities across manufacturers and research facilities, fostering standardization in equipment procurement and performance evaluation.
• Method Validation: Laboratories developing or validating analytical techniques involving HREM can use the protocol to ensure the accuracy of sub-nanometre structural observations.
• Research & Development: Assists researchers in materials science, nanotechnology, and life sciences to accurately characterize and report on ultra-fine material structures, forming the basis for reproducible and credible scientific investigations.

Related Standards

When applying ISO 25387:2026 in analytical electron microscopy, these related international standards may also be relevant:

• ISO 15932: Terminology for electron microscopy and microbeam analysis-critical for consistent definitions used in ISO 25387:2026.
• ISO 20263: Imaging methods for scanning transmission electron microscopes (STEM).
• Other ISO TC 202 Standards: Various documents on sample preparation, calibration, and performance criteria for electron microscopes.
• IEC Electropedia & ISO Online Browsing Platform: Reference sources for standard terms and abbreviations used in the field.

Conclusion

By adopting ISO 25387:2026, laboratories and researchers in analytical electron microscopy can confidently determine and report the point resolution of their HREM instruments. This standard delivers transparency, comparability, and precision for advanced imaging applications vital to material science, nanotechnology, and related fields, supporting global best practices in ultra-high-resolution electron microscopy.