Overview
ISO/TS 24597:2011 provides standardized procedures for evaluating the sharpness of digitized images produced by a scanning electron microscope (SEM). It defines three objective evaluation methods - Fourier transform (FT), contrast-to-gradient (CG) and derivative (DR) - and gives acquisition, data and reporting requirements to ensure consistent, repeatable measurements of SEM image sharpness.
Keywords: SEM image sharpness, image sharpness evaluation, microbeam analysis, Fourier transform method, contrast-to-gradient, derivative method.
Key topics and requirements
- Evaluation methods
- FT method: compares Fourier transform profiles of the SEM image to those of model (convoluted) images.
- CG method: uses weighted harmonic mean gradients of the 2D brightness distribution; note that elements of the CG method are claimed under patent by Hitachi (see document introduction).
- DR method: fits error-function profiles to gradient directional-edge profiles of particles.
- Image acquisition and specimen
- Use a flat, smooth specimen area with circular particles (see Annex G for specimen preparation).
- Set specimen tilt to 0° (±3° tolerated).
- Choose a field of view that avoids structured or uneven foregrounds.
- Pixel size and calibration
- Calibrate magnification/scale per ISO 16700.
- Determine pixel size from field-of-view or scale marker and set pixel size to about 40% of the expected image sharpness (example: 0.8 nm pixel for expected 2 nm sharpness).
- Evaluation results are computed in pixels and converted to nm using calibrated pixel size.
- Image quality requirements
- Minimum contrast-to-noise ratio (CNR) = 10 for valid sharpness evaluation.
- Image data: uncompressed bitmap/TIFF, at least 8-bit greyscale; do not use scanned/printed images.
- Minimize astigmatism, charging, vibration and magnetic distortion; optimize probe current and acquisition time.
- Data area and reporting
- Use a square image area of at least 256 × 256 pixels, free of overlays (scale bars, annotations).
- Clause 7 defines required contents of the test report and examples (Annex H).
Applications and users
ISO/TS 24597:2011 is aimed at practical, objective assessment of SEM imaging performance and is useful for:
- SEM operators and laboratory technicians performing quality control of imaging conditions.
- Microbeam analysis and materials characterization labs validating image fidelity.
- Instrument manufacturers and service engineers verifying focus, probe settings and detector performance.
- Accredited testing and calibration labs (ISO/IEC 17025 context) documenting SEM image sharpness for reports and comparisons.
Related standards
- ISO 16700:2004 - SEM magnification calibration (referenced for pixel-size and scale calibration).
- ISO/IEC 17025:2005 - competence of testing and calibration laboratories.
- ISO 22493 - SEM vocabulary (terms and definitions used).
ISO/TS 24597:2011 supplies a practical framework for rigorous, repeatable SEM sharpness measurements that support routine QC, instrument evaluation and standardized reporting.