ISO 27911:2011

Overview

ISO 27911:2011 defines a reproducible method for determining the lateral (spatial) resolution of aperture-type near-field scanning optical microscopes (NSOM / SNOM). The standard targets apertured NSOMs operated in transmission, reflection, collection or illumination/collection modes and uses the practical approach of imaging a test object much smaller than the expected resolution. ISO 27911 emphasizes use of the point spread function (PSF) concept and provides guidance suitable for non-expert operators performing resolution calibration and characterization.

Key technical topics and requirements

• Scope and applicability
  • Applies only to apertured NSOMs (not apertureless/scattering NSOMs).
  • Operative in transmission, reflection, collection, or illumination/collection modes.
• Measurement principle
  • Lateral resolution is determined by imaging a very small object and extracting the NSOM’s PSF (e.g., FWHM of the PSF).
  • Recognizes that NSOM PSFs can be complex (not strictly Gaussian) and depend on aperture geometry, coating and polarization.
• Specimen selection and preparation
  • Recommends isolated nano-sized fluorescent objects on non-luminescent substrates to avoid topographic and background artefacts.
  • Limits topographic height of objects to one-tenth of the expected lateral resolution to reduce crosstalk between topography and optical signal.
  • Annex A provides examples (line-profile and quantum-dot specimens); Annex B gives specimen-preparation guidance.
• Instrument and experiment parameters
  • Factors influencing resolution: aperture size, metal-coating condition, tip–sample gap control (shear-force or cantilever methods), signal-to-noise, pixilation, and contrast mechanism (fluorescence preferred to reduce background).
  • Requires documenting experimental conditions (probe type, imaging mode, specimen properties, feedback method) when reporting resolution.
• Data collection, analysis and recording
  • Covers setting parameters prior to operation, collecting optical images, analyzing PSF/line-profiles and recording results for traceability.

Practical applications and who uses this standard

ISO 27911:2011 is valuable for:

• Metrology laboratories calibrating NSOM instruments and validating lateral resolution claims.
• Instrument manufacturers establishing performance specifications and QA procedures.
• Research labs in nanophotonics, surface chemical analysis, materials science and semiconductor inspection using NSOM for high-resolution optical imaging.
• Standards bodies and regulatory organizations needing consistent resolution measurement methods.

Benefits include improved comparability of NSOM performance data, reduced artefacts in optical imaging, and standardized reporting of lateral resolution.

Related standards and resources

• Normative reference: ISO 18115-2 (vocabulary for scanning-probe microscopy).
• Prepared by ISO/TC 201 (Surface chemical analysis), SC 9 (Scanning probe microscopy).