Overview
ISO 23124:2024 specifies a standardized method to measure the spatial resolution - both lateral and axial - of a Raman microscope. Focused on the narrow line method, the standard defines how to acquire and analyse Raman images of small or line-shaped specimens to determine resolution using full width at half maximum (FWHM) metrics and confocal imaging principles.
Key topics and requirements
- Measurement method
- Uses the narrow line method (not the straight-edge or grating methods).
- Spatial resolution is evaluated from the FWHM of line profiles (line spread function) or confocal point spread function.
- Specimen requirements
- Use small or line-shaped objects (e.g., carbon nanotubes, semiconductor nanowires, suspended graphene) that give strong Raman signals.
- Object size (diameter, thickness, penetration depth) must be less than one-quarter of the expected resolution to ensure negligible broadening.
- Reference sample sizes should be verified by traceable methods (AFM, SEM, interferometry).
- Instrumental parameters to record
- Numerical aperture (NA) of the objective lens.
- Confocal pinhole size (or equivalent optics) - smaller pinholes improve axial/lateral resolution but reduce detected signal.
- Calibration of x–y stage or laser scanning precision prior to measurement.
- Operator-set parameters: laser power, spectral width, polarization, integration time (typical tens to hundreds of ms/pixel), and pixel size.
- Data acquisition and analysis
- Line-profile length should exceed three times the expected resolution.
- Signal-to-noise ratio (SNR) > 5 for reliable FWHM fitting; baseline subtraction method is specified.
- For axial scans, pixel step size must be smaller than one-quarter of the expected axial resolution.
- Documentation
- Full instrument configuration and experimental settings must be recorded with the resolution result.
Applications and users
- Laboratories and facilities that perform Raman imaging and spectroscopy.
- Manufacturers of Raman microscopes and confocal micro-spectroscopy systems for product specification and validation.
- Metrology and quality assurance teams requiring traceable spatial resolution measurement.
- Researchers using Raman mapping for nanoscale materials (CNTs, graphene, semiconductor nanowires) who need quantified lateral/axial resolution for data interpretation.
Related standards and resources
- Produced by ISO/TC 201 (Surface chemical analysis).
- Terminology and referenced definitions draw on related ISO documents (e.g., ISO 10934, ISO 25178-607, ISO 18115-1) cited in the standard.
- Annex A provides a case study using dispersed carbon nanotubes and suspended graphene to illustrate the procedure.
Keywords: ISO 23124, Raman microscope, spatial resolution, lateral resolution, axial resolution, narrow line method, confocal pinhole, FWHM, numerical aperture.