Overview
ISO 21227-2:2006 specifies an evaluation procedure for multi-impact stone‑chipping tests on coatings using digital optical imaging. It defines how to acquire, calibrate and analyse images of stone‑chip damage so that the damaged area is quantified and converted to a rating compatible with ISO 20567-1. The method provides a more objective, reproducible alternative to purely visual assessment for paints and varnishes.
Key topics and requirements
- Principle: convert a digital grey‑scale or colour image to a binary image (thresholding) where damaged areas are identified as dark patches and undamaged areas as light patches; calculate damaged‑area percentage and assign a rating.
- Illumination: use illumination that clearly separates damaged from undamaged areas - diffuse bright‑field illumination is recommended to avoid detecting unrelated surface disturbances.
- Region of interest (ROI): preferred ROI is 70 mm × 70 mm to avoid edge effects; smaller ROIs may be used only if differences vs. total area are within acceptable limits (≤ half a rating).
- Resolution: minimum imaging resolution is 6 pixels/mm; different resolutions can affect measured damaged area.
- Calibration: calibrate the imaging system with reference panels prepared as in ISO 20567-1. Calibration fixes acquisition parameters and links image‑analysis outputs to assigned reference ratings.
- Evaluation and rating: report the percentage damaged area and convert it to a 0–5.0 rating scale (half‑point increments allowed) per ISO 20567-1. Extended evaluation can identify damage by layer (substrate, primer, ED coat).
- Precision: international round‑robin testing established reproducibility limits for typical damage levels (example results reported in the standard).
- Test report: include identification of coating, reference to ISO 21227-2, illumination and acquisition details (resolution, grey/colour scale, gamma), image‑processing steps, results, deviations and operator information.
Practical applications and users
ISO 21227-2 is practical for:
- Coating manufacturers and R&D labs validating stone‑chip resistance.
- Automotive and transport OEMs assessing protective coatings subject to road debris.
- Independent test laboratories performing standardized, traceable acceptance tests.
- Quality assurance teams requiring objective, quantitative defect metrics for production control.
The method supports improved comparability, quantitative reporting and digital record‑keeping compared with visual assessment.
Related standards
- ISO 20567-1 - specifies the multi‑impact stone‑chip test method and reference images used for rating.
- ISO 21227-1 - general guidance for evaluation of coated surfaces using optical imaging.
Keywords: ISO 21227-2, optical imaging, stone chipping, multi-impact test, paints and varnishes, calibration, ROI, thresholding, damaged‑area rating.