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ISO 25178-602: Design and Characteristics of Non-Contact (Confocal Chromatic Probe) Instruments
Geometrical Product Specifications (GPS) — Surface Texture: Areal — Part 602
1. Principles of Confocal Chromatic Profilometry
ISO 25178-602:2025 (second edition) specifies the design and metrological characteristics of non-contact instruments using confocal chromatic probes based on axial chromatic aberration of white light. This technology encodes surface height into the spectral domain: different wavelengths focus at different distances from the chromatic objective, and the reflected wavelength identifies the surface height. The idea of using axial chromatic dispersion to code surface heights was developed in the 1980s and has since been implemented in commercial scanning profilometers for areal surface topography measurement and even roundness measurement on coordinate measuring machines (CMMs).
The confocal chromatic probe consists of three basic elements: an optoelectronic controller housing the light source and spectrometer, a linking fibre optic cable, and a chromatic objective (sometimes called an “optical pen”). In practice, the pinholes required for confocal operation are realized using a fibre optic coupler that acts as both the source pinhole and the discrimination pinhole. This fibre-coupled design offers the significant advantage of separating the optical pen from the electronics, enabling measurements in confined spaces and harsh environments where traditional optical heads cannot operate.
Confocal chromatic probes offer distinct advantages: no mechanical z-scanning (reducing noise and increasing speed), ability to measure through transparent layers, and compatibility with high-slope surfaces. Vertical range is determined by the axial chromatic aberration and can range from tens of micrometers to several millimeters. The technique achieves vertical ranges exceeding those of PSI while maintaining good resolution.
| Component | Function | Design Consideration |
|---|---|---|
| Light source | Broadband white light (halogen, xenon, LED) | Spectral range must match optics and detector |
| Chromatic objective (optical pen) | Generates axial chromatic aberration | Determines vertical range and working distance |
| Spectrometer | Decodes reflected wavelength to height | Resolution determines height discrimination |
| Fiber optic coupler | Acts as source and discrimination pinholes | Enables remote optical head placement |
2. Measurement Capabilities and Error Sources
The confocal chromatic probe senses one surface point at a time, requiring lateral scanning for areal measurements. Line sensor variants use an image detector for simultaneous multipoint measurement along a line, enabling much faster areal coverage when combined with a single y-axis scanning stage. The vertical range of the sensor equals the axial chromatic aberration observed between the shortest and longest detectable wavelengths. Spectrometer-based systems achieve vertical ranges from tens of micrometers to several millimeters, depending on the objective lens design. The relative height at any surface point is obtained by identifying the peak wavelength in the spectrometer curve and converting it to a distance using calibration data.
The standard identifies nine major influence quantities that affect measurement uncertainty. Spot size limits lateral resolution and is determined by the pinhole size and numerical aperture. Numerical aperture limits the maximum measurable local slope that can be detected before light falls outside the collection cone. Chromatic aberration power affects the linearity of the height-to-wavelength relationship. Local slope variations can cause non-measured points when the reflected beam misses the discrimination pinhole. Dark signal and stray light from the detector electronics produce systematic bias. The sampling interval determines the smallest features that can be resolved, following the Nyquist criterion. Surface absorption can distort spectral peak positions, particularly for dark or colored materials. Transparent layer thickness creates multiple spectral peaks corresponding to different interfaces, which can be exploited for subsurface measurements.
Non-measured points occur when the spectrometer cannot identify a clear spectral peak. Common causes include surfaces too dark (low reflectivity), too shiny (saturation), steep local slopes exceeding the numerical aperture limit, or surface height outside the chromatic range. Plan measurement strategies accordingly and account for non-measured points in data analysis.
3. Practical Engineering Applications
Confocal chromatic instruments excel where contact measurement is impractical. They are ideal for soft or delicate surfaces such as photoresist coatings, polymer films, and biological samples that would be damaged by a physical stylus. The non-contact nature enables high-speed in-process measurements on production lines where scan speed is critical. The ability to operate through transparent media enables unique applications such as measuring surface topography beneath oil films, protective coatings, or encapsulating materials. By detecting multiple spectral peaks from different interfaces within a transparent layer, the technique can measure layer thickness and subsurface topography simultaneously.
The fiber-coupled design allows the optical pen to be separated from the optoelectronic controller, enabling measurements in confined spaces and integration with CMMs for form measurement. Practical implementations include point sensors on CMMs for precise dimensional metrology, line sensors for high-speed scanning of large areas, and rotary configurations for non-contact roundness measurement. The absence of mechanical z-scanning in the sensor head reduces measurement noise and enables faster data acquisition compared to point autofocus probe instruments. For optimal results, engineers should ensure adequate light intensity, select objectives with numerical aperture matched to the expected slope distribution, and use the through-transparency capability for reference surface compensation when available.
For optimal confocal chromatic measurements: ensure adequate light intensity, choose objectives with appropriate numerical aperture for expected slopes (higher NA for steeper features), account for surface absorption effects on dark or colored samples, and use the through-transparency capability for reference surface compensation. Regular calibration using certified step-height standards is essential for maintaining traceability.
4. Frequently Asked Questions
Q: What is the main advantage of confocal chromatic probes over stylus instruments?
A: Non-contact operation eliminates surface damage, enables higher scan speeds (no physical dragging), and allows measurement of soft, delicate, or sticky materials that cannot be measured with a contacting stylus.
A: Non-contact operation eliminates surface damage, enables higher scan speeds (no physical dragging), and allows measurement of soft, delicate, or sticky materials that cannot be measured with a contacting stylus.
Q: Can confocal chromatic probes measure through transparent layers?
A: Yes. The probe detects multiple spectral peaks from different interfaces within a transparent layer, enabling simultaneous measurement of surface topography beneath oil films, protective coatings, or encapsulating materials, as well as layer thickness.
A: Yes. The probe detects multiple spectral peaks from different interfaces within a transparent layer, enabling simultaneous measurement of surface topography beneath oil films, protective coatings, or encapsulating materials, as well as layer thickness.
Q: What causes non-measured points in confocal chromatic measurements?
A: Low reflectivity (dark surfaces), detector saturation (shiny surfaces), steep local slopes exceeding the numerical aperture acceptance cone, or surface height outside the calibrated measurement range prevent reliable wavelength peak identification.
A: Low reflectivity (dark surfaces), detector saturation (shiny surfaces), steep local slopes exceeding the numerical aperture acceptance cone, or surface height outside the calibrated measurement range prevent reliable wavelength peak identification.
Q: How is vertical range determined in confocal chromatic probes?
A: By the axial chromatic aberration distance between the focal points of the shortest and longest detectable wavelengths, determined by the chromatic objective design. Ranges from tens of micrometers to several millimeters are achievable depending on the objective.
A: By the axial chromatic aberration distance between the focal points of the shortest and longest detectable wavelengths, determined by the chromatic objective design. Ranges from tens of micrometers to several millimeters are achievable depending on the objective.
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