Geometrical product specification (GPS) - Surface texture: Areal - Part 606: Nominal characteristics of non-contact (focus variation) instruments (ISO 25178-606:2015); German version EN ISO 25178-606:2015
1Key Takeaways
This part of ISO 25178 specifies the measurement characteristics of a particular method for non-contact measurement of surface topography using a focal change (FV) sensor. It describes the measurement characteristics of a focal change microscope used to create images of surface topography.
2Expert Interpretation
This in-depth interpretation of the ISO 25178-606:2015 standard focuses on the optical surface measurement principles, instrument characteristics, and technical specifications for variable-speed technology. This systematic analysis includes key parameters such as measurement uncertainty, lateral resolution, and transfer function, providing a standardized basis for precision manufacturing and surface quality control.
Standard Framework and Technology Evolution Background
ISO 25178-606:2015, as an important part of the Geometric Product Specification (GPS) system, specifically specifies the nominal characteristics of non-contact surface topography measurement instruments based on the principle of focus change. This standard was adopted by the European Committee for Standardization (CEN) in June 2015, marking an important progress in the field of industrial standardization of optical three-dimensional surface measurement technology.
Core Terminology System and Definition Analysis
The standard establishes a complete terminology system, in which key concepts require professional interpretation:
| Terminology | Definition | Metallistic Significance |
|---|---|---|
| Measurement Coordinate System | Right-handed orthogonal coordinate system (x, y, z), where the z-axis is parallel to the optical axis | Establishes a spatial reference for optical measurement |
| Transfer Function | A function that describes the relationship between actual dimensions and measured values | Characterizes the measurement response characteristics of an instrument's systematic nature | < /tr>
| Lateral Period Limit | The spatial period of a sinusoidal profile, at which the instrument's height response drops to 50% | Determines the instrument's spatial resolution limit |
| Width Limits Full-Height Transmission | The minimum width of a rectangular groove, while maintaining the same measured height | Reflects the instrument's ability to measure small features |
Focus Variation Technology: A Deep Dive
Focus Variation Technology combines the optical system's narrow depth of field with a vertical scanning mechanism to provide topographic information by analyzing focus variations. The core technology utilizes a white light source to illuminate the sample and continuously collects data by vertically moving the optical system. Each object region is in sharp focus at a specific vertical position within the scanner.
An algorithm converts the acquired sensor data into 3D information and a full-depth color image. 3D information is calculated by analyzing the focus information (curve) along the vertical axis. Unlike traditional coaxial illumination technology, focus variation technology can use a variety of illumination sources (such as ring light sources) and can measure surface features with local slope angles up to 90°.
Instrument metrology characteristics requirements
Key performance parameter specifications
| Characteristic parameters | Symbol | Requirements | Test methods |
|---|---|---|---|
| Gain coefficient | κx, κy, κz | Ideal value is 1 | Transfer function linear regression slope |
| Linearity deviation | δx, δy, δz | Maximum local difference | Polynomial approximation evaluation |
| Measurement noise | NM | Standard deviation expression | Repeatability measurement evaluation |
| Lateral resolution | RL | Minimum distance between two distinguishable geometric elements | Standard test sample verification |
Optical system characteristic requirements
The standard puts forward clear requirements for the optical system of the focus variation sensor: the numerical aperture (AN) should be as large as possible because it directly affects the depth of field; the objective lens used should be corrected for chromatic aberration to ensure accuracy in white light measurement; the working distance should be long enough to allow measurement of tall and rough samples.
Analysis of Factors Influencing Measurement Uncertainty
The standard details 21 key factors influencing measurement results, including optical measurement wavelength (λ0), optical measurement bandwidth (B0), illumination angle range, polarization state, and numerical aperture. These factors collectively determine the instrument's overall measurement performance.
In particular, ambient vibration (NVIB) and required integration time (TII) significantly impact measurement noise. While focus variation techniques are relatively robust to small or infrequent vibrations, strictly controlled environmental conditions are still required for high-precision measurements.
Implementation Recommendations and Application Guide
Instrument Selection and Configuration Recommendations
Select the appropriate focus variation measurement instrument configuration based on your measurement requirements: For highly reflective metal surfaces, it is recommended to use a polarization filter (polarizer and analyzer) to eliminate specular reflections. For complex geometries, an instrument with a ring-shaped illumination system should be selected.
Measurement Procedure Optimization
The standard recommends fitting the focus information curve using a polynomial or more complex function. The maximum location is determined by calculating the peak of the fitted polynomial or function. This method strikes a good balance between accuracy and speed.
Calibration and Verification Program
Establish a regular calibration system and verify the instrument's transfer function, linearity, and resolution characteristics using standard reference materials (SRMs). In particular, key parameters such as the lateral period limit (DLIM) and width-limited full-height transmission (W) should be regularly verified.
Technical Limitations and Applicability Analysis
The focus variation technique is only applicable to surfaces where the surface image undergoes sufficient variation during the vertical scan. Transparent samples or components with minimal local roughness will exhibit a focus curve without a distinct peak, making it impossible to determine the location of the maximum focus.
Typically, the focus variation technique provides repeatable measurement results for surfaces with local Sa values in the 5nm to 15nm range. The filter cutoff frequency calculated from this Sa value (depending on the sampling interval) ranges from 1μm to 3μm.
Industrial Application Case Studies
Focus variation technology is widely used in industrial quality assurance and R&D activities. Key application areas include surface analysis and characterization in the cutting tool industry, precision manufacturing, the automotive industry, materials science, corrosion and tribology, electronics, medical device development, and the paper and printing industry.
Typical applications include 3D measurement of gearboxes (as shown in Figure A.3 of the standard), topography measurement of precision components, and surface roughness analysis. These applications fully demonstrate the unique advantages of focus variation technology in providing both true and false color information.
Technical Challenges and Solutions for Standard Implementation
Major challenges in implementing ISO 25178-606 include adaptability to measurement of complex surfaces, strict control of environmental conditions, and the complexity of instrument calibration. Solutions include developing advanced focus analysis algorithms, establishing strict environmental control standards, and establishing detailed calibration procedures.
Future technology development directions include improving measurement speed, enhancing the measurement capability of transparent and semi-transparent materials, and developing more advanced polarization control technology. These developments will further expand the application scope of focus variation technology in industrial measurement.