In Video Measuring Systems (VMS), the selection and usage of calibration standards directly affect measurement accuracy and long-term stability. Glass scales and grating scales are two commonly used calibration elements. Although both establish length references and correct system errors, they differ significantly in structure, working principle, application scenarios, and maintenance requirements. This article analyzes these differences in detail, covering technical features, applications, calibration content, maintenance, and typical use cases.
1. Application Differences
Glass scales and grating scales focus on different measurement dimensions:
Glass Scale: Primarily used for pixel calibration and 2D geometric calibration, suitable for visual calibration and field of view (FOV) uniformity verification. High-contrast etched lines enable sub-pixel image accuracy correction.
Grating Scale: Mainly used for platform linear accuracy and long-travel repeatability, ideal for large-stroke VMS and mechanical stage error compensation. Periodic lines provide precise linear displacement measurements, ensuring accurate positioning over long travel ranges.
Example: On a 100×100 mm measurement stage, if verifying linear accuracy of ±2 μm is required, a grating scale offers better precision and stability. For lens distortion, pixel size, or FOV uniformity calibration, a glass scale provides more accurate optical reference.
2. Calibration Content
Pixel Calibration
Determines the physical size of pixels:
Glass Scale: Used for local pixel size measurement
Grating Scale: Used for pixel consistency verification over large areas
Geometric Calibration
Compensates lens aberrations and distortion:
Glass Scale: Uniform etch lines allow distortion model creation
Grating Scale: Not suitable for distortion analysis; mainly used for linear displacement correction
Platform Linear and Repeatability Verification
Generates linear error compensation tables to improve long-travel positioning accuracy:
Grating Scale: Stable periodic lines suitable for displacement data collection across different positions
3. Maintenance and Usage Requirements
1. Cleaning and Maintenance
Use lint-free cloth and anhydrous alcohol to clean glass surfaces
Avoid strong solvents and hard objects contacting the scale
Do not blow with compressed air directly
2. Storage
Store in a constant-temperature environment to prevent thermal expansion/contraction
Use shockproof containers
Avoid direct sunlight and UV exposure
3. Usage Notes
Ensure calibration standard and equipment are at the same temperature before measurement
Use fixtures or stages to secure position stability
Avoid tilt or misalignment to prevent image distortion
4. Typical Application Examples
Example 1: 2D Plane Calibration with 5× Lens
Use glass scale for pixel calibration and FOV distortion correction
Suitable for PCB, precision stamping parts, and mobile device components
Example 2: Linear Compensation on 300 mm XYZ Stage
Use grating scale to sample multiple positions on the stage
Generate error compensation table
Suitable for precision fixtures, semiconductor carriers, and metal parts for long-travel high-precision measurement
5. Conclusion
Glass scales and grating scales serve different functions in video measuring systems:
Glass Scale: Optical calibration, ensures pixel size and distortion correction
Grating Scale: Mechanical displacement calibration, ensures linear accuracy and repeatability of long-travel stages
In practical engineering applications, combining both achieves comprehensive optical and mechanical accuracy. Proper selection and maintenance of calibration standards are key to ensuring long-term system stability and reliability.