Glass Scale vs. Grating Scale: Key Differences in Applications for Video Measuring Systems

In Video Measuring Systems (VMS), the selection and use of calibration standards have a direct impact on measurement accuracy and long-term system stability. Glass scales and grating scales are two commonly used calibration standards. Although both establish length references and compensate for system errors, they differ significantly in structure, operating principles, application contexts, and maintenance requirements.

This article analyzes these differences in detail, including technical characteristics, applications, calibration content, maintenance, and typical use cases.

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1. Differences in Applications

Glass scales and grating scales serve different measurement purposes:

Glass Scale

• Primarily used for pixel calibration and 2D geometric calibration

• Suitable for image-based calibration and evaluating Field of View (FOV) uniformity

• High-contrast etched lines enable sub-pixel level image correction

Grating Scale

• Primarily used for verifying linear accuracy of the stage and repeatability over long travel ranges

• Ideal for VMS systems with large travel ranges and the need for mechanical error compensation

• Periodic lines provide accurate linear displacement data to ensure stable positioning across the measuring volume

Example:
On a 100 × 100 mm measuring table, if linear accuracy of ±2 μm is required, a grating scale provides higher stability and precision.
Conversely, for calibrating lens distortion, pixel size, or FOV uniformity, a glass scale offers a more accurate optical reference.

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2. Calibration Content

Pixel Calibration

Defines the physical size of each pixel:

• Glass Scale: Measures local pixel size

• Grating Scale: Checks pixel consistency over a large area

Geometric Calibration

Corrects lens distortion and geometric errors:

• Glass Scale: Uniform grid enables distortion modeling

• Grating Scale: Not suitable for distortion analysis; mostly used for correcting linear displacement

Linear Accuracy and Repeatability of Stage Movement

Creates a linear compensation map to improve positioning accuracy over long travel distances:

• Grating Scale: Periodic lines provide stable data for displacement sampling at multiple positions

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3. Maintenance and Usage Requirements

1. Cleaning and Care

• Use lint-free wipes and absolute alcohol to clean glass surfaces

• Avoid strong solvents or abrasive contact

• Do not blow compressed air directly

2. Storage

• Maintain stable temperature to avoid thermal expansion or contraction

• Store in shock-resistant containers

• Avoid direct sunlight and UV exposure

3. Usage Precautions

• Ensure calibration standard and equipment are at the same temperature before measurement

• Use fixtures or stable platforms to secure positioning

• Avoid tilt or misalignment to prevent imaging distortion

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4. Typical Application Examples

Example 1: 2D Calibration with 5× Lens

• Use a glass scale for pixel calibration and FOV distortion correction

• Suitable for PCB, precision-stamped components, and mobile device parts

Example 2: Linear Compensation on a 300 mm XYZ Stage

• Use a grating scale to sample multiple positions across the stage

• Generate a linear error compensation map

• Suitable for precision fixtures, semiconductor tooling, and metal components requiring high-accuracy long travel measurements

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5. Conclusion

Glass scales and grating scales play different roles in video measurement systems:

• Glass Scale: Optical calibration for pixel size and distortion correction

• Grating Scale: Mechanical displacement calibration for linear accuracy and repeatability over long travel

In practical engineering applications, combining both standards enables comprehensive accuracy in both optical and mechanical aspects. Proper selection and maintenance of calibration standards are critical to ensuring long-term system stability and reliability.