Application example
Shaft diameter after grinding
- Challenge
- Manual sampling missed drift between checks.
- Setup
- Thru-beam telecentric sensor measuring every shaft in-line.
- Result
- 100% measurement with trend data for tool-wear compensation.
Components · Measurement
Optical Measurement
Non-contact optical measurement checks dimensions at production speed without touching or wearing the part. Choosing the right principle — and understanding accuracy, repeatability and resolution — is the key to measurements you can trust.
Đo quang học không tiếp xúc kiểm tra kích thước ở tốc độ sản xuất mà không chạm vào sản phẩm. Hiểu đúng độ chính xác, độ lặp lại và độ phân giải là chìa khóa.
How it works
Shadow measurement with telecentric optics
A thru-beam measurement sensor sends parallel (collimated) light from an emitter to a telecentric receiver. The part casts a sharp shadow; the receiver finds the shadow edges with sub-pixel precision and reports diameter, gap or position.
Because both sides are telecentric, the measured size does not change when the part moves slightly toward or away from the sensor — ideal for shafts, pins, wires and gaps on moving lines.
For any measurement system, three terms matter: resolution (smallest change it can display), repeatability (how consistent repeated readings are) and accuracy (how close readings are to the true value). A common rule is that measurement resolution should be about one tenth of the tolerance.
Types
Measurement principles in detail
Open each type for how it works, where it fits, its limitations and the specifications to check.
01 Thru-beam telecentric measurement sensorFast, precise dimension measurement from a shadow.
How it works. Collimated LED light and a telecentric receiver with a high-resolution sensor detect shadow edges; multiple edges allow diameter, gap, position and multi-feature measurements in one shot.
Best for
- Shafts, pins, wires and cables
- Gap and position checks
- In-process measurement during grinding or extrusion
Limitations
- Measures the outline only
- Requires a clear path through the part area
Key specs to check
- Measuring range
- Repeatability
- Sampling rate
- Minimum detectable object
02 Vision-based 2D gaugingMany dimensions at once from one image.
How it works. Camera with telecentric lens and collimated backlight; caliper, line, circle and distance tools measure many features simultaneously with sub-pixel precision.
Best for
- Stamped and machined parts
- Multiple dimensions per part
- Position of holes and features
Limitations
- Field of view limited by telecentric lens size
- Needs calibration and temperature control for tight tolerances
Key specs to check
- Pixel size on part
- Sub-pixel repeatability
- Lens distortion and telecentricity
03 Laser displacement sensorPoint measurement of distance and height.
How it works. Laser triangulation to a single spot. Two opposing sensors measure thickness; scanning gives profiles.
Best for
- Height, thickness, run-out
- Positioning and warpage checks
Limitations
- One point per sensor
- Sensitive to surface angle and gloss
Key specs to check
- Measuring range
- Linearity
- Repeatability
- Spot size
- Sampling rate
04 Confocal displacement sensorPrecise distance on glass, mirrors and multilayers.
How it works. Chromatic confocal optics measure distance by wavelength; can detect several transparent layers in one measurement.
Best for
- Glass and film thickness
- Mirror-like surfaces
- Very small features
Limitations
- Short measuring range
- Small spot — careful positioning
Key specs to check
- Range
- Resolution
- Angle tolerance
- Number of layers
05 3D profilers for dimensionsWidth, height, step and volume across a profile.
How it works. Laser line profilers measure full cross-sections; see the 3D Vision page for details.
Best for
- Bead, seam and step measurement
- Coplanarity and flatness
Limitations
- Occlusion and reflections
Key specs to check
- X/Z resolution
- Profile rate
Compare
Resolution, repeatability and accuracy
| Term | Meaning | How it is checked |
|---|---|---|
| Resolution | Smallest change the system can detect or display | Datasheet; pixel size and sub-pixel method |
| Repeatability | Spread of repeated measurements of the same part | Measure one part many times; report standard deviation |
| Accuracy | Closeness of the result to the true value | Compare with a certified reference (gauge block, pin) |
| Linearity | Accuracy across the whole measuring range | Reference parts at several sizes or positions |
| Gauge R&R | Share of tolerance used by measurement variation | Several parts, operators/repeats, statistical study |
Specifications
Key specifications explained
| Specification | What it means | How to choose |
|---|---|---|
| Measuring range | Span of dimensions or distances the sensor covers. | Must include part variation and position tolerance. |
| Repeatability | Consistency of repeated results (often ±σ or 3σ). | Target roughly 1/10 of the tolerance. |
| Accuracy / linearity | Deviation from the true value across the range. | Verify with certified references. |
| Sampling rate | Measurements per second. | Enough samples per part at line speed. |
| Temperature drift | Change of results with temperature. | Important for tight tolerances; control or compensate. |
| Minimum object / edge | Smallest object or gap detectable. | Check against your smallest feature. |
| Outputs | Analog, digital, Ethernet, judgement outputs. | Match PLC and SPC integration. |
Engineering tools
Calculate before you choose
Quick estimates based on standard formulas. Always confirm with a test on real parts.
Tolerance → Resolution Calculator
Enter your tolerance and field of view. The calculator estimates the measurement resolution and pixel size needed for a vision-based measurement.
Selection guide
Which principle for which task?
| Requirement | Recommended | Why / note |
|---|---|---|
| Shaft or wire diameter on a moving line | Thru-beam telecentric sensor | Fast, insensitive to position. |
| Many dimensions on flat parts | Camera + telecentric lens + backlight | All features in one image. |
| Thickness of sheets or plates | Two opposing displacement sensors | Difference gives thickness. |
| Glass or film thickness | Confocal sensor | Multi-layer capable. |
| Bead, step or seam profile | 3D laser profiler | Full cross-section. |
| Height of a single point | Laser displacement sensor | Simple and fast. |
Selection steps
- List the dimensions and their tolerances.
- Choose the principle: shadow, imaging, triangulation or confocal.
- Check required resolution and repeatability against tolerance (1/10 rule).
- Plan fixturing and part presentation — position variation adds error.
- Control the environment: temperature, vibration, contamination.
- Calibrate with certified references and verify with a gauge study.
- Integrate results with PLC and statistical process control.
Examples
Typical application examples
Illustrative setups showing how the components are combined in practice.
Application example
Stamped bracket dimensions
- Challenge
- Twelve dimensions measured manually with calipers.
- Setup
- Telecentric lens, collimated backlight, vision gauging tools.
- Result
- All dimensions measured in one image within the cycle time.
Application example
Cable diameter during extrusion
- Challenge
- Diameter variations detected too late.
- Setup
- Thru-beam sensor with continuous output to the line controller.
- Result
- Real-time feedback for extrusion speed control.
Common mistakes to avoid
- Confusing resolution with accuracy — a display with many digits is not proof of accuracy.
- Ignoring temperature: parts and fixtures expand.
- Poor part presentation: tilt and position errors add directly to the result.
- Dirty windows and lenses in coolant or dust environments.
- No calibration routine with traceable references.
FAQ
Frequently asked questions
What is sub-pixel measurement?
Edge positions are calculated from the gray-level transition across several pixels, giving a position finer than one pixel. Achievable sub-pixel precision depends on optics, lighting and noise.
What is the 10:1 rule?
A common guideline that the measurement resolution should be about one tenth of the tolerance range, so measurement variation uses only a small part of the tolerance.
Can vision replace a CMM?
For in-line 2D and some 3D dimensions, yes. For complex 3D geometry with very tight tolerances, a coordinate measuring machine remains the reference.
How often should I calibrate?
Define a routine based on risk: at startup, after changes, and at regular intervals using certified references.
Continue exploring
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