Light Source and Illumination Selection: Where 80% of Defect Detectability Is Decided
Machine vision light source selection guide: it explains which defects each common illumination method suits: diffuse light, low-angle light, coaxial light, backlight and stripe light; and how to combine illumination schemes according to material and defect characteristics.
Clarify One Thing First
The Light Source Is Not an Accessory but Part of the Imaging Solution
The choice of machine vision light source depends on the defect type and material characteristics: diffuse light suits color differences and stains, low-angle light suits scratches and indentations, backlight (transmitted) suits holes and damage, coaxial light suits highly reflective and mirror-like surfaces, and striped light combined with line scanning can enhance bump and dent features. Real solutions are usually a combination of several illumination methods.
In visual inspection projects, the light source is often treated as an "accessory" and specified last. But in many failure cases the root cause is exactly the light source: the defect has no contrast at all in the image, and no amount of algorithm tuning can detect it.
Another way to look at it: the camera determines "how fine a detail can be resolved", and the light source determines "whether it can be seen". If something cannot be seen, no amount of resolution helps.
So the sensible order is: first define which defects must be inspected, then find for each defect type the illumination that makes it form a clear image, and only then decide which camera and algorithm to use.
selection Factors
Front Diffuse Light
Uniform, non-directional illumination. Suitable for defects that rely on overall grayscale and color differences, such as color differences, stains and color classification.
Low-Angle Ring Light
Light grazes the surface at a large angle. This highlights surface relief such as scratches, indentations and wrinkles, because the sloped faces of these features reflect light strongly.
Coaxial Light
The optical axis is coaxial with the camera and reflected light returns perpendicularly. Suitable for highly reflective, mirror-like surfaces; it suppresses stray reflections and makes surface defects form a clear image.
Backlight / Transmitted Light
The light source is behind the material. Suitable for holes, damage, short shot and edge contours——the light-transmitting parts form high-contrast bright spots.
Fringe / Structured Light
Projects a fringe pattern and reflects surface form through fringe distortion. Combined with line scan it can detect fine relief and is also commonly used to suppress reflections.
Multispectral / Specific Wavelength Bands
The wavelength band is selected according to the reflection characteristics of a specific material. This is commonly used for defects on coatings, adhesive layers and transparent films that conventional illumination struggles to form a clear image of.
Field of View and Pixel Sampling
Do Not Mistake Pixel Scale for Inspection Accuracy
The illumination result cannot be extrapolated from a specification sheet, only measured. The same type of defect may need completely different illumination on different materials: the best incident angle for stains on dark fabric and for stains on light fabric is often opposite.
Therefore "sample trial before finalizing the solution" is not a cumbersome process but a necessary step.
| Defect Types | Recommended Illumination | Causes |
|---|---|---|
| Hole / Damage | Backlight / Transmitted Light | Transmitted-light areas form high-contrast bright spots |
| Scratch / Indentation | Low-Angle Light | Strong reflection on the bevel forms a clear image of scratches |
| Color Difference / Stain | Front Diffuse Light | Relies on overall grayscale and color differences |
| Defects on highly reflective surfaces | Coaxial Light / Polarization | Suppress stray reflections |
| Slight Unevenness | Fringe Light + Line Scan | Fringe distortion reflects shape change |
| Coating / Adhesive Layer | Specific wavelength bands / multispectral | Differences in material reflection characteristics |
Light Source and illumination
Light Source Stability
Brightness drift makes judgement thresholds ineffective. The light source's thermal characteristics and aging must be taken into account, and where necessary a controller should provide constant-current drive and brightness feedback.
Ambient Light Interference
Workshop lighting and natural light cause interference. This is usually suppressed with light shields, strobe synchronization or differential imaging.
Strobe Synchronization
High-speed applications commonly use strobing: the light source is lit only during the exposure instant, which raises instantaneous brightness and reduces heat. It must be strictly synchronized with the camera trigger.
Combined Illumination
When one defect type needs one kind of illumination and another needs a different one, multiple imaging passes (changing the illumination each time) followed by fused judgement are usually used.
algorithm Selection
Only after the illumination solution is fixed can the algorithm be targeted. If two defect types in the same image require opposite processing, that means two separate images should be taken rather than forcing one image to do the job.
Multiple Regions per Image
Under the same illumination, different regions use different thresholds.
- Suited to scenarios with similar defect types
- Simple to Implement
Multi-Image Fusion
Capture one image per illumination type, judge each separately, then merge.
- Covers more defect types
- The cost is longer imaging time
Image Differencing
Subtract the inspection image from the qualified reference image to highlight changed regions.
- Suited to scenes with a stable background
- Requires high positioning accuracy
Channel Operations
Use differences between wavebands or channels to construct a feature map.
- Commonly used for coating and adhesive layer inspection
- Requires capturing multiple channels at the imaging stage
Communication and interlocking
The light source must be driven by a controller, which handles brightness setting, strobe triggering, and channel switching. If a solution involves switching between multiple light channels, the timing must be scheduled centrally in the control system to avoid mismatches such as "the camera has already exposed while the light has not fired".
- 01 List the defect types to be inspected
- 02 Test illumination for each defect type
- 03 Determine the illumination combinations that can be covered
- 04 Design the number of light sources and their mounting positions
- 05 Determine the controller and strobe timing
- 06 Synchronized tuning with the camera trigger
- 07 Compare by measurement with samples
- 08 Freeze recipe parameters
Common Question
Why are defects sometimes detected after changing the light source?
Can Dark and Light Materials Use the Same Light Source?
What Are the Limits of Backlight Inspection?
How Do You Light Scratches on Transparent Materials?
Does Workshop Lighting Interfere with Inspection?
What Information Do You Need to Provide?
Submit sample testing
The illumination solution must be validated on physical samples. Please provide samples containing typical defects and we will run comparative tests of multiple illumination methods.
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Need to assess imaging conditions, defect criteria and cycle time item by item? Go to the Full Requirement Assessment →
The Most Effective Step in Selection: Test Your Own Sample
For the same workpiece, the inspection result differs greatly with different lenses, light sources, mounting distances and algorithm combinations. Sending us samples for measurement is more reliable than extrapolating from a specification table.