Choosing a lens for a line scan camera is not only about focal length.
We see this quite often: the customer already has a 4K, 8K or higher-resolution line scan camera, but the final image is still not good enough.
Sometimes the sensor is too long for the lens. Sometimes the pixel size is very small, but the lens resolution cannot really support it. In other projects, the problem comes from field of view, working distance, distortion, lighting or mechanical installation.
So before choosing the lens, we normally check the camera and the real inspection condition together.
Line scan cameras are widely used for wafer inspection, PCB inspection, display panels, metal surfaces, films, printed materials, batteries, textiles and other continuous production lines.
Below are the main points we usually check.
1. Start with Sensor Length and Pixel Size
For a line scan camera, sensor length and pixel size are two basic parameters.
Sensor length tells us how much image width the lens must cover.
For example, a 4K line scan camera with 5 μm pixels has an active sensor length of about:
4096 × 5 μm = 20.48 mm
This means the lens should provide useful image quality across at least this image width.
If the camera uses a longer sensor, the optical design becomes more difficult. It is not enough for the center to look sharp. The image quality near both ends of the line also matters.
Pixel size is another important point.
Smaller pixels can provide more sampling resolution, but only when the lens can support it.
This is why we normally ask for the camera model or sensor specification first.
2. Field of View and Working Distance Should Be Confirmed Together
The next question is simple:
How wide is the area you need to inspect?
It may be 50 mm, 100 mm, 300 mm or even wider.
The required field of view determines the optical magnification together with the sensor length.
A simple first estimate is:
Magnification ≈ Sensor Length ÷ Object Field of View
For example, if the sensor length is 20 mm and the object field is 100 mm, the magnification is around 0.2×.
But this number alone is not enough.
The same field of view may need a different optical design when the working distance changes.
For a real machine, we also need to know how much space is available between the lens and the object.
So when sending an RFQ, it is better to provide both the field of view and working distance.
3. Do Not Judge the Lens Only by “Megapixels”
This is another issue we see quite often.
A camera has high resolution, so the customer looks for a lens marked “high resolution” or “10 MP”.
For line scan systems, this is usually not enough information.
Pixel size gives us a better idea of what the optical system needs to support.
As a simple reference:
| Pixel Size | Sensor Nyquist Frequency |
|---|---|
| 5 μm | about 100 lp/mm |
| 3.45 μm | about 145 lp/mm |
| 2 μm | about 250 lp/mm |
This does not mean the lens must have perfect MTF at these frequencies.
However, it clearly shows why a 2 μm pixel camera puts much more pressure on the lens than a 5 μm pixel camera.
A lens that works well with 5 μm pixels may become the limitation when used with 2 μm pixels.
For line scan inspection, we also care about MTF across the full sensor length.
If the center is sharp but the two ends become soft, defect detection or measurement can become unstable across the inspection width.
4. Distortion and Edge Quality Matter More Than Many People Expect
For long sensors, flat-field performance becomes more important because the lens needs to keep useful image quality from the center to both ends of the line.For simple defect detection, a small amount of distortion may not be a serious problem.
For dimensional measurement, wafer inspection, PCB inspection or other precision applications, the situation is different.
Distortion can directly influence measurement results.
Field curvature can also cause trouble.
If the best focus is curved, the center may look good while the edge becomes blurred on a flat sensor.
This becomes more obvious when the sensor is long or the pixel size is small.
So for a line scan lens, we normally look at more than center resolution.
Depending on the application, we may also check:
- edge MTF
- distortion
- field curvature
- lateral color
- illumination uniformity
These parameters often tell us more than a simple megapixel rating.
5. High Line Rate Also Means More Pressure on Lighting
A line scan camera normally works while the object or camera is moving.
When the line rate is high, exposure time becomes shorter.
That means less light reaches the sensor during each exposure.
In practice, this can become a bigger problem than expected.
A good lens cannot compensate for poor lighting.
For many line scan systems, a bright line light is used because it concentrates light on the narrow inspection area.
But the lighting method still depends on the surface.
Reflective metal, PCB solder, glass, film and matte surfaces behave very differently.
For this reason, we prefer to look at the lens and lighting together, especially for high-speed inspection.
6. Do Not Forget the Mechanical Space
A lens may work optically but still be difficult to install.
For this reason, mechanical space should be confirmed early.
Useful information includes:
- camera mount
- available lens diameter
- maximum lens length
- distance to the object
- installation direction
- nearby mechanical parts
Line scan cameras may use C-mount, F-mount, M42, M58 or other interfaces.
Larger sensors often need larger mounts. Otherwise, vignetting may appear.
Mechanical stability is also important.
With long sensors and small pixels, camera tilt or lens decenter can quickly reduce edge image quality.
In some systems, the lens itself is not the problem. The problem appears only after the complete system is assembled.
7. When Do You Need a Custom Line Scan Lens?
A standard lens is enough for many normal inspection systems.
Custom design becomes more useful when several difficult requirements appear together.
For example:
- long sensor
- small pixel size
- wide field of view
- low distortion
- high edge resolution
- limited working distance
- special wavelength
- restricted mechanical space
In this situation, using a catalogue lens may require too many compromises.
A custom line scan lens can be designed around the actual camera, field of view, working distance and resolution target.
At VY Optics, we can support optical design, optical component manufacturing, lens assembly, alignment and opto-mechanical integration for industrial inspection systems.
What Information Should You Send Us?
For a new line scan lens project, the following information is usually enough for the first evaluation:
- camera model
- sensor length
- pixel size
- required field of view
- working distance
- wavelength
- minimum defect size
- distortion requirement
- camera mount
- available mechanical space
If you already have a camera datasheet, drawing or sample image, please send it together.
The specification does not need to be complete at the beginning.
We can first check whether the optical direction is practical, then confirm the missing parameters step by step.
Final Thoughts
For line scan inspection, the camera and lens should be considered as one system.
A 4K, 8K or 12K camera does not automatically mean the system can use all of that resolution.
The lens still needs to support the sensor length, pixel size, field of view and edge image quality.
Before choosing a lens, we suggest starting with five questions:
What camera are you using?
What is the pixel size?
What field of view do you need?
What is the working distance?
What is the smallest defect you need to see?
Once these points are clear, it becomes much easier to decide whether a standard line scan lens is enough or whether a custom design is worth considering.
Need a Line Scan Lens for Your Inspection System?
If you are working on wafer, PCB, display, battery, metal surface, film or other line scan inspection equipment, you can send us your camera information and application requirements.
VY Optics can help review the optical setup and discuss a suitable line scan lens or customized optical solution.
