Sometimes a line scan imaging system produces a very clear image in the center, but the image becomes blurry toward one or both ends of the sensor.
The first reaction may be that the lens resolution is not good enough.
But this is not always the reason.
The problem may come from the optical design, sensor size, working distance, or mechanical alignment. Sometimes several factors are involved.
Before changing the lens, we normally suggest checking where the blur appears and whether it is the same on both sides.
These details can help us understand the problem.
1. First, Check the Sensor Length
A line scan camera may use a 4K, 8K or even longer sensor.
For example, a 4K camera with 5 μm pixels has a sensor length of 20.48 mm.
An 8K camera with the same pixel size has a sensor length of 40.96 mm.
If the lens was originally selected for a smaller sensor, it may not provide enough image quality across the longer sensor.
Sometimes the image is still visible at both ends, but the sharpness is much lower than in the center.
This is why checking only the image circle is not enough.
We also need to know how the lens performs at the actual image height used by the camera.
2. Field Curvature May Be One Reason
A lens does not always form its sharpest image on a perfectly flat surface.
In some optical systems, the best-focus surface is slightly curved. This is called field curvature.
The camera sensor, however, is flat.
If we focus the lens at the center, the ends of the sensor may not be at their best-focus positions.
The result is a sharp center and softer edges.
Sometimes changing the focus can improve the edges, but the center becomes less sharp.
This is a useful clue.
For applications that need consistent image quality across a long sensor, field curvature should be considered during optical design.
3. Astigmatism Can Also Affect Edge Sharpness
Field curvature and astigmatism are different optical problems, although they may appear together.
Astigmatism means that details in different directions do not come into focus at exactly the same position.
For example, lines in one direction may look relatively sharp, while lines in another direction remain blurred.
This can be more noticeable toward the edge of the image.
In optical design software, we can evaluate these aberrations and check how they affect image quality at different field positions.
MTF analysis is also useful here.
Instead of looking only at the center MTF, we can compare the calculated performance across the active sensor length.
However, good simulated MTF does not automatically guarantee the same performance after manufacturing and assembly.
The mechanical structure and assembly accuracy also matter.
4. What If Only One Side Is Blurry?
This is another situation worth checking.
If both ends show similar blur, field-dependent optical performance or focus position may be involved.
But if the left side is sharp and the right side is blurry, we would also check the alignment.
For example:
- Is the camera sensor tilted relative to the lens image plane?
- Is the lens properly mounted?
- Is there any mechanical tilt or decenter?
- Has the working distance or camera position changed?
A small alignment error can cause a noticeable focus difference across a long sensor.
This does not always mean the lens itself is defective.
Sometimes the lens performs well on its own, but the complete imaging system needs adjustment.
5. Before Replacing the Lens, Try These Checks
If your line scan image is sharp in the center but blurry at the edges, there are a few things worth checking first.
Check the focus.
Adjust the focus slightly and observe whether the sharpest area moves from the center toward the edges.
If the best focus is different across the sensor, this gives us useful information about the optical system.
Compare the left and right sides.
Are both ends equally blurry, or is one side noticeably worse?
A difference between the two sides may suggest an alignment problem, although other causes are possible.
Check the camera and lens specifications.
Confirm the sensor length, pixel size, lens image field and working distance.
The lens may be suitable for a smaller camera but not for the current sensor.
Check the inspection target and lighting.
Low contrast, uneven illumination, motion blur and incorrect synchronization can also make fine details difficult to see.
These problems should not be confused with optical defocus.
It is better to identify the cause before deciding to replace the lens.
6. How Can Optical Design Help?
When developing a lens for a line scan camera, we can evaluate the image quality at different field positions during the design stage.
For example, we may analyze:
- MTF at the center and different image heights
- Field curvature and astigmatism
- Distortion
- Working distance and magnification
- Sensitivity to lens decenter, tilt and spacing errors
These calculations help us understand whether the optical design can meet the imaging requirements.
They also help us determine which manufacturing and assembly tolerances need more attention.
For a long sensor or high-resolution inspection system, this becomes especially important.
At VY Optics, optical design, precision component manufacturing, lens assembly and alignment are considered together.
Our goal is not only to achieve good performance in the optical design model, but also to make the design practical for manufacturing and assembly.
7. When Is a Custom Lens Necessary?
Not every edge-blur problem requires a new lens.
Sometimes adjusting the focus, camera position or mechanical alignment is enough.
In other cases, the existing lens may not be suitable for the sensor length or inspection requirements.
For example, the customer may need a long line scan sensor, small pixels, high edge resolution and a limited working distance at the same time.
A standard lens may not provide the required performance under these conditions.
A custom optical design allows us to consider the sensor, field of view, working distance, image quality and mechanical limitations together.
However, before starting a new design, we prefer to understand what is actually causing the problem.
What Information Should You Send Us?
If you are having image quality problems with a line scan camera, please send us the camera model and lens specifications.
It is also helpful to provide:
- Sensor length and pixel size
- Field of view and working distance
- Images showing the center and edge performance
- Operating wavelength and F-number
- Mechanical installation drawing, if available
- Minimum defect size or resolution requirement
With this information, we can review the optical requirements and discuss possible causes.
If a new lens is needed, we can also evaluate a customized optical design.
Final Thoughts
A sharp center and blurry edges do not always mean the lens has poor resolution.
Field curvature, astigmatism, sensor coverage and mechanical alignment can all affect the result.
The important thing is to identify the cause before making changes.
For line scan inspection, we suggest paying attention to image quality across the full sensor length, not only at the center.
Need help with your line scan imaging system?
VY Optics provides custom optical design, precision optical component manufacturing, lens assembly and alignment for industrial imaging applications.
If you are developing a line scan inspection system or having difficulty with image quality, feel free to share your camera specifications and application requirements with us.
We would be glad to review the project and discuss a suitable optical solution.
