Line Scan Lens vs Area Scan Lens: What Is Different in Optical Design?

When customers ask us about a line scan lens, sometimes they already have an area scan lens and want to know whether it can be used with their new camera.

Actually, it may work.

We do not think every line scan camera needs a specially designed lens. The important thing is whether the existing lens can cover the sensor and provide enough image quality.

For example, a 4K line scan camera and an 8K line scan camera may have the same pixel size, but their sensor lengths are very different.

This can make a big difference when choosing the lens.

Can an Area Scan Lens Be Used with a Line Scan Camera?

Yes, but we need to check the actual parameters.

An area scan camera captures a complete 2D image at one time. A line scan camera captures one line, then builds the image as the object or camera moves.

For the lens, the first thing we check is the sensor size.

A standard area scan lens may work with a line scan camera if its usable image field is large enough.

But covering the sensor is only the first step.

We also need to check resolution, distortion, illumination and image quality at both ends of the sensor.

If these parameters meet the application requirements, there may be no need to change the lens.

Sensor Length Makes a Big Difference

Let’s take two cameras as examples.

Both cameras use 5 μm pixels.

The 8K sensor is twice as long as the 4K sensor.

A lens designed for a smaller image field may work well with the 4K camera but cannot necessarily support the 8K camera.

Sometimes the problem is obvious, such as dark corners or insufficient image coverage.

In other cases, the image is still visible across the sensor, but the resolution near the ends becomes poor.

For this reason, we normally ask customers to provide the camera model before recommending a lens.

Good Center MTF Does Not Mean Good Edge MTF

This is something we pay attention to during optical design.

A lens may have very good resolution in the center, but the MTF drops toward the edge.

For an area scan camera, we need to check image quality across the two-dimensional field.

For a line scan camera, we focus on the active sensor line, especially the performance at both ends.

Imagine a customer inspecting a wide metal strip.

The center looks clear, but the edges are slightly blurred.

If the customer only needs to detect large scratches, the result may still be acceptable.

But if the system needs to detect small defects across the full width, the lens may not meet the requirement.

So we cannot judge the lens only by center MTF.

We need to know the required MTF at the actual image height.

What About Distortion and Field Curvature?

These two parameters are also important, but they are not the same thing.

Distortion changes the position of image features. This can affect dimensional measurement.

Field curvature means the best-focus surface is curved instead of flat.

For example, the center may be in focus while the ends of the sensor become slightly blurred.

For simple defect inspection, a certain amount of distortion may be acceptable.

However, for PCB measurement, wafer inspection or other precision applications, the requirements may be much stricter.

We normally discuss the acceptable distortion and MTF with the customer before starting a custom optical design.

There is no need to request extremely low distortion if the application does not require it. That would only make the optical design more complicated and may increase the cost.

Does a Smaller Pixel Size Always Need a Better Lens?

Generally, smaller pixels place higher demands on lens resolution.

For reference:

These values are calculated from the camera pixel size.

They do not mean the lens must achieve perfect MTF at these frequencies.

For example, a lens working well with 5 μm pixels may not provide enough contrast when used with a 2 μm pixel camera.

But we still need to consider the minimum defect size, object-side resolution, magnification and required image contrast.

A smaller pixel does not automatically mean the inspection system can detect smaller defects.

The camera and lens must work together.

When Do We Suggest a Custom Line Scan Lens?

We normally check whether an existing lens can meet the requirements first.

If a standard lens works, there is no reason to spend extra money on custom design.

However, some projects have several difficult requirements at the same time.

For example, the customer may need an 8K or 12K line scan camera, small pixels, low distortion and good edge MTF.

At the same time, the working distance or mechanical space may be limited.

In this situation, a standard lens may not be suitable.

A custom design allows us to consider these requirements together from the beginning.

At VY Optics, our work covers optical design, precision optical component manufacturing, lens assembly and alignment.

We also pay attention to the mechanical structure because good optical design alone does not guarantee good performance after assembly.

What Information Do We Need?

If you already have a line scan camera, please send us the camera model or datasheet.

We also need to know the field of view, working distance, wavelength, minimum defect size and available mechanical space.

If you have an MTF or distortion requirement, please include it.

For a new inspection system, a drawing or photo of the inspection area is also helpful.

We can first evaluate whether an existing optical design is suitable. If not, we can discuss a customized solution.

Final Thoughts

An area scan lens is not automatically unsuitable for a line scan camera.

The important thing is whether it can support the sensor length, pixel size and required image quality.

For many normal inspection applications, a standard lens may be enough.

For long sensors, small pixels and high-precision measurement, the optical requirements become more difficult.

Before deciding on a lens, we suggest checking the camera and inspection conditions together.

Need Help with Your Line Scan Lens?

If you are developing a line scan inspection system for PCB, wafer, display, battery, metal surface or other industrial applications, please send us your camera information and optical requirements.

VY Optics can help evaluate the lens selection and discuss whether a standard or customized optical solution is more suitable.