Line-Scan Imaging: Two Faces, One Continuous Scan

Overview

A line-scan camera has a sensor that is one row of pixels wide. It builds an image by acquiring one line at a time while the part moves past it, so the second dimension of the image comes from motion rather than from the sensor.

That arrangement sounds like a complication, and on many parts it is. On a long, flat, machined surface it is the simplest way to get an image with even illumination and constant scale across the whole area, because the part passes under a fixed light and a fixed sensor at a constant speed.

This article explains how a line-scan station is built around a machined face, why the synchronisation between motion and exposure is the part that decides image quality, and where an area camera remains the better choice. Big Bird Industrial (formerly Shimada Big Bird Industrial) has been building cleaning and deburring systems for automotive powertrain components since 2001, and the arrangement described here follows inspection stations for cylinder head sealing faces and base planes.

1. How The Image Is Built

The mechanics are simple and the timing is not.

One Line At A Time

Each exposure captures a narrow strip of the surface, and consecutive strips are joined into a two-dimensional image. The strip width is one pixel, so the resolution along the direction of travel is set by how far the part moves between exposures.

The Synchronisation Requirement

If the part moves faster than the exposure rate assumes, the image is stretched and every feature is elongated along the direction of travel. If it moves slower, the image is compressed. Both change the measured size of a defect without changing its appearance, which makes a synchronisation fault look like a process change.

2. When It Beats An Area Camera

The advantage is uniformity, and it appears on surfaces that are long relative to the field of view.

Even Illumination Across The Length

A line light illuminates one narrow strip at a time, and every strip receives the same illumination because the geometry never changes. An area camera covering the same length has to illuminate a wide field, and the illumination falls off towards the edges of that field. On a sealing face where a defect at one end must be judged the same as a defect at the other, that uniformity matters.

Constant Scale

A line-scan image has the same scale everywhere, because every part of it was acquired at the same working distance. An area camera covering a long surface has a perspective, with the scale changing across the field, which complicates any size-based defect criterion.

3. Two Faces On One Part

A cylinder head presents a sealing face on the top and a base plane on the bottom, and both carry defects that matter for different reasons.

The Sealing Face

The face that seals against the gasket carries scratches, dents, porosity and machining marks, and any of them can create a leak path. The criterion is usually a maximum defect size in a defined region, which requires a uniform scale across the full face.

The Base Plane

The base plane carries similar defect classes but a different acceptance, because it locates against a mating surface rather than sealing. It also tends to collect machining debris at its edges, which is why the inspection often runs after cleaning rather than before.

4. Lighting A Line

The light has to be matched to the line, and several geometries are used depending on the defect class.

Bright Field And Dark Field In Line Form

A line light directly above the sensor produces a bright field, which shows colour and coverage. A line light at a low angle produces a dark field, which shows scratches and machining marks against a dark background. Both are available as line sources, and a station often carries two and switches between them.

Why Coaxial Is Difficult

Coaxial illumination through a beam splitter is straightforward for an area camera and awkward for a line camera, because the splitter has to be aligned along a one-pixel-wide axis. Where pits and pores are the target, this is a real limitation of the line-scan approach and a reason to use an area camera instead.

5. Motion, Encoder And Repeatability

The motion system is part of the measurement, which is what makes a line-scan station different from a static one.

Encoder Versus Timed Exposure

Driving the exposure from an encoder on the motion removes the dependence on speed stability, because the exposure is triggered by distance rather than by time. That is the arrangement that makes a line-scan image dimensionally meaningful: a feature measured on the image corresponds to a distance on the part.

Straightness And Vibration

The part has to travel in a straight line under the sensor within a small tolerance, because any deviation changes the working distance and therefore the focus. Vibration along the direction of travel is less serious than vibration across it, which is one reason the handling is designed around a rigid rail rather than a belt.

6. What Limits A Line-Scan Station

The limits are mechanical rather than optical. A part that cannot be moved in a straight line at a stable distance cannot be inspected this way, and a part with features that face away from the sensor needs a second pass or a different method.

The other limit is time. A line-scan image is built while the part moves, so the cycle includes the travel. On a head that is short compared with the scan length, the travel is negligible; on a long casting it may not be.

7. Verifying The Setup

Verification uses a target with features of known size in both directions, scanned at the production speed and compared against the nominal dimensions.

If the feature is correct in one direction and wrong in the other, the synchronisation is wrong. That single test distinguishes a scaling problem from an optical one, and it is the first check to run when a defect starts appearing larger or smaller than it used to.

8. Colour And Coating Variation

A machined face is not optically uniform. Casting skin at the edges, coolant staining in the middle and a different surface at the bolt pads all change how the light returns, and a line-scan image records those differences faithfully.

Why That Is Both Useful And Difficult

The variation is useful because a coolant stain is a real finding. It is difficult because a threshold set on one region of the face will flag the normal variation in another. The practical answer is to normalise per region rather than per image, so that each area of the face is judged against the appearance that area normally has.

That normalisation needs a reference set from production rather than from a single good part, because the range of normal appearance is what the threshold has to sit outside.

9. Maintenance And Drift

A line-scan station has more places to drift than a static one, because the motion is part of the measurement.

The Three Items To Watch

Encoder alignment and condition, because a slipping encoder changes the scale silently. Rail straightness, because a rail that has settled changes the working distance and therefore the focus. Line light output, because a lamp that has dimmed reduces contrast uniformly, which shows as a slow rise in missed defects rather than as an obvious fault.

All three are caught by scanning the two-direction target at the production speed and comparing the measured dimensions against nominal. A single number that has grown in one direction and not the other points at the motion; a number that has fallen in contrast points at the light.

Frequently Asked Questions

Can A Line-Scan Camera Inspect A Stationary Part?

Only by moving either the camera or the part. A line camera with a static part produces a single row of pixels, which is not an image.

Does It Need Special Lighting?

It needs a line light matched to the sensor axis. An area light used with a line camera produces uneven illumination and wasted energy.

What Happens If The Speed Varies?

With an encoder, nothing, because the exposure follows distance. Without one, the image is stretched or compressed along the direction of travel and every measured size changes.

Is Line Scan Better Than Area For All Flat Surfaces?

No. A short face that fits in one area view is simpler and faster to inspect with an area camera, and coaxial illumination for pits is easier with it.

Conclusion

A line-scan camera builds an image from motion, which gives constant scale and even illumination across a long face. That is exactly what a sealing surface needs, and it is why this arrangement is common on machined planes.

The measurement depends on the motion being synchronised with the exposure, so the motion system is part of the instrument. A station verified with a two-direction target separates a scaling fault from an optical one in a single test.