Line scan system design: balancing camera, lens, light and motion

A line scan station fails at its weakest component, and the weak one is usually whatever was sized last. Cameras get picked by resolution, lenses by whatever mounts, lights by length, and the encoder is whatever the drive already had spare. The cure is to size all seven components — camera, lens, light, motion, encoder, structure, processing — against one shared number: the spatial sampling in millimetres per pixel. That value sets the line rate, the lens magnification, the light length, the encoder counts and the interface load at the same time. This guide walks the components in the order they constrain each other, with the arithmetic in the open, and closes with a worked 300 mm web you can copy.

The number that sizes the whole station

Sampling is millimetres per pixel across the web. You fix it once from the defect, and everything else inherits it:

Two pixels across a defect is the sampling floor; features smaller than that fold into neighbouring pixels. Three pixels is the working minimum for clear defects, four to five for low-contrast ones. Commit to 3 px on high-contrast flaws and 4-5 px where contrast is weak, then let that number pick the hardware.

On a 300 mm web moving at 0.3 m/s, an 8K line of 8192 pixels samples at 300 ÷ 8192 = 36.6 μm per pixel, and the line rate is 8192 × 300 ÷ 300 = 8.2 kHz. The ladder across sensor sizes:

Halving the sampling doubles the line rate, and doubling the pixel count on top of that quadruples the data. The compounding is why 16K over a wide web is an interface decision before it is a sensor decision.

Drawn as a station, it is one camera looking across a moving web, a line light throwing a bright stripe on the inspection line, and an encoder on the transport reporting position to the camera.

Once the rate is fixed, setting the line rate of a line scan camera covers the camera-side settings; this guide stays at design time.

Camera: buy pixels across the width

Line length follows the web plus its wander: give the field 5-10% more width than the nominal web so tracking error cannot walk an edge out of view. Then the interface, not the sensor, sets the ceiling, because data rate = line rate × pixels × bytes. A GigE link carries roughly 100 MB/s of image payload, which caps 8K at 8-bit near 12 kHz. 10 GigE moves about 1 GB/s. Camera Link reaches 255 MB/s on Base, 510 MB/s on Medium and 680 MB/s on Full.

The same ladder in three catalogue steps:

All 3 share: Sensor type CMOS; Reverse image Supports horizontal reverse image output; Trigger mode External trigger, internal trigger; Humidity 5% RH to 90% RH (no condensation); Certification CE, RoHS, KC. Values as printed in each product's data sheet; open a part for its drawing and full specifications.

The MV-CL042-91GC-V2 is the 4K step: a colour GigE camera with 7 μm pixels and a 29 mm image circle on an M42 mount. The MV-CL081-41CM is the 8K step, a monochrome Camera Link camera with 7 μm pixels, a 57 mm image circle and an M72 mount. The MV-CL166-91F2C takes 16K on XoF with 5 μm pixels, an 82 mm image circle and an F-mount.

Filter line scan cameras by line length and interface in the camera finder. If the motion side is still open, read when to use a line scan camera first — it decides whether the rest of this page applies.

Lens: image circle, then magnification, then f-number

The first gate is image circle. 8192 pixels at 7 μm make a 57.3 mm line, and the lens must cover it or both ends of the web go dark. That failure looks like a lighting fault but is not: lens vignetting causes and fixes separates image-circle falloff from natural roll-off, and line scan illumination uniformity covers the lighting twin of the same symptom.

The second gate is magnification. For the example web, m = 57.34 ÷ 300 = 0.19x. With a 40 mm lens the working distance follows: WD = f × (1 + m) ÷ m = 40 × 1.191 ÷ 0.191 = 250 mm. Run it the other way when mounting height is fixed: at a 400 mm ceiling, f = 400 × 0.191 ÷ 1.191 = 64 mm, so the next size up is a 65 mm lens. The line scan camera field of view calculation carries the same algebra with more cases.

The third gate is f-number. A 7 μm pixel stops resolving past 1 ÷ (2 × 0.007 mm) = 71 lp/mm, and the aperture decides whether the lens delivers that. The Airy disc is 2.44 × λ × N: at 550 nm and f/5.6 it is 7.5 μm, one pixel wide already; at f/8 it is 10.7 μm, 1.5 pixels of blur from diffraction alone. Work between f/4 and f/5.6 with 7 μm pixels, and open up only when the light budget forces it. Lens resolution vs pixel size and line scan lens selection for 4K, 8K and 16K cameras go deeper on MTF matching; the stock sits in line scan lenses and the lens finder.

Light: as long as the field, bright at the shortest exposure

Length comes first. The bar must span the full field, and 1.3-1.6 times the field when it sits close or fires at an angle, because the web ends see a finite bar at a shallower angle than its centre. A 300 mm field takes a 400-500 mm light. At the compact end, a bar like the LS-LSC-0527 is 300 × 17 mm, rated 87 W red / 108 W white. Under the same 1.3× rule that bar serves a field of about 230 mm.

Brightness is set by the exposure ceiling, not by taste. Motion blur equals speed × exposure; holding it to half a pixel means 18.3 μm ÷ 300,000 μm/s = 61 μs maximum. Run 50 μs: blur is 300,000 μm/s × 50 μs = 15 μm, or 0.41 px. The line period at 8.2 kHz is 122 μs, so the duty cycle is 41% — at these rates strobing buys almost nothing, and the light must be sized for continuous duty and its heat. Line scan light power sizing works the photon budget, lighting controllers for line scan cameras covers driving long bars, and edge-controlled external sync exposure shows the per-line gating.

Long bars at continuous duty run hot: fan-cooled housings are normal from about half a metre up, and LED machine vision light overheating covers the thermal design. If the light and optics share an enclosure, treat the fans as one airflow problem from the start; balancing a multi-fan enclosure ventilation system works through the plenum-versus-tapered-duct choice. Browse line scan lights by length and colour in the light finder.

Motion and encoder: one count per pixel

If the transport holds speed within ±1% and a 1% scale error is acceptable, free-running the camera at a fixed line rate is legitimate: every 1% of speed error stretches or compresses the image by 1%. The moment speed ramps, coasts or jitters, trigger from an encoder sized by the same sampling. The encoder needs at least one count per pixel: the example web samples at 8192 ÷ 300 = 27.3 px/mm, and a 100 mm drive wheel with a 2500 pulse-per-revolution encoder in ×4 quadrature delivers 10,000 counts per 314 mm of travel — 31.8 counts/mm, or 0.86 px per count.

Encoder ticks per pixel for line scan sets the scale register, wiring quadrature encoder inputs covers the signal side, and making the line rate follow machine speed at runtime handles drives whose setpoint changes mid-run.

Structure and data path

Two mechanical budgets fall straight out of the sampling. Cross-web wobble stays under about one pixel of object-side motion — 40 μm in the example. Lateral vibration scales by magnification: 0.1 mm of shake becomes 19 μm on a 7 μm sensor, 2.7 pixels of smear no software recovers. Bolt camera and light to one stiff frame, off the rollers, and look at a straight edge across the web at full speed before accepting the station.

Out-of-plane motion stays inside the depth of field: DOF = 2 × c × N × (1 + m) ÷ m², with c the pixel, gives 2 × 0.007 mm × 5.6 × 32.6 = 2.6 mm total, or ±1.3 mm of flutter at f/5.6. A web that flaps harder needs a flatter path, or a smaller aperture traded against the diffraction limit above.

The data path closes the loop: 8192 Hz × 8192 px × 1 byte = 67 MB/s, comfortable on GigE and inside Camera Link Base; at 10-bit it is 84 MB/s, still within both. The same web at 1 m/s pushes 224 MB/s and rules GigE out. On the Camera Link route, frame grabber compatibility for line scan lists what must match, and once the mechanics settle, calibration with a moving checkerboard turns pixels back into millimetres.

Worked example: 300 mm web at 0.3 m/s, 0.15 mm defects

The whole design on one page. The requirement: catch 0.15 mm defects with at least three pixels across each.

The camera step:

The lens step:

The light step:

The pairings line up: the lens carries M58/M72 threads and the camera takes M72, so it screws on directly. The light's 484 mm length spans the 300 mm field with the ends well inside its throw.

Checks before you order

• Field of view at least the web width plus 5-10% wander margin.
• Lens image circle at least the sensor line length; mount threads identical.
• Camera maximum line rate above speed ÷ sampling, with 20% headroom.
• Interface payload above line rate × pixels × bytes, counting bits beyond 8.
• Light at least as long as the field, 1.3-1.6× when close or angled, on a continuous-duty thermal plan.
• Encoder at one count per pixel or finer, ×4 quadrature counted.
• Exposure ceiling 0.5 px ÷ speed, inside the camera's shortest exposure.
• Cross-web wobble under 1 px object-side; web flutter inside the depth of field.
• Processing rated for the sustained data rate, not the peak.

When the usual answer is wrong

• When the required line rate falls to a few hundred hertz on a narrow web, an area sensor in line-scan mode costs less: area scan cameras with a line scan mode gives the crossover logic.
• Colour on moving parts: one monochrome camera with sequenced RGB light beats a tri-linear sensor when objects slew under the line — colour from a monochrome line scan camera shows the timing.
• Cylindrical parts break the flat-web rules at the edges: uneven back lighting on line scan cylinder edges covers the fixes.
• At 8K the ceiling is often processing, not optics: deep learning defect inspection on large 8K line scan images sizes the compute side. Webs wider than one sensor run two cameras with a few tens of millimetres of overlap and a stitch.

Start the chain in the system configurator, or send the three inputs — web width, line speed, defect size — through the contact page and we will send this table back filled in for your line. The systems topic collects the related sync-and-scale guides.