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Assessment
Application // 08

Web and line-scan inspection

Film, foil, paper and board, nonwovens, coil and sheet.

Technical review: Frank GuoReviewed

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// Short answer

Web inspection uses line-scan cameras to image continuous material, such as film, foil, paper, board, nonwovens, coil and sheet, across its width as it moves. An encoder on a roller triggers each image line, so the image keeps its scale when speed changes. Line lights make holes, spots, streaks and coating faults visible. There is no single part to reject, so defects go to a defect map and are marked, or the line is slowed or stopped. We confirm feasibility on your material first.

// What it checks

Which web defects a line-scan station can find

Whether a defect shows depends on the light and its size compared with the pixel.

01 // Holes

Holes and tears

Openings in the web, which let light through.

Usually needsTransmitted light (backlight)
02 // Spots

Spots, gels and inclusions

Dark or bright spots, gels in film, dirt and foreign particles.

Usually needsTransmitted or diffuse light
03 // Streaks

Streaks and coating faults

Lines along the web, scratches, coating skips and uneven coating.

Usually needsReflected light at a chosen angle
04 // Edges

Wrinkles and edge faults

Creases, folds, edge cracks and edge position.

Usually needsLow-angle light or an edge view

Lighting decides what is visible

Lighting geometries for web inspection
LightingHow it worksMakes visible
Transmitted (backlight)Light shines through the web.Holes, gels, thin spots, inclusions.
Reflected (bright-field)The surface mirrors light to the camera.Coating faults, streaks, stains on foil, coil and board.
Low-angle (dark-field)Light skims the surface.Scratches, creases, wrinkles.
// How it works

Line-scan camera, encoder and line light

Each part is chosen from your web width, speed and defects.

01

Line-scan camera

Images one thin line across the web at a time and stacks the lines into a continuous image. Wide webs use several cameras side by side.

02

Encoder triggering

An encoder on a roller sends a pulse per fixed length of travel; each pulse triggers one line, so scale holds when speed changes.

03

Line light

Exposure per line is very short, so light is concentrated into a bright, even line on the scan line.

How resolution and speed are set

How cross-web and along-web resolution and speed are set
DirectionWhat sets itWhat it means
Cross-webWeb width divided by the pixels across all cameras.Wider webs or smaller defects need more pixels or cameras.
Along-webWeb travel per encoder pulse (per image line).Usually matched to cross-web so defects are not stretched.
SpeedLines per second needed at top web speed.Faster webs need a higher line rate and brighter light.
Smallest defectMust cover several pixels in both directions.Decides both pixel sizes; confirmed on your material.
// Station layout

How a web inspection station is laid out

Side view of a typical station; the real layout follows your web path.

[Schematic]Side view · not to scale

Swipe sideways to see the whole drawing →

Side view of a line-scan web inspection station A continuous web runs from an unwind roll over rollers to a rewind roll. An encoder on the first roller sends pulses that trigger each image line of a line-scan camera above an imaging roller. An angled line light lights the scan line. Defects are logged on a defect map of the roll, and a marker downstream flags them at the web edge or signals the line to stop. PULSES → LINE TRIGGER UNWIND REWIND WEB DIRECTION → [3] ENCODER ON ROLLER [1] LINE-SCAN CAMERA [2] LINE LIGHT [5] MARKER / STOP SIGNAL [4] DEFECT MAP OF ROLL ALONG-WEB →
[1] Line-scan camera: images one line across the full web width at a time.
[2] Line light: a bright, even line of light on the scan line.
[3] Encoder: one pulse per length of travel triggers each line.
[4] Defect map: each defect logged by type and position on the roll.
[5] Marker / stop: flags the defect at the edge or signals the line.
// Pass and flag[Illustrative examples]

What a clean web and a flagged defect look like

Illustrations, not images from a real line. Which defects count, and how small, is agreed on your material during the sample test.

EVEN WEB
PASS
Example // Film, backlit

Web within limit

Even texture under transmitted light, matching the good material.

SURFACE: IN LIMITACTION: NONE
HOLE FOUND
HOLE
Example // Film, backlit

Hole in the web

Light through the hole shows bright; it is logged and marked.

DEFECT: ✗ HOLEACTION: MAP + MARK
SAME GAPREPEATING DEFECT
REPEATING
Example // Defect map

Repeating defect

An equal gap between defects usually points to a damaged roller.

PATTERN: ✗ REPEATSACTION: ALARM / STOP
// After detection

Defect maps, marking and stopping instead of rejects

A continuous web has no single product to push off the line.

How a web inspection station responds to defects
ResponseWhat happensNeeds
Defect mapEach defect logged with type, cross-web position and distance along the roll.Encoder position and a roll reference.
MarkingA label, flag or ink mark at the web edge, found again at rewinding or slitting.A marker downstream of the camera.
Alarm, slow or stopSerious or repeating defects alert operators or stop the line.Signals agreed with the PLC.
Roll reportLets the slitter or next process cut out defects or grade the roll.Agreed format.
// Limits

Where web inspection stops

We agree these boundaries before a project starts.

Not detectable with a camera

  • Thickness and coating weight. These need gauging sensors, not a camera.
  • Faults inside opaque layers. A fault buried in an opaque laminate, board or coil cannot be seen.
  • Strength and adhesion. Tensile strength, bond and seal strength are not visible.

Works reliably when

  • The web path is stable. Imaging over a roller keeps the web in focus without flutter.
  • Defects are defined. A defect catalogue with samples of each type that matters.
Line conditions for a web inspection station
ConditionWhat we need to knowWhy it mattersConfirmed during
Material and widthMaterial, finish and width of every product.Decides lighting and number of cameras.Sample test
Speed rangeLowest and highest web speed.Sets line rate and light power.Site survey
Web pathRollers, free spans and flutter.The web must stay in focus.Site survey
PLC and downstreamPLC, rewinder or slitter, existing marker.Decides how maps, marks and stops are used.Design review
// Acceptance

How the station is accepted

01

False alarms

Good material flagged, measured on agreed good lengths.

02

Missed defects

Known defects that pass unflagged, measured with marked samples.

03

Position check

Map positions and marks compared with known defect positions.

// Before we start

What to prepare for the sample test

We image your material under different lighting first.

Sample preparation guide

Good material

Lengths from different batches and finishes.

Defect samples

Lengths with each defect type marked.

Material details

Width, finish and your defect list.

Line information

Speed range, web path, PLC, rewinder or slitter.

// FAQ

Web and line-scan questions

What is the difference between a line-scan and an area-scan camera?
An area-scan camera takes a whole frame at once and suits separate products. A line-scan camera takes one line at a time and builds a continuous image as material moves, so it suits webs of any length.
Why does a line-scan camera need an encoder?
Each image line is triggered by an encoder pulse for a fixed length of travel, so lines cover the same length at any speed. Without it the image stretches or squashes as speed changes, and defect positions along the roll are lost.
How is the resolution of a web inspection system worked out?
From the smallest defect that matters, which must cover several pixels. Cross-web pixel size comes from web width and camera pixels; along-web from travel per encoder pulse. The camera must then reach the line rate needed at top speed.
What happens when a defect is found on a moving web?
It is logged on a defect map with its position, then marked at the web edge, passed to the slitter in a roll report, or used to stop the line.
Can a web inspection system measure thickness or coating weight?
No. Those need gauging sensors. A camera can show where coating is missing or visibly uneven.
Which cameras and software do you use?
We build stations on HIKROBOT VisionMaster. HIKROBOT GigE line-scan cameras (CL series) are stated compatible with GigE Vision and GenICam, with line, frame and combined line-and-frame trigger modes (vendor-stated). This website does not claim any official HIKROBOT partnership or distributor status.
Related:
Surface defect detection →Plastics & packaging →Building materials →Machine vision lighting →PLC & reject integration →Technology →How we deliver →

Sources

  1. HIKROBOT GigE line-scan camera datasheet (CL series): GigE Vision, GenICam, trigger modes (vendor datasheet, checked 2026-09-24)
// Web & line-scan

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Tell us your material, web width, speed range and the defects that matter. We will tell you what a line-scan station can find.

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