Line scan cameras

How to choose a line scan camera

A line scan camera has one, or a few, rows of pixels. It builds the image line by line while the part, web or drum moves past it, so the image can be as long as the product itself. Use it for continuous material (film, foil, paper, steel, glass, textiles, battery electrodes), for very wide or very long parts, and for cylinders rotating in front of the camera. The design rests on four numbers: pixels across, pixel size on the object, line rate and light.

1. Pixels across the width

Divide the width to inspect by the resolution you need on the object. A 600 mm web at 0.1 mm per pixel needs 6,000 pixels, so an 8K camera (8,192 pixels) with some margin. If one camera cannot cover the width at the needed resolution, use two or more with an overlap and stitch the images.

2. Line rate from the speed

Each line must cover one object pixel of travel: line rate = speed / object pixel size. At 1 m/s and 0.1 mm per pixel that is 10 kHz; at 3 m/s and 0.05 mm it is 60 kHz. Pick a camera whose maximum line rate is above that at the bit depth you will use - many cameras reach their top rate only at 8 bit. The line rate calculator works it out, including the exposure that is left per line.

3. Trigger from an encoder

Speed changes stretch or squash the image if the camera free-runs. Feed the camera, or the frame grabber, with a rotary encoder on the conveyor or roller so each line is taken at a fixed travel distance. Use a frame (start) trigger from a photoelectric sensor to cut the stream into one image per part, and allow overlap between frames when defects can sit on a frame boundary - see line scan image overlap across frame boundaries.

4. Light: enough of it, and TDI when there is not

At high line rates the exposure per line is a few microseconds, so line scan systems need intense, narrow line scan lights focused on the scan line. When the light is still short, TDI (time delay integration) cameras sum the same object line over 2, 3, 4 or more sensor rows as it moves, which multiplies the signal without a longer exposure. TDI needs the encoder trigger to match the motion exactly.

5. Pixel size, sensor length and lens

Sensor length = pixels x pixel size: 4K x 7 µm is about 29 mm, 8K x 7 µm or 16K x 3.5 µm about 57 mm. Long sensors need lenses with a large image circle, usually M42, M72 or larger mounts, or F-mount through an adapter. Choose a line scan lens whose image circle covers the sensor length and whose resolution suits the pixel size; small pixels (3.5-5 µm) are less forgiving of the lens than 7 µm pixels.

6. Interface

Data rate = pixels x line rate x bytes per pixel: an 8K camera at 50 kHz and 8 bit sends about 410 MB/s, more than GigE (about 115 MB/s) can carry. GigE and 2.5GigE suit 2K-4K cameras at moderate rates; 10GigE, CoaXPress, Camera Link and fibre-optic links carry the fast 8K and 16K models. CoaXPress, Camera Link and fibre need a frame grabber; check the vision PC has the PCIe slot and the memory bandwidth.

What this range covers

• Line length: 512 pixels (SWIR) through 1K, 2K, 4K, 8K and 12K to 16K, plus contact image sensor (CIS) modules with 330 mm and 660 mm scan width.
• Pixel size: mostly 3.5, 5 and 7 µm; 12.5 and 25 µm on SWIR models; 14 µm on the CIS modules.
• Line rate: from about 2 kHz on small cameras to 200 kHz and more on fast models; UV TDI models reach up to 1,000 kHz where stated.
• Imaging modes: single line, dual-line and multi-row TDI; mono, colour and NIR.
• Spectrum: visible, NIR, UV and SWIR (InGaAs).
• Interfaces: GigE, 2.5GigE, 10GigE, USB3, CoaXPress, Camera Link and fibre (LC ports).
• Mounts: C-mount, M42, M58, M72, M90, M95 and F-mount; the CIS modules have integrated optics.

Typical applications

• Web inspection of film, foil, nonwovens, paper and battery electrode coating.
• Glass and display panel inspection with TDI for low-light, high-speed scanning.
• Surface inspection of steel strip, wood and printed sheets.
• Unrolled images of bottles, cans, bearings and cable by rotating the part in front of the camera.
• Sorting food, recycled material and seeds on fast belts, in colour or SWIR.
• Flat document and PCB scanning with a CIS module at a 15 mm working distance.

Common mistakes

• Free-running the camera on a conveyor whose speed varies: the image is stretched and squashed.
• Computing the line rate at 8 bit and then running 10 or 12 bit, which halves the rate on many cameras.
• A lens whose image circle is shorter than the sensor: the ends of the line go dark and soft.
• Using an area light: line scan needs a bright, narrow line of light exactly on the scan line.
• Choosing GigE for an 8K camera at high line rate without checking the bandwidth.

Questions

Line scan or area scan?
Line scan for continuous or very long material and rotating parts; it gives very high resolution across the width and no stitching along the length. Area scan cameras are simpler for discrete parts that fit in one image.

What is TDI and do I need it?
TDI adds the signal from several rows as the object passes, for the same effect as a longer exposure without blur. You need it when a single-line camera would be too dark at your line rate, for example on fast glass or wafer lines.

Mono or colour?
Mono for defects that show as brightness changes: scratches, holes, coating gaps, contamination on most webs. Colour only when the defect is a hue change, such as print register or colour sorting. Colour line scan cameras use two or more sensor lines, so they also depend on a steady encoder signal.

Where do I start a complete system?
Size the camera with the steps above, then pick lens and light to match. The system configurator checks that the parts fit together.