Camera fps and shutter speed for moving web inspection

A wallpaper machine does not need a fast camera to be photographed once a minute. It needs a camera that can freeze 2.17 m/s of web, because shutter speed, not frames per second, decides whether the image is usable. This guide gives you both numbers: the fps answer, which is almost free, and the exposure time that keeps motion blur under half a pixel. It then takes the fork most web jobs hit: if the goal is inspection rather than a periodic archive image, a colour line scan camera whose line rate matches web speed is the standard answer. All sizing below assumes a 1.4 m wide web; scale linearly to yours.

Frames per second: the easy half of the spec

One image per minute is a 1 fps requirement, and every industrial camera clears that by two orders of magnitude. Data volume is equally trivial. A 2.5 MP colour frame is about 7.5 MB of raw pixels and a 20 MP frame about 60 MB. One frame per minute therefore averages 0.125 to 1 MB/s against the roughly 100 MB/s a single Gigabit Ethernet link carries. Spec 1 fps minimum with a colour sensor and nothing in the fps column rules a camera out — anything in the area scan cameras category qualifies on that count.

The 1024 × 768 screen does not set the camera spec

The monitor rescales whatever the camera sends; its 1024 × 768 resolution and 32-bit colour say nothing about the sensor. Size the camera to the smallest defect worth catching instead, and put at least three pixels across it. A 1 mm defect then needs an object-side pixel of 0.33 mm or finer. Over a 1.4 m web that means 4,242 or more pixels across the line. A 4K sensor delivers 2.9 pixels on that defect and an 8K sensor 5.9. In a vision build the screen is never the limiting spec; defect size and web width are.

Shutter speed: where 130 m/min bites

Convert the speed once and keep the units: 130 m/min ÷ 60 = 2.17 m/s = 2167 mm/s. Every microsecond of exposure smears the web by speed × exposure:

The half-pixel rule holds edges crisp enough for defect detection and measurement alike: keep exposure at or below half the object pixel divided by speed. At 1 mm per pixel that allows 231 µs; at 0.4 mm per pixel, 92 µs; at 0.2 mm per pixel, 46 µs. The generic trade-off is in exposure time vs shutter speed for moving-part inspection, and machine vision motion blur: diagnose soft edges shows what blur looks like once it lands in the image.

Worked example: the snapshot path with an area camera

Take 0.4 mm per pixel as the resolution target — three pixels on a 1.2 mm defect. The blur limit is 0.5 × 0.4 mm ÷ 2167 mm/s = 92 µs. Specify 80 µs for margin: the smear is 2167 mm/s × 80 µs = 0.17 mm, which is 0.43 pixels and visibly crisp. Ambient light will not support 80 µs at a useful f-number, so plan a strobed LED light driven hard for the pulse — the machine vision lights finder starts that selection, and line scan light: how much power a fast line really needs shows the power arithmetic behind short pulses.

Use a global shutter sensor for the shot. A rolling shutter reads rows sequentially, and a web at 2.17 m/s skews those rows into a parallelogram before the frame finishes. A 2.5 MP colour GigE camera with a global shutter covers a once-a-minute record image:

Snapshot or continuous: the fork that decides the camera type

Between two one-per-minute snapshots, 130 m of web goes past. Coverage is the problem. A 5 MP camera at 0.5 mm per pixel sees a patch about 1.3 × 1.0 m, so one snapshot inspects roughly one percent of the web that passed in its minute. If the minute-by-minute image is a record for the archive, that is fine and the area camera above is the cheapest answer. If the goal is catching defects wherever they occur, the camera must watch every centimetre, and that is what a line scan camera does: it exposes one pixel line at a time and builds the image as the web carries each line past the lens.

The sizing rule is one division:

Each line period then advances the image by one pixel of web travel, and exposure time is set independently — hold it to half the line period to keep blur at half a pixel.

Sizing the line scan camera to a 1.4 m web

Pixel size over web width is the field-of-view calculation in line scan camera field of view: how to calculate it; here it is for four sensor lengths at 2.17 m/s:

For wallpaper, colour fidelity counts as much as resolution, and the 4K row is the sweet spot: 2.9 pixels on a 1 mm defect, a friendly 6.3 kHz line rate, a 79 µs exposure and a data rate a plain GigE link carries. The colour camera that fits this math is a 4K colour GigE line scan camera with 7 µm pixels and a 29 mm image circle:

The lens arithmetic: 4096 pixels × 7 µm = 28.7 mm of sensor line, inside that 29 mm image circle with margin. Magnification is 28.7 ÷ 1400 = 0.0205. A 50 mm lens then focuses at about a 2.5 m working distance and a 25 mm lens at about 1.25 m; check your own numbers in the lens focal length calculator and pick glass with line scan lens selection for 4K, 8K and 16K cameras.

If machine speed varies, clock the lines from the drive encoder: encoder-triggered line scan capture: wiring quadrature inputs shows the wiring, how to set the line rate of a line scan camera the configuration, and make line scan line rate follow machine speed at runtime the runtime control loop.

Need line-rate headroom or a low-cost camera to prove the concept first? The LS-LCM-0016 is a 2K GigE line scan camera with 7 µm pixels, TDI and a 58 kHz line rate:

At the 2K pixel size over this web the requirement is 3.2 kHz, so 58 kHz leaves an 18× speed margin, and TDI sums several lines of signal to buy exposure headroom in dim light. Size the drive with lighting controllers for line scan cameras: what they fix.

Bandwidth and the Ethernet run

Continuous colour at 4K produces 4096 pixels × 3 bytes × 6339 lines/s ≈ 78 MB/s, about 0.62 Gbit/s. That fits one GigE Vision link with a dedicated network interface card and jumbo frames enabled; gige vision for a high speed camera: how much frame rate fits walks the budget line by line, and gige vision camera selection: bandwidth, cables and software covers the software side. The 16K case later in this guide raises the number to about 1.25 GB/s, which is 10-Gigabit-class data.

Cabling is the last physical decision. On this kind of install the camera hangs 1.25 to 2.5 m from the web, so the run to the electrical cabinet is realistic. A fixed-mount 15 m RJ45 camera data cable with locking studs spans it with slack for routing:

For longer or moving runs, pick from the camera data cables category in shielded, high-flex or extended lengths to match the route.

When the usual answer is wrong

A genuine archive image. If one colour image per minute for the production log is honestly the whole requirement, stop at the global-shutter area camera and the strobe. A line scan set-up adds a line light, an encoder and configuration effort that a periodic snapshot does not need.

Print-quality wallpaper detail. If the job is checking print registration or fine pattern repeats rather than gross flaws, move to 16K. The MV-CL166-91F2C is a 16K RGB line scan camera with 5 µm pixels on XoFLink over two SFP+ connections, with an F-mount and an 82 mm image circle:

Over the same 1.4 m web that resolves 85 µm per pixel, which needs a 25.4 kHz line rate, a 20 µs exposure and about 1.25 GB/s of data rate. The multi-line RGB readout keeps colour registration clean at speed and the TDI stages restore signal at the short exposure; the light budget is the hard part, so commit to an overdriven line light before committing to the camera.

Speed that varies. A fixed line rate on a changing speed stretches or compresses the image along the web, which distorts every measurement. The encoder-clocked line rate above is what keeps geometry true; if the machine has no encoder, a retrofit one is ordinary practice.

Checks before you order

• Web width at the camera position. Measure it where the camera sits, not at the winder; every pixel size, line rate and working distance above scales linearly with width.
• Smallest defect that matters. Fix three pixels across it, and let that choose 2K, 4K, 8K or 16K.
• Exposure and light together. Read the exposure from the half-pixel rule first, then size the line light and its duty cycle.
• Encoder granularity. One encoder pulse should advance the image by no more than one pixel.
• Lens image circle. It must cover the sensor line with margin — the 28.7 mm against 29 mm check above is the pattern.
• Network hardware. One dedicated interface card per camera, jumbo frames on, cable length matched to the route.

The decision

• Frames per second: 1 fps. Any colour industrial camera qualifies; do not pay for frame rate.
• Shutter speed: 80 µs at 0.4 mm per pixel (the half-pixel limit computes to 92 µs); scale exposure inversely with pixel size.
• One image per minute, for the record: a 2.5 MP global-shutter colour GigE camera and a 50 µs strobe — done.
• Continuous full-width inspection of the 1.4 m web: a 4K colour line scan camera clocked by the encoder at about 6.3 kHz with a 79 µs exposure and a line light sized for it; the data rides one GigE link at 0.62 Gbit/s.
• Print-level detail: 16K RGB on XoFLink at 25.4 kHz and 20 µs, with the light budget committed first.

Put your web width, speed and defect size into the camera finder or start at the camera step of the system configurator; if you would rather have the sizing checked, /contact/ reaches the applications team. More web and conveyor installs are collected under the applications guides.