A line scan camera builds its image one line at a time while the part moves past it. The camera must take a new line every time the part moves one pixel width, or the image stretches or squashes. Enter the speed in any common unit, the field of view across the motion and the pixels per line. Optionally add the camera's maximum line rate and the encoder's pulses per revolution and roller diameter. The calculator returns the pixel size on the part, the line rate, the longest exposure, the data rate and the encoder setting, and shows the line rate at slower and faster line speeds so you can see how much headroom the camera has.
How the line rate calculation works
Work in consistent units: speed in mm/s (m/min × 1000 / 60), the field and pixel in mm, the rate in lines per second. The formula gives square pixels, the same size across and along the motion, which keeps circles round and measurements simple.
Worked example: a 300 mm web at 30 m/min
- Speed: 30 m/min = 30 × 1000 / 60 = 500 mm/s.
- Object pixel with a 4K (4096 pixel) camera: 300 / 4096 = 0.0732 mm.
- Line rate: 500 / 0.0732 = 6,830 lines per second, or 6.8 kHz.
- Longest exposure: 1 / 6,830 = 146 µs.
The same web at higher speeds:
| Web speed | Line rate with 4K over 300 mm | Longest exposure |
|---|---|---|
| 10 m/min | 2.3 kHz | 439 µs |
| 30 m/min | 6.8 kHz | 146 µs |
| 60 m/min | 13.7 kHz | 73 µs |
| 120 m/min | 27.3 kHz | 37 µs |
| 300 m/min | 68.3 kHz | 15 µs |
At high speeds the line rate is rarely the hard limit. Light is: at 15 µs the sensor gets a tenth of the light it gets at 146 µs, so fast lines need high-intensity line scan lights, focused on the scan line.
Choosing pixels per line
More pixels per line give a finer object pixel but a higher line rate for the same speed, and more data. Size the pixels per line from the smallest feature first, the same way as for an area camera (the camera resolution calculator explains the pixels-per-feature rules), then check the line rate.
If the rate is beyond the camera, there are three ways back:
- Use a wider object pixel along the motion only (non-square pixels), when features in that direction are larger.
- Split the width between two cameras, each covering half the field at half the pixels.
- Choose a camera with a higher maximum line rate, or one with time delay integration (TDI), which adds several lines together for more light at the same speed.
Triggering from an encoder
With a free-running line rate, any speed change distorts the image. An encoder fixes this by triggering lines from the motion itself. The required setting is the travel per line, which is the object pixel:
For example, a 2,500-pulse encoder on a roller of 200 mm circumference gives 0.08 mm per pulse. With a 0.0732 mm object pixel that is 1.09 lines per pulse, so the camera's or frame grabber's encoder input must multiply the pulse train by 1.09. Use a quadrature (A/B) encoder when the line can stop, vibrate or reverse, so the camera ignores backward motion.
The lens and the light
A line scan sensor is long: 4096 pixels of 7 µm make a line 28.7 mm long, larger than most area-scan lenses cover. Use line scan lenses with an image circle at least as long as the sensor line, and calculate the working distance from the sensor length and the field with the lens focal length calculator (enter the line length as the sensor width).
The line light must be at least as long as the field plus a margin at each end, because intensity falls off at the ends of the bar. The line scan image overlap guide covers how to stitch lines into frames without gaps or duplicates.
Next steps
Browse line scan cameras by line length in the camera finder, or configure the whole station, camera, lens, light and cables, in the configurator.
Questions
How do I calculate the line rate for a line scan camera?
Divide the field of view across the web by the number of pixels per line to get the object pixel size. Then divide the web or conveyor speed by that pixel size. For square pixels on a 300 mm field with 4096 pixels at 500 mm/s, the object pixel is 0.073 mm and the line rate is about 6,830 lines per second, or 6.8 kHz.
What happens if the line rate is too low?
Each line then covers more travel than one pixel width, so the image is squashed along the direction of motion and small features in that direction can be missed completely. If the camera cannot reach the needed rate, use fewer pixels per line, a wider object pixel, or a faster camera.
Do I need an encoder for a line scan camera?
Use one whenever the speed is not perfectly constant. An encoder on the conveyor or a roller triggers one line per fixed distance of travel, so the image keeps its proportions when the line speeds up, slows down or stops. A free-running line rate only works on processes with a steady, well-controlled speed.
How long can the exposure be on a line scan camera?
At most one line period, which is one divided by the line rate. At 6.8 kHz that is 146 microseconds. In practice leave some margin for the camera's readout overhead, and shorten the exposure further when motion blur within a line must be reduced. Short exposures need bright, focused line lights.