Line scan lenses

How to choose a line scan lens

A line scan camera builds the image one line at a time while the part or web moves past. Its sensor is a single long row of pixels, often 30-80 mm long, so the lens has to cover a much larger image circle than a C-mount lens for an area camera, with low distortion and even brightness from one end of the line to the other. Choose by sensor length, then magnification, mount and resolution.

1. Image circle: cover the full sensor line

The image circle must be at least as long as the sensor line (pixels x pixel size). Typical lines and the lens classes in this range:

Some product pages also list the field of view the lens gives with a 4K, 8K or 16K camera, which saves the calculation.

2. Magnification from the field of view

Magnification = sensor line length / field of view width. A 57.3 mm line (8K x 7 µm) imaging a 573 mm wide web needs 0.1x; the same camera on a 57 mm wide strip needs 1x. Each pixel then covers pixel size / magnification on the part: 70 µm at 0.1x, 7 µm at 1x. Line scan lenses are optimised for a magnification or a magnification range, from a few hundredths of a x for wide webs to 1x-3x for wafers, PCBs and fine print. Pick the lens whose stated range includes your value; outside it sharpness and distortion degrade. The line rate calculator turns the pixel size on the part and the transport speed into the line rate the camera must run.

3. Mount and focus

Large line sensors use M42, M58, M72 or F-mount cameras; check the camera's mount and flange distance. Many line scan lenses come in V-mount (the barrel is clamped by its diameter, e.g. V38 or 56 mm) and reach the camera through an adapter and focusing tubes or a helicoid; others are listed with mount options (M42, M58, M72, M90, M95). At higher magnification the lens sits farther from the sensor, so the tube length changes with the magnification. See V-mount line scan lenses, M42-mount line scan lenses and mount adapters.

4. Resolution and distortion

The lens must resolve the pixel: 1000 / (2 x pixel size) gives about 71 lp/mm for 7 µm, 100 lp/mm for 5 µm and 143 lp/mm for 3.5 µm pixels. Central resolution in this range reaches 200 lp/mm. Compare the edge value as well, since the ends of a long line are where lenses lose contrast. Distortion is specified as low as below 0.001% on the best lenses, which matters when the line image is used for measurement or when several cameras are stitched across a wide web.

5. Aperture and light

Line scan exposures are short: at 50 kHz the camera has at most 20 µs per line. Line scan lenses typically open to F2.8-F4.5, and stopping down for depth of field costs light quickly, so plan the line scan light together with the lens. Brightness falls toward the ends of the line (relative illumination); the camera's flat-field correction evens it out, but only within the dynamic range left over.

Typical applications

• Web inspection of film, paper, nonwovens, foil and battery electrode coating at low magnification.
• Glass, display and solar panel inspection across a wide conveyor.
• PCB, wafer and print inspection at 0.5x-3x, where pixels on the part are a few micrometres.
• Unrolling cylindrical parts (cans, bottles, rollers) by rotating them under the camera.
• Food and recycling sorting on fast belts.

Common mistakes

• Fitting a C-mount area-scan lens to an 8K or 16K camera: the image circle is too small and the line ends go dark.
• Using a lens far outside its design magnification range and blaming the camera for soft edges.
• Forgetting the focusing tubes or helicoid a V-mount lens needs to reach the chosen magnification.
• Choosing the aperture for depth of field before checking that the light is bright enough for the line period.
• Cutting the line stream into frames with no overlap, so a defect on a frame boundary is split in two; see line scan image overlap across frame boundaries.

Questions

Can I use an area-scan lens on a line scan camera?
Only for short lines. A 1.1" C-mount lens covers about 17.5 mm, enough for a 2K x 7 µm (14.3 mm) line but not for 4K x 7 µm (28.7 mm) and up.

What does the magnification range of a line scan lens mean?
The range over which the maker specifies resolution and distortion. You set the magnification with the working distance and the tube length between lens and camera.

Do I need a telecentric line scan lens?
When the part height varies and the measured size must not change with it, as on thick or warped parts. Some lenses here specify telecentricity below 0.05°.

Which camera goes with these lenses?
See line scan cameras, or start a matched set in the configurator.