Line-Scan Lens Selection for 16K Sensors: Magnification, Line Rate and Mount

Area-scan lens selection starts from sensor format. Line-scan selection starts somewhere else: from the sensor line length in millimetres, because there is no second dimension to cover. Get that number wrong and everything downstream — magnification, field of view, working distance — is wrong with it.

This article covers the POMEAS 16K line-scan lens series, whose published values are used throughout. It is a selection method, not a compatibility claim: no statement is made that any POMEAS lens replaces an optic supplied with a specific camera, and no third-party specification is quoted.

Step 1: Establish the sensor line length

A 16K line-scan sensor has 16,384 pixels along the line. With a 5 µm pixel pitch:

16,384 × 5 µm = 81.92 mm

That figure matches the published 82 mm maximum image circle of the POMEAS 16K series, which is the first check: the lens image circle has to be at least the sensor line length. A lens built for a shorter line will not fill the sensor, and the outer pixels receive no image.

If the camera's pixel pitch is not 5 µm, recompute — 16,384 × pitch — and compare that against 82 mm before going further.

Step 2: Pick magnification from the field of view

On a line-scan station the field of view across the web is:

field of view width = sensor line length / magnification

All models in the POMEAS 16K series share a 146 mm focal length, an 82 mm maximum image circle, an M72 mount, F4.0–F16 aperture and a published distortion of < 0.002 %, rated −10 °C to +50 °C. What changes between models is magnification, and each model is published with a three-point magnification range rather than a single figure:

The model code carries the nominal magnification: LLC026 = 0.26X, LLC05 = 0.5X, LLC075 = 0.75X, LLC1 = 1X, LLC2 = 2X — consistent with the published three-point ranges above. The series also includes PMS-LLC033, PMS-LLC133 and PMS-LLC3 for 0.33X, 1.33X and 3X nominal.

Worked example with an 81.92 mm line length:

- At 0.26X: 81.92 / 0.26 ≈ 315 mm of web width. - At 1.0X: 81.92 / 1.0 ≈ 82 mm. - At 2.0X: 81.92 / 2.0 ≈ 41 mm.

Three points matter more than one number: the range means the lens can be set anywhere between them, and the working distance moves with the setting. That is why the working-distance column has three values — these are not tolerances, they are the positions the lens is specified at.

Step 3: Calculate the line rate the station actually needs

Line rate is not a lens specification, but it is the number that decides whether a web inspection station can run at all, and it is fixed by two process values:

line rate = web speed / required resolution along the travel direction

Published worked examples from the POMEAS line-scan documentation:

Check the arithmetic on the first row: 200 mm/s ÷ 15 µm = 200,000 µm/s ÷ 15 µm ≈ 13,333 lines/s = 13.3 kHz. The relationship is inverse — halving the required resolution halves the line rate; doubling the speed doubles it.

Two practical consequences follow. First, resolution along travel and resolution across the line are different constraints: across-line resolution is set by magnification and pixel pitch, along-travel resolution by line rate and speed. Second, a lens that resolves beautifully is useless if the camera cannot run at the required line rate, and vice versa — both have to be checked against the same speed.

Step 4: Mount and mechanical checks

The POMEAS 16K series uses M72, not C-mount. M72 is a larger industrial thread used on large-format and line-scan lenses because the rear element and the image circle are larger than a C-mount can serve. Two checks:

- Does the camera have an M72 mount, or is an adapter required — and does an adapter preserve the flange distance? - Is there enough mechanical support? A 146 mm focal length lens is physically long and heavy compared with a 25 mm FA lens; cantilevering it off a camera mount without support invites vibration, which on a line-scan station shows up as banding along the web.

Step 5: Aperture, depth of field and the light budget

F4.0–F16 is a narrow range next to an FA lens, and it is deliberate: line-scan lenses trade maximum aperture for flatness and uniformity across a long line. The practical consequence is a light budget — at F4.0 on a fast web, exposure time is short, and illumination has to be sized for it. Line-scan stations normally use a line light or a bar light running continuously rather than a strobed area light, precisely because every line has to be lit.

Depth of field across the web also has to be checked: a web that flutters, or a product with height variation on a conveyor, moves in and out of the focal plane, and along a line-scan image that appears as alternating sharp and soft bands rather than a uniformly soft frame.

When a line-scan lens is the wrong answer

If the product is discrete and can be stopped, an area-scan camera with an FA or telecentric lens is simpler and cheaper. Line-scan is justified when the product is continuous (web, film, foil, paper, sheet), when the field is very wide relative to the resolution required, or when the process cannot stop. POMEAS publishes both paths — the 16K line-scan series above and the FA range for area sensors — and the decision is made on the product, not the optics.

What this method does not cover

It does not cover encoder synchronisation, which is what keeps the pixel spacing constant when web speed varies; nor camera-side items such as exposure mode, gain and the interface bandwidth needed to sustain the line rate. Illumination geometry is its own selection and is not settled by choosing a lens.

FAQ

How is a line-scan lens different from an area-scan lens?

It is specified for line length rather than sensor format, and it is corrected across one long dimension. The POMEAS 16K series is published with an 82 mm image circle for exactly that reason.

What magnification do I need?

Field of view = sensor line length ÷ magnification. With an 81.92 mm line, 0.26X gives about 315 mm and 2.0X about 41 mm.

Why is each model listed with three magnifications?

Because the lens can be set across a range, and the working distance changes with the setting. The three values are specified positions, not tolerances.

What line rate do I need?

Line rate = web speed ÷ required resolution along travel. 200 mm/s at 15 µm needs 13.3 kHz; 500 mm/s at 10 µm needs 50 kHz.

Is M72 the same as C-mount?

No. M72 is a larger industrial mount used for large image circles. Check the camera mount and any adapter's flange distance.

Can I use a line-scan lens on an area camera?

That is not what it is specified for. The image circle is sized for a line sensor, and the correction is optimised across the line.

Next Step

Send the line-scan camera pixel count and pixel pitch, the web or conveyor speed, the required resolution along travel and the field of view width, and POMEAS will return the 16K line-scan model with the calculated magnification, working distance and required line rate for your line.