Telecentric Lens for Battery Electrode and Tab Measurement

Electrode and tab measurement is a field-of-view problem before it is anything else. The features are small — a tab edge, a coating boundary, a few tenths of a millimetre of alignment error — but they sit on a web or a stack that is tens or hundreds of millimetres wide. That combination is what makes the magnification choice the whole design.

The tension between width and tolerance

A 200 mm wide electrode on a 2/3″ sensor needs about 11 / 200 ≈ 0.055X. At that magnification a 3.45 μm pixel resolves roughly 63 μm per pixel, so a ±0.2 mm tab-position tolerance spans about six pixels. Workable, but a ±0.05 mm coating-alignment tolerance spans under two pixels, which is not.

The honest conclusion is that one station cannot hold both. Wide-web measurement and tight-tolerance measurement are different magnifications, and production lines that need both run two stations with different optics.

Where low-magnification telecentric models sit

Below about 0.3X the telecentric lens becomes physically large and expensive, because the front element has to be at least as large as the field of view. This is the practical reason very wide electrodes are often measured with line-scan or multi-camera stitching instead of a single telecentric view.

The models below start at 0.061X, which covers roughly 180 mm on a 2/3″ sensor. Beyond that, consider whether the measurement can be decomposed — measure tab pitch locally at higher magnification and derive the web-level geometry from encoder or stage position.

Low-magnification telecentric models for wide-field electrode work

Magnification from 0.061X to 0.3X, ordered by resolution. These are the models that reach the field of view an electrode web or a stack edge requires. Note what the resolution column shows: at this magnification a given pixel size resolves tens of microns per pixel, which is the real limit on tight alignment tolerances.

What is actually measured

• Electrode width — a two-edge measurement across the web, robust if the edges are clean.
• Coating width and its offset to the foil — a contrast boundary, not a geometric edge. Its extracted position depends on illumination consistency more than on optics.
• Tab position and pitch — relative to a reference edge or to the neighbouring tab.
• Winding or stacking alignment — layer-to-layer offset, usually measured at the stack edge.

Of these, coating boundary is the one that most often produces disputes, because the "edge" is defined by a contrast threshold that drifts with illumination and with the coating formulation.

What is out of reach

• Coating thickness. Not a lateral measurement. Needs a confocal, interferometric or beta/X-ray gauge.
• Electrode thickness or stack thickness. Same reason.
• Subsurface defects. Invisible to any optical silhouette method.
• Tab height or weld quality in Z. A 2D view gives a projection.

Web motion and the exposure question

On a moving web, motion blur destroys the edge before resolution does. The constraint is exposure time × web speed < sub-pixel distance, which at low magnification is a generous number and at high magnification is not. If the line cannot be stopped or strobed, the achievable resolution is set by the strobe, not by the lens.

Practical configuration

Backlight for the silhouette edges (electrode and tab outer boundary), and a separate front or coaxial station for coating-boundary contrast. Trying to do both in one image forces a lighting compromise that degrades the more demanding measurement. The lighting guide covers the selection.

Questions engineers ask about this setup

Can one telecentric lens measure a whole electrode web to tight tolerance?

Usually not. A 200 mm electrode needs roughly 0.055X on a 2/3″ sensor, which resolves about 60 μm per pixel with a 3.45 μm pixel. A tolerance under about two pixels is not reliably measurable, so wide coverage and tight tolerance need separate stations.

Why are low-magnification telecentric lenses so large?

The front element must be at least as large as the field of view, so a lens covering 180 mm has a front element of similar scale. This is why very wide webs are often measured with line-scan or multi-camera stitching instead.

How is coating width measured?

As a contrast boundary between coated and uncoated foil rather than a geometric edge. Its extracted position depends on illumination consistency, so backlight silhouette methods that work for the foil edge do not transfer directly to the coating boundary.

Can coating or electrode thickness be measured optically in this way?

No. Thickness is not a lateral measurement and does not appear in a silhouette. It needs a confocal, interferometric, or beta/X-ray gauge depending on the material and the required range.

What limits resolution on a moving web?

Motion blur before optics. The requirement is exposure time × web speed below one sub-pixel distance. If the line cannot be stopped or strobed, the strobe capability sets the achievable resolution rather than the lens.

Is telecentricity needed if the web height is stable?

Less so, but web flutter and stack height variation are real on most lines. Object-side telecentricity removes the magnification term that flutter would otherwise introduce, which is usually worth having when the measurement is a width or a pitch.

Related reading

• Telecentric lens selection guide
• Magnification and FOV calculation
• Lighting for telecentric measurement
• What a telecentric lens is and when to use one
• When a telecentric lens is the wrong choice
• Spring: wire, pitch and free length
• Gear pitch and bearing dimensions
• Shaft and sleeve outer diameter

Send one part and we will tell you what the measurement can hold

Specification arithmetic gets you to a shortlist, not to a verdict. Send one part with the drawing and the tolerance you have to hold, and we will run it and report what the system returns — including the features that come back as unmeasurable. A written feasibility note is worth more than another datasheet.