Telecentric Lens for Spring Measurement: Wire, Pitch and Free Length

Springs are the clearest case where depth of field, not resolution, decides the optics. A compression spring is a three-dimensional helix: the wire you want to measure at the front of the coil is tens of millimetres nearer the lens than the wire at the back. If both are not within the depth of field, the measurement is of a blurred edge.

Why depth of field is the binding constraint

For a spring of coil diameter D imaged side-on, the near wire and the far wire are separated by roughly D along the optical axis. A 12 mm coil therefore needs the depth of field to span about 12 mm, or the far wire is out of focus while the near wire is sharp.

Depth of field decreases with magnification and with aperture. This is the direct conflict: the magnification that resolves a 0.02 mm wire tolerance is the magnification that will not hold the coil in focus. The models below are ordered by depth of field for exactly this reason.

What a single side view can hold

• Free length — a two-edge measurement, robust, and the most common spring inspection.
• Outer coil diameter — also two-edge, robust, provided the spring is not tilted.
• Coil pitch — measurable on the near side of the coil where the wire is in focus.
• Wire diameter — measurable on the near wire only. The far wire reads wider because it is blurred.
• Verticality / bend — derivable from the axis of the silhouette.

Telecentric models ordered by depth of field for coiled parts

Magnification from 0.2X to 0.8X. Ordered by depth of field, largest first, because on a coiled part the near-to-far wire separation — roughly the coil diameter — has to fit inside it. Note the trade: the models with the highest resolution here are not the ones with the most depth of field.

What it cannot hold

• Total coil count when the far side is occluded by the near side — common on tightly wound springs.
• Inside diameter — the inner boundary of the silhouette is the wire, not the bore, and on a tight coil the bore is not visible at all.
• Wire diameter on the far side — blur makes it read oversize consistently, which is easy to mistake for a real trend.
• Spring rate or load — not an optical measurement.
• End-ground squareness — a 3D datum relation.

See when a telecentric lens is the wrong choice.

Aperture: the trade you actually make

Stopping down increases depth of field and costs diffraction-limited resolution and light. On springs you almost always stop down, because a sharp image of half the coil is less useful than an adequate image of the whole coil. The practical consequence is that spring stations need more illumination than the same magnification would on a flat part.

Presentation and fixturing

Springs roll. A spring resting on a flat plate contacts at two wire points and its axis is rarely parallel to the sensor. A V-block or a locating pin through the bore makes the presentation repeatable, and repeatable presentation is what makes free length measurable at all. On very small springs, hanging the part from a pin avoids contact deformation entirely.

Lighting

Backlight for the silhouette. Springs are usually metallic and curved, so a collimated backlight matters: with a diffuse backlight the curved wire edges produce a gradient rather than a step, and the extracted edge position then drifts with threshold. The lighting guide covers this.

Questions engineers ask about this setup

Why does wire diameter read differently front to back?

Because the far wire is outside the depth of field and therefore blurred, and a blurred silhouette edge reads wider. The error is systematic rather than random, which makes it easy to mistake for a genuine dimensional trend. Measure the near wire, or increase depth of field by stopping down.

Can I measure the inside diameter of a spring?

Only if the bore is actually visible through the coil. On tightly wound springs the near wire occludes it, and the inner boundary of the silhouette is the wire, not the bore.

How do I choose magnification for a spring?

Start from depth of field, not resolution. The near and far wires are separated by roughly the coil diameter along the optical axis, so the depth of field has to cover that first; magnification and aperture then follow, and resolution is what is left over.

Should I stop the aperture down for springs?

Usually yes. A sharp image of half the coil is less useful than an adequate image of the whole coil. The cost is diffraction-limited resolution and light, so plan for more illumination than the same magnification would need on a flat part.

Can free length be measured reliably?

Yes, it is the most robust spring measurement — a two-edge distance. The prerequisite is repeatable presentation: springs roll, and a part resting on a flat plate rarely has its axis parallel to the sensor. Use a V-block or a locating pin.

Is coil count measurable?

Only when the far side of the coil is not occluded by the near side. On tightly wound springs it usually is, and the count has to come from a different view or from the manufacturing record.

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
• Gear pitch and bearing dimensions
• Shaft and sleeve outer diameter
• Lead-frame coplanarity

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.