Telecentric Measurement Drifts When Workpiece Height Changes

What drift you are actually seeing

When a workpiece moves closer to or further from the lens, a conventional lens changes magnification, so every measured dimension scales slightly. A telecentric lens is designed to hold magnification constant over a limited depth range, which is why it is specified for height-varying parts. Drift that survives that design is usually not magnification error at all. It is either the part leaving the telecentric depth range, or the edge-detection threshold shifting because focus changed.

The depth range is finite and it is measurable

Telecentricity is only held within the depth of field. Across the POMEAS telecentric range, depth of field spans 0.017 mm to 171.97 mm depending on model. A high-magnification model has a very small depth of field, so a height change that is trivial for a low-magnification model will push a high-magnification model outside its telecentric zone. This is the single most common reason a system that worked on the bench drifts on the line: the bench parts were nominally flat, the production parts are not.

Separating magnification drift from focus drift

Measure a known artefact at three heights without changing anything else. If all dimensions scale together, you are seeing magnification drift and the part is leaving the telecentric range. If some edges shift while others do not, or if the measured edge becomes softer, you are seeing focus drift affecting edge detection. The two require opposite fixes, so this test comes before any hardware change.

Fix for magnification drift

Either reduce the height variation with a locating fixture, or select a model with a larger depth of field at the required magnification. Because depth of field and resolution trade against each other, a model with larger depth of field will have coarser object-space resolution; across the range, resolution spans 1.1 to 398 micrometres. The correct choice is the model whose resolution is still sufficient while its depth of field covers the real height variation, and that combination has to be checked per part family.

Fix for focus drift

If magnification is stable but edges soften, the issue is that the part moves outside the sharp focus zone even though it stays inside the telecentric zone. Fixing it means either adding autofocus or fixing the light so the edge contrast does not depend on focus. A common cause is that the illumination is angled, so a height change alters where the shadow falls on the edge and therefore what the detector calls the edge position.

What telecentricity figures do and do not tell you

The specified telecentricity, which across the POMEAS range is 0.02 to 0.04 degrees on the models where it is quoted, describes how parallel the chief rays are. It bounds the magnification change per millimetre of height. It does not tell you the depth range over which that holds, and it does not cover focus effects. Both must be taken from the depth of field and resolution of the specific model.

Frequently asked questions

Should a telecentric lens eliminate measurement drift with height?

It reduces magnification drift, but only inside its telecentric depth range. Outside that range drift returns.

How do I know if the part left the telecentric range?

Measure a known artefact at several heights. If all dimensions scale together, the part is outside the telecentric range.

What is the depth of field of a POMEAS telecentric lens?

It depends on the model, spanning 0.017 mm to 171.97 mm across the range. Depth of field and resolution trade against each other.

Can I just add autofocus to fix drift?

Only for focus drift. If magnification drift is present, autofocus does not correct the scale error; the part must stay inside the telecentric range.

Why do edges soften even when magnification is stable?

The part may still be moving out of the sharp focus zone, or the illumination geometry changes with height so the detected edge position shifts.

Does a lower telecentricity figure guarantee no drift?

No. Telecentricity bounds magnification change per millimetre of height but says nothing about the depth range over which it holds.