Measuring Parts in Motion: 35 µs Exposure and What It Buys You on a Line

The usual reason measurement cannot move onto a production line is motion blur: the part is travelling, the image smears, and the edge you need to locate is no longer an edge. One way round it is to stop the line for each measurement, which costs throughput. The other is to shorten the exposure until the part is effectively stationary during the capture. The POMEAS HM series online image measuring heads use a 35 microsecond exposure, which is short enough that a product moving at production speed is frozen without stopping the line. This page explains what that number means, what it costs you optically, and where the approach stops working.

How Much Motion Does 35 µs Actually Freeze?

Motion blur is displacement during exposure. At 35 µs, a product moving at 1 metre per second travels 0.035 mm — 35 micrometres — while the shutter is open. At 0.3 m/s it travels about 10 µm. Whether that matters depends entirely on the tolerance you are holding: 35 µm of blur is irrelevant against a 50 µm tolerance and fatal against a 2 µm one. This is the calculation to do first, before any discussion of which instrument to buy, and it is a straight multiplication — speed times exposure time — not a vendor specification.

The Cost of a Short Exposure

A short exposure admits less light. If you shorten it by a factor of a hundred without changing anything else, the image darkens by the same factor and the edge contrast collapses — which destroys the measurement just as effectively as blur would. The HM design compensates by supplying its own illumination: a collimated green LED source in the transmitter, matched to the receiver, so the exposure can be short and the shadow still crisp. This is why online projection heads are specified as a transmitter-and-receiver pair with a defined separation, rather than as a camera you aim at the line.

Why Collimated Light Matters as Much as Shutter Speed

Freezing the part solves blur. It does not solve the second problem, which is that a conventional lens changes apparent size as the object moves closer or further away. On a line, the pass-line always wanders a little. The HM heads use telecentric optics on both the transmitter and receiver side, so the accepted light is parallel to the optical axis and apparent size does not change with position within the depth of field. The practical result is that a part which floats a little along the beam still measures the same. A partly telecentric arrangement — telecentric on one side only — does not give you this, because some rays still arrive at an angle. The underlying optics are explained in measuring shaft outer diameter with a telecentric lens.

Where the Approach Reaches Its Limit

Three boundaries are worth stating plainly:

• Speed versus tolerance. Work out speed × exposure before anything else. Above a certain combination, no exposure short enough to freeze the part leaves enough light for a usable edge, and the honest answer is to measure a slower section of the line or to sample instead of measuring 100%.
• Silhouette only. Freezing the part lets you read its outline. It does not give you the surface. Flatness, step height normal to the view and internal geometry need a different principle, as set out in online gauge, confocal and vision compared.
• Presentation still governs. A frozen image of a part at the wrong angle is a sharp image of the wrong thing. Fixturing, pass-line stability and part orientation remain the dominant error source on most installations.

Sampling Versus 100% Coverage

The ability to measure in transit changes what is economical, not what is correct. If the process is stable and the failure mode is gradual tool wear, sampling at a bench may give you the same control at lower cost and with easier calibration. If the failure mode is intermittent and the cost of escape is high, 100% inline is justified. The decision factors are worked through in inline versus offline optical measurement, and the throughput arithmetic for a given target rate is in cycle time and station count.

Freezing Is One Part of a Stable Reading

A sharp shadow is necessary but not sufficient. Long-term stability on a line is dominated by things that have nothing to do with shutter speed: contamination on the optical windows, thermal growth in the fixture, and drift in the calibration. Verification against a certified artefact on a defined interval is what turns a good first-week result into a trustworthy sixth-month result. See calibration and measurement stability.

FAQ

How do I know if 35 µs is short enough for my line?

Multiply line speed by exposure. At 35 µs and 1 m/s the part moves 35 µm during the capture; at 0.3 m/s it moves about 10 µm. Compare that displacement with the tolerance you must hold. If the blur is a small fraction of the tolerance, the exposure is adequate; if it is comparable to the tolerance, it is not.

Does a shorter exposure always give a better measurement?

No. Shorter exposure means less light, and below some point the edge contrast collapses and the measurement degrades. The useful exposure is the shortest one that still returns a crisp, well-exposed shadow — which is why online heads use a dedicated collimated LED source rather than relying on ambient light.

Do I still need to stop or index the line?

Not for the capture itself. The 35 µs exposure on the POMEAS HM series heads freezes the product in transit, so the line runs continuously through measurement.

Why telecentric optics, and why on both sides?

Telecentric optics keep apparent size constant as the object moves along the axis, which removes the error that pass-line wander would otherwise introduce. Putting them on the transmitter as well as the receiver is what makes that hold across the whole beam; a single-sided arrangement still lets some rays arrive at an angle and the size still shifts.

What if my part is moving faster than the calculation allows?

Either measure on a slower section of the line, or accept sampling rather than 100% coverage. Extending the exposure to gather more light reintroduces blur. There is no setting that defeats the arithmetic.

Can a frozen silhouette detect surface defects?

Only defects that change the outline — a burr, a chip, foreign matter on an edge. A scratch or discolouration inside the silhouette does not change the shadow.

Next Step

Give us the line speed, the tolerance you must hold, the material and the largest dimension in one view. We can tell you whether in-transit measurement covers it, which head fits, or whether sampling at a bench such as IMAGE 3 is the better-matched answer. The method selection guide is a good place to start the comparison.