Laser Focal Point Measurement Methods Compared: Pulse Engraving, Direct Beam Measurement, and focusfinder.pro

Several methods are available for determining the laser beam focal position. They differ in complexity — and consequently in precision and equipment cost. The spread is remarkable: from solutions costing a few euros to direct measurement systems costing tens of thousands.

Method 1 — Pulsed engraving: the simplest approach

Short laser pulses are generated while incrementally increasing the distance between the welding head and a test plate (typically anodised aluminium), producing spots of varying diameter. Identifying the smallest spot indicates the distance at which the plate sat at the focal point.

This is an indirect method — it measures the effect on material, not the beam itself — and its accuracy is limited by three factors: thermal effects that enlarge the spot beyond the true focal diameter, subjective visual judgement of which spot is smallest, and the Rayleigh effect, which forces pulse spacing of roughly 0.5–1.5 mm or more to see any difference at all.

Accuracy: roughly 0.5–1.5 mm or worse. Cost: a few euros.

Method 2 — Direct beam measurement: the high end

Precision optical setups with cameras, filters and beam absorbers measure the beam directly on a sensor, at several to a dozen-plus positions along the beam axis, then fit the results to the theoretical Rayleigh beam-waist profile.

Accuracy: better than 10 µm. The limitations are practical: high implementation cost, additional machine space, and permanent per-machine installation — a separate unit for every machine in the plant.

Cost: up to tens of thousands of euros — per machine.

Method 3 — focusfinder.pro: a practical compromise

focusfinder.pro engraves a specially designed line pattern onto a test material at multiple positions with different focal offsets. The engraved plate is then scanned with a dedicated high-resolution scanner, and the resulting image is numerically fitted to an ideal Gaussian beam curve — the same principle as direct measurement, applied to an engraved pattern instead of the beam itself.

The output is an automatically determined focal position — eliminating subjective assessment by the process engineer or operator, while remaining simple to use. A single focusfinder.pro can also be used across multiple machines, reducing implementation and operating cost versus permanently installed units.

Accuracy: 50–100 µm, and up to 50 µm depending on laser and welding-head optics — sufficient for a stable welding process on demanding battery materials.

Which method should you choose?

	Pulsed engraving	Direct beam measurement	focusfinder.pro
Measures	Effect on material (indirect)	The beam itself (direct)	Effect on material, numerically processed
Accuracy	~0.5–1.5 mm or more	Better than 10 µm	50–100 µm (up to 50 µm)
Result determined by	Visual inspection	Automatic	Automatic
Cost	A few euros	Tens of thousands of euros	Rational implementation cost
Machine space	None required	Additional space required	None required
Multi-machine use	—	One unit per machine	One unit, multiple machines

For battery module production — joining bus bars to cells in copper and other difficult-to-weld materials at 250–300 µm thickness — a precision of at least 100–150 µm is required, which rules out pulsed engraving. Direct beam measurement clears the bar with room to spare, but at a cost and space penalty repeated for every machine.

The optimal solution, combining high measurement accuracy with a rational implementation cost, is focusfinder.pro.

Based on the technical article "RMA FocusFinder — A Reliable Method for Determining the Focal Point of a Single-Mode Laser" by Rajmund Pobereżny, Project Manager, RMA.