Beam Steered Lasers
Key Takeaways
- Mirrors move, not the laser or the part — that is what makes high-speed marking possible.
- Two galvanometers, one per axis, provide beam motion within the marking field.
- A flat-field lens focuses the beam to achieve high power density across the whole field.
- Scan speed, field of view and beam quality are the three limits that bound what a scan head can do.
Beam Steered Lasers
A beam steered laser marking system moves the focused laser spot across a stationary part using fast, computer-controlled mirrors rather than moving the part or the laser itself. Two lightweight mirrors mounted on galvanometer motors deflect the beam in X and Y, and a flat-field lens keeps it focused across the whole marking area. The part sits still; only the beam moves.
This is the architecture behind essentially all modern industrial marking, and it is why laser marking became viable for high-speed production in the first place.
How the Optical Train Works
- Beam expander. Enlarges the raw beam so that the focusing lens can produce a smaller spot — a larger beam at the lens gives a tighter focus.
- X and Y galvanometer mirrors. Very low inertia mirrors driven by closed-loop galvo motors, capable of repositioning in fractions of a millisecond.
- F-theta lens. The critical component. An ordinary lens focuses onto a curved surface, which would put the mark out of focus away from centre. An f-theta lens is corrected so the focal plane is flat and so displacement is linearly proportional to scan angle, keeping geometry undistorted.
Field Size, Spot Size and the Trade-off
The f-theta lens focal length sets the marking field, and it cannot be chosen without consequence:
| F-theta focal length | Marking field | Focused spot | Best for |
|---|---|---|---|
| Short (e.g. 100 mm) | Small | Smallest | Fine codes, micro-marking, high resolution |
| Medium (e.g. 160–254 mm) | Moderate | Moderate | General production marking |
| Long (e.g. 330 mm+) | Large | Largest | Large parts, at reduced resolution and irradiance |
A larger field always costs resolution and power density. Because irradiance is power divided by spot area, a bigger spot spreads the same wattage more thinly — which on a marginally absorbing polymer can be the difference between a mark and no mark. Specifying an oversized field “for flexibility” is a common and costly mistake.
Three-Axis and Dynamic Focusing
A standard two-axis system marks on a flat plane. Adding a motorised focusing module before the galvos — a third axis — lets focal height be varied on the fly, so contoured, stepped and cylindrical plastic parts can be marked in focus throughout. For moulded components with significant draft or curvature this is often essential rather than optional.
Parameters the Operator Controls
- Marking speed — mirror traverse rate, which together with power and repetition rate sets energy delivered per unit length.
- Jump speed and delays — how fast the beam repositions between marked segments. Incorrect jump, mark and polygon delays cause the classic defects: burned corners, rounded starts and tails on vectors.
- Hatch pattern and line spacing — for filled areas, controlling both appearance and cumulative heat input.
- Focus offset — deliberate defocusing is sometimes used to widen the spot and soften the mark.
On plastics these timing parameters matter more than on metals, because polymers are far less tolerant of the localised heat accumulation that poor delay settings produce. Troubleshooting guidance is collected in troubleshooting common laser engraving problems on plastics.
Delay Parameters and Why Marks Distort
Galvanometer mirrors have mass, so they lag the command signal. Every scan head therefore
exposes a set of delay parameters that synchronise the laser against the mirrors. These are
the settings responsible for most geometric mark defects, and they are frequently left at
factory defaults:
| Parameter | What it does | Symptom when wrong |
|---|---|---|
| Laser-on delay | Waits after the mirrors start moving before firing | Too short leaves a burnt blob at the start of each vector; too long leaves the start of the line missing |
| Laser-off delay | Holds the beam on as the mirrors decelerate | Too short truncates the end of a line; too long overshoots past the intended endpoint |
| Mark delay | Pauses at the end of a vector before the next move | Too short rounds off corners and drags the end of the stroke |
| Jump delay | Settling time after a fast non-marking move | Too short causes a wavy or displaced start to the next feature |
| Polygon delay | Settling between connected vectors within one shape | Too short rounds corners; too long burns a heavy dot at each vertex |
| Jump speed | Speed of non-marking moves | Excessive speed induces mirror ringing that carries into the next mark |
These interact with marking speed, so a recipe tuned at one speed will show corner and
line-end defects when the speed changes. Retune the delays whenever marking speed changes
materially, rather than treating them as machine constants.
Field Calibration and Distortion Correction
An f-theta lens maps angular mirror deflection to a flat focal plane, but the mapping is
never perfect, and it degrades toward the edges of the field:
- Correction files are lens-specific. The scan head applies a distortion
correction table matched to the installed lens. Fitting a different focal length without
loading the corresponding file produces marks that are dimensionally wrong at the field
edges while looking correct at the centre. - Barrel and pincushion distortion show as straight lines bowing near
the field perimeter. This is a calibration issue, not a mechanical one. - Verify with a grid, not a part. Marking a full-field grid and
measuring it reveals scale error, skew and distortion that a single small mark at the
centre will never show. - Keep the working area inside the rated field. Accuracy, spot size and
focus all degrade toward the edges, so a part placed at the extreme of the field will mark
worse than the same part at the centre. - Recalibrate after any optical change — lens, protective window
or head replacement all warrant a fresh grid check.
Troubleshooting Scan Head Artefacts
| Symptom | Likely cause | Correction |
|---|---|---|
| Heavy dot at the start or end of every line | Laser-on or laser-off delay mistuned | Adjust the delay pair. This is a timing fault, not a power fault, and reducing power will only thin the whole mark. |
| Corners rounded on squares and characters | Polygon or mark delay too short for the marking speed | Increase the delay, or reduce marking speed. Retune whenever speed changes materially. |
| Wavy lines at the start of a feature | Mirror ringing after a fast jump | Increase jump delay or reduce jump speed. |
| Marks dimensionally correct at centre, wrong at the edges | Wrong or missing lens correction file | Load the correction file matched to the installed lens, then verify with a full-field grid. |
| Fill areas showing visible banding | Hatch spacing against spot size, or bidirectional scan offset | Adjust hatch spacing, and check the bidirectional compensation if alternate lines are offset. |
| Focus good at centre, soft at the edges | Field flatness limit of the lens, or a three-dimensional part | Reduce the used field, or specify a three-axis head with dynamic focus. |
| Position drifts during the first hour of a shift | Galvanometer thermal drift before warm-up completes | Allow the specified warm-up. Do not calibrate on a cold head. |
Related Terms and Reading
- Plastics laser marking solutions
- Laser irradiance
- How to mark plastic with a laser
- Laser marking and machine vision codes for traceability
- Laser irradiance
Applying this in production
The Sabreen Group provides independent engineering support for galvanometer marking system specification, field selection and parameter development. If you are specifying a process, qualifying a material or troubleshooting a production problem, our engineering services team can help. Contact us to discuss your application.
Frequently Asked Questions
What is a beam steered laser?
A laser system using dynamic optical components — galvanometer mirrors or MEMS scanners — to rapidly direct and control the beam position without moving the laser source or the workpiece. It is the standard architecture for industrial marking.
How does beam steering actually work?
Adjustable mirrors tilt or rotate to change the beam direction, with one galvanometer handling the X axis and one the Y axis to provide motion within the marking field. A flat-field lens assembly then focuses the light to achieve high power density at the surface.
Why is beam steering faster than moving the part?
Because mirrors have far less mass than a workpiece or a laser head. Rapid beam movement allows high processing rates in engraving, marking, cutting and additive manufacturing that mechanical motion systems cannot match.
What limits the performance of a scan head?
Scan speed and acceleration limits of the mirror or scanner movement cap processing speed. Field of view is set by the range of mirror angles and the optical setup. And maintaining beam focus across the whole field constrains achievable quality.
What is a flat-field lens for?
It focuses the laser to maintain high power density across the entire marking field, compensating for the fact that a steered beam would otherwise travel different distances to different points, defocusing toward the field edges.
Why do characters come out with blobs at the ends of strokes?
Mistuned laser-on and laser-off delays. Galvanometer mirrors have mass and lag the command signal, so the head needs delay parameters to synchronise the beam against mirror motion. A blob means the beam is firing while the mirrors are still nearly stationary. It is a timing fault, so reducing power only thins the entire mark instead of fixing it.
Do scan head delays need retuning when marking speed changes?
Yes, whenever the change is material. The delays compensate for mirror settling, and the settling requirement scales with how fast the mirrors are being driven. A recipe tuned at one speed will show rounded corners and line-end artefacts at another, which is why factory-default delays often produce acceptable marks in one job and poor ones in the next.
Why are marks dimensionally correct in the middle of the field but wrong at the edges?
Almost always a lens correction file that does not match the installed lens. The scan head applies a distortion correction table specific to the focal length fitted, and changing lenses without loading the corresponding file leaves the centre accurate while the edges bow and shift. Verify with a full-field grid rather than a single central mark, which will never reveal the error.
