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F-theta scan lenses: why a normal lens will not do
A galvanometer scanner deflects a beam through an angle, but a conventional lens places the focused spot at a distance proportional to the tangent of that angle. An f-theta lens is designed so the displacement is proportional to the angle itself, which makes the scan linear and the control electronics simple.
THE SHORT ANSWER
A galvanometer scanner deflects a beam through an angle, but a conventional lens places the focused spot at a distance proportional to the tangent of that angle. An f-theta lens is designed so the displacement is proportional to the angle itself, which makes the scan linear and the control electronics simple.
| Design property | Image height proportional to field angle |
|---|---|
| Conventional lens | Image height proportional to tan of angle |
| Field | Flat focal plane required |
| Typical use | Laser marking, engraving, welding |
| Spot size driver | Filled entrance aperture |
| Coating | Matched to the laser line |
Point a scanner at a normal focusing lens and mark a straight line. The marks bunch up in the centre and stretch at the edges, because a well-behaved lens places its image at a height proportional to the tangent of the field angle, while the scanner sweeps in equal angular steps.
Designing the distortion in
An f-theta lens deliberately introduces the distortion that cancels this. Image height becomes proportional to the angle itself, so equal steps of scanner drive give equal steps of spot position. The linearisation lives in the glass rather than in a correction table, which is why these lenses exist as a category.
The three-way trade
Focal length sets the working field and the working distance. Spot size scales with focal length and inversely with the diameter of the beam entering the lens. Increase the field and the spot grows; shrink the spot and the field shrinks. There is no combination that gives a large field and a small spot from a modest beam, and asking for one is the most common specification error in a marking system.
Filling the aperture
Quoted spot sizes assume the entrance aperture is filled, which is usually why a beam expander sits between the laser and the scanner. Fit the system without one and the spot will be larger than the datasheet promised.
Field edges
Performance at the corner of the field is always worse than at the centre. Specify the field you actually need rather than the largest one available, and read the spot size at the corner.
When to use it
- Galvanometer scanning where position must be linear in the drive signal
- Laser marking and engraving over a defined working field
- Any process needing a consistent spot size across the whole field
- Applications where a flat focal plane matters more than absolute aberration correction
What to watch out for
- Working field, focal length and spot size are locked together: you cannot choose all three
- Spot size degrades toward the field edge; check the specification at the corner, not the centre
- The entrance aperture must be filled or the spot will be larger than quoted
- These lenses are coated for a specific laser line and are not general-purpose optics
Still deciding?
Tell us the wavelength, the aperture and what you are measuring. We answer with a part number, a price and a lead time, usually the same working day.