LINOS F-Theta-Ronar Field Lenses & Beam Expanders 2026: How to Choose Focal Length and Match Expanders to Your Scanner

A scanner's two mirrors deflect the beam at constant angular velocity. Focus that beam with a plain spherical lens and the laser spots land "dense in the centre, sparse at the edges" — the larger the working area, the bigger the deviation in edge linear velocity and energy density.

An f-Theta field lens makes the focused spot position linear with the scan angle (the "f-theta" relationship), converting the scanner's constant angular velocity into constant linear velocity on the workpiece, so spot size and energy density stay uniform across the entire scan field.

The two official calculation relationships (source: LINOS official brochure, Basic Calculations section):

• Spot diameter: Spot-Ø = 1.83 × λ × FL / Beam-Ø
  • where 1.83 is the apodization factor, λ is wavelength, FL is focal length (mm), and Beam-Ø is the input beam diameter (mm)
• Scan length: 2y = FL × 2Θy × π/180, 2x = FL × 2Θx × π/180
  • where FL is focal length (mm) and Θx, Θy are scan angles (°)

Official description of the telecentric versions: the maximum scan field does not exceed the lens aperture, but spot roundness and incidence angle remain constant across the whole scan field.

Per official technical explanations:

• Low-absorption coating: fused silica optics in the 400–1080 nm range ship with the manufacturer's low-absorption coating as standard —
  • absorption ≤ 20 ppm @1064 nm (typically better than 10 ppm)
  • absorption ≤ 30 ppm @532 nm (typically better than 15 ppm)
  • identification rule: fused silica lenses whose part number ends in "-26" carry this coating
• Coating processes cover UV / VIS / NIR, from conventional deposition up to ion-beam sputtering;
• Angle-optimized coatings are used to achieve excellent and uniform transmission across the entire scan field;
• Damage threshold testing: all coatings are measured per DIN EN ISO 11254-2 using the multi-pulse (S-on-1) method;
• Thermal focus shift is countered, per official documentation, by using lower-absorbing materials (specifically selected high-grade fused silica) plus low-absorption coatings, monitored batch by batch with the manufacturer's in-house absorption measurement equipment.

Why it matters: short-pulse (femtosecond/picosecond) LIDT values differ from nanosecond values by two orders of magnitude (0.9 J/cm² vs 40 J/cm²). Selecting optics against the nanosecond threshold for a femtosecond process is a classic source of cascading damage.

Per official description, beam expanders are beam-shaping optical systems for laser material processing:

• They change beam diameter and divergence simultaneously, optimizing focal spot diameter, focal position, and beam propagation characteristics;
• By changing the input beam divergence, they can adjust the back focal length of F-Theta-Ronar / Focus-Ronar field lenses;
• All LINOS beam expanders can be used in reverse as beam reducers;
• Official documentation explicitly recommends pairing them with LINOS F-Theta-Ronar field lenses.

Common mechanical parameters (official brochure):

• Mechanical maximum output beam diameter: 31 mm
• Maximum length: 165.5 mm (including divergence-correction travel)
• Maximum outer diameter: Ø 42.0 mm; mounting diameter Ø 37.6 −0.1 mm (datum face A)
• Maximum permitted input angle skew: 0.2° (without vignetting); output beam tilt is amplified by the inverse of the magnification
• Official approximation: max input beam diameter = 31 mm ÷ magnification (higher magnification permits a smaller input aperture)

• Continuous variable 1x…4x and 2x…8x; wavelength bands 340–360 nm / 515–540 nm / 1030–1080 nm; all fused silica design;
• Maximum input beam diameter up to 8 mm; mechanical maximum output 31 mm;
• 10 independent presets (magnification and divergence); officially stated to enable automatic magnification switching that shortens machine setup time while maintaining laser safety class during adjustment;
• Interfaces: SubD9/RS232, USB 2.0, Phoenix Contact/RS232; part-number suffix rule −20 = SubD9/RS232, −21 = Phoenix Contact/RS232, −22 = USB 2.0;
• Baud rate 9600 bit/s; supply 7–12 V (7–24 V for the Phoenix Contact interface); CE / RoHS compliant; IP 20; Windows (Win10) control software.

Individual datasheet examples (for verifying selection details):

Official positioning: the first off-the-shelf low-outgassing standard UV beam expander, with a stainless steel housing and fused silica optics, aimed at semiconductor, wafer processing, and electronic display industries. The official press release (2024-04-11) launched it together with the low-outgassing UV F-Theta Ronar lens (340–360 nm).

Officially, both field lenses and beam expanders use replaceable fused silica protective glass, and with the protective glass fitted on the exit side the assembly is protected to IP6X dust standards. Protective glass is ordered separately, for example:

Step 1 — Fix the working area and spot requirement

• Work backwards with the official formulas: Spot-Ø = 1.83 × λ × FL / Beam-Ø and 2y = FL × 2Θy × π/180;
• Remember that scan angle is limited by the scanner itself (maximum scan angles differ by model — check the scanner datasheet).

Step 2 — Fix the focal length and check working distance

• Pick candidate focal lengths from the tables in Section 2 and read off the working distance WD at the same time — insufficient WD causes crashes into the workpiece and is more often overlooked than spot size.

Step 3 — Fix the material (fused silica vs optical glass)

• Wavelength in 340–360 / 440–460 / 515–540 / 1030–1080 / 1900–2000 nm → fused silica series;
• 532–1064 nm at conventional power → optical glass series;
• Short pulse or high power → prefer fused silica with low-absorption coating (the "-26" part numbers).

Step 4 — Verify LIDT

• Match the actual pulse duration and power density against LIDT (nanosecond and femtosecond values differ by two orders of magnitude — see Section 4);
• Officially: actual beam expander LIDT and thermal focus shift depend on the specific system setup and laser parameters; testing before use is recommended.

Step 5 — Decide whether you need a beam expander

• To reduce spot size → expand the input beam diameter (using an expander to approach the field lens's design input aperture yields the smallest spot);
• To adjust focus position / back focal length → change input divergence with an expander;
• To reduce rather than expand → officially, LINOS expanders can be used in reverse as beam reducers.

Step 6 — Fix interfaces, presets and consumables

• For motorized versions, confirm the interface suffix (−20 / −21 / −22) and whether 10-preset automatic switching is needed;
• Verify the field lens thread (M85×1, with individual models at M76×1 / M110×1) and the expander mounting diameter (Ø37.6 −0.1 mm);
• Order matched protective glass in the same purchase.

• Pitfall 1: Looking only at scan field, not working distance. A long focal length gives a large area but also a long WD (e.g. 420 mm focal length → WD 510.9 mm). If the machine structure cannot accommodate it, rule it out at step one.
• Pitfall 2: Selecting optics on nanosecond LIDT for a femtosecond process. Official data: fused silica field lenses show 40 J/cm² at 12 ns but only 0.9 J/cm² at 291 fs. Selecting against the nanosecond value leads to cascading damage in short order.
• Pitfall 3: Confusing "mechanical aperture" with "1/e² optical aperture." A beam expander's mechanical maximum output is 31 mm, while a specific model may be 25 mm or 30.9 mm at the 1/e² definition; calculating the spot from 31 mm gives the wrong answer.
• Pitfall 4: Forgetting that expander input aperture shrinks with magnification. Official approximation: max input beam diameter = 31 mm ÷ magnification. Selecting a high magnification without checking the input aperture produces vignetting or clipping.
• Pitfall 5: Mixing telecentric and non-telecentric optics within one process. Telecentric versions keep spot roundness and incidence angle constant across the field; non-telecentric versions vary with position. Mixing them on one machine produces inconsistent quality between stations.
• Pitfall 6: Ignoring the 0.2° input angle skew limit. Officially, a maximum input angle skew of 0.2° is permitted without vignetting, and output tilt is amplified by the inverse of the magnification — so the higher the magnification, the more a sloppy beam path hurts.
• Pitfall 7: Not stocking protective glass. Protective glass is a replaceable consumable ordered separately; running out stops the line.

9. Product Tie-in: Sourcing LINOS Field Lenses and Beam Expanders

Fradwell supplies original German LINOS field lenses (F-Theta-Ronar) and beam expanders, with selection support by focal length / wavelength / scan field, datasheet verification, and worldwide shipping:

• 1030–1080 nm fused silica low-absorption field lenses (70 / 100 / 118 / 163 / 167 / 251 / 255 / 270 / 340 / 420 mm): [product page placeholder];
• 340–360 nm UV fused silica field lenses (70 / 100 / 160 / 167 / 255 mm): [product page placeholder];
• Manual variable magnification beam expanders (1x-4x, 2x-8x, multiple wavelength bands): [product page placeholder];
• Motorized variable magnification beam expanders (1x-4x, 2x-8x, 10 presets): [product page placeholder];
• Low-outgassing UV beam expanders (1x-4x, semiconductor/wafer/display): [product page placeholder];
• Matched protective glass (PG series): [product page placeholder].

For selection support, send us: laser wavelength, average/peak power and pulse duration, target scan field, target spot diameter, working distance constraints, and scanner model with its maximum scan angle.

10. Conclusion

LINOS field lens and beam expander selection in three sentences:

1. Focal length is the single primary variable — it fixes scan field, spot size, and working distance together, and locking it locks the relationship between all three (official formulas in Section 1);
2. Material and coating follow from wavelength, power, and pulse duration — fused silica for UV and short pulses, low-absorption coating (part numbers ending "-26") for demanding 1030–1080 nm work, with LIDT verified against the actual pulse duration;
3. The beam expander is your precision lever — the official standard line is continuous variable magnification (1x-4x / 2x-8x), usable forward to expand, in reverse to reduce, and to adjust the field lens's back focal length and spot size.

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