Lab 5 · Galvo field correction

Goal#

Map galvo field distortion over a 200 × 200 mm field, build a correction table, and verify the corrected residual.

Steps#

1. Calibration → Galvo field, 9×9 grid, 200 mm field.
2. Look at the residual heat map: distortion should be small at the centre and largest in the corners (pincushion or barrel).
3. Record residual RMS and max before and after correction.
4. Re-run with a 5×5 grid. Compare corner residuals — too coarse a grid cannot flatten the corners.
5. Re-run with 15×15. See whether the residual keeps falling, and what it costs in acquisition time.
6. Use the correction table to mark a 180 mm square and measure the difference between its diagonals.

Pass criteria#

Item	Criterion
Residual RMS before correction	typically 80–150 µm
Residual RMS after 9×9 correction	< 15 µm
Corner max residual after 5×5	clearly worse than 9×9
Improvement of 15×15 over 9×9	limited (diminishing returns)
Diagonal difference of the square	< 0.1 mm

Common mistakes#

• Grid covering only the centre. The corners are never measured, interpolation becomes extrapolation, and the error explodes.
• Watching RMS but not max. A good RMS hides one corner that is 3× over tolerance — and that corner is where the line will fail.
• Correction table out of step with pixel size. Field correction depends on C1; re-calibrate C1 and you must re-calibrate the field.
• Ignoring jump and mark delays. Wrong values leave tails on marked lines that are easily confused with field distortion.

Review questions#

1. Where does field distortion come from? (Hint: residual design error in the f-θ lens, and the two mirrors not being co-planar.)
2. Going from 9×9 to 15×15 multiplies acquisition by 2.8× for a small gain. What metric tells you where to stop?
3. How does the residual change if the field grows from 200 mm to 400 mm? Why are large fields harder?
4. Bilinear interpolation versus a 2D polynomial fit for the correction table: what does each buy you, and which is more robust to acquisition noise?