7.1.4 Correction of Marking Field Distortion

Beam path in a two-axis scan system.

Deflecting a laser beam with a two-axis scan system gives rise to three main distortion effects:

Due to the mirrors’ physical arrangement, the combined length of the beam segments between the 1st reflection point R₁ and the marking field (i.e. B₁ + B₂), varies with the deflection angles of both mirrors.
The greater the deflection angles, the longer the deflected beam becomes.
For example, in the figure above: (B₁ + B₂) < (B₁' + B₂').
The image size on the marking field varies in direct proportion not to the deflection angle itself, but rather to the tangent of the deflection angle.
The beam's focal point lies on an approximately spherical surface, and not in the plane of the marking field (except at the field origin).

These distortion effects can be corrected by a combination of optical and algorithmic means.

Optical Correction of Distortion using F-Theta Lens

Distortion effect (3), mentioned above, can be corrected by focusing the deflected laser beam with an F-Theta lens, which effectively flattens the beam focus surface so that it lies in the plane of the marking field.

However, the F-Theta lens introduces its own barrel-shaped distortion, resulting in a composite pin-cushion/barrel distortion of the marking field:

Field Distortion Correction with F-Theta Lens

Algorithmic Correction of Composite Marking Field Distortion

With the help of an appropriate algorithm, the SP-ICE-3 Card can compensate for the composite field distortion, and make the image size on the marking field directly proportional to the deflection angle.

By extracting values from a look-up table, provided by a Correction File ( refer to HeadCorrectionAPI for more details), and using an interpolation procedure, the card adjusts all position co-ordinates sent to the scanners, so that an undistorted image of the correct size is produced on the marking field.