RTC
®
5 PC Interface Board
Rev. 1.9 e
7 Basic Functions for Scan Head and Laser Control
121
innovators for industry
By focusing the deflected laser beam with an F-Theta
objective, the two last-named effects can be avoided:
• A direct proportionality between scan angle and
scanned distance in the image field is obtained.
• Additionally, the focus lies on a flat surface.
The F-Theta objective, however, causes a barrel-
shaped distortion of the image field. This distortion is
added to the pillow-shaped distortion caused by the
arrangement of the mirrors, resulting in a so-called
“barrel-pillow-shaped“ distortion of the image field
(see
).
Field Correction Algorithm
The RTC
®
5 board provides a correction algorithm to
compensate for the above mentioned field distortion.
The algorithm is based on a correction table.
An orthogonal grid of 257 x 257 points is superim-
posed on the ideal square image field. The adjusted
X and Y coordinates for a corrected output of these
grid points are stored in a correction table. To move
the focus to any point within the image field, the
RTC
®
5 calculates the correct coordinates by interpo-
lating from the grid points in the correction table. The
result is transmitted to the scan head. The correction
algorithm is executed for every single microstep.
(See the section
.)
By default, SCANLAB creates an individual correction
table for every delivered system. It is stored in a
correction file named (
*.CT5
) in the RTC
®
5 software
package. Also see the command
The calculation of the correction table is based solely
on general system data (such as mirror geometry,
calibration and the objective’s specifications).
Standard correction files therefore reflect neither the
unique properties possessed by the customer’s indi-
vidual system, nor alignment errors.
For those customers requiring more accurate
correction tables tailored to the unique properties of
their particular scan systems, SCANLAB offers the
correXion5 program. This program generates RTC
®
5
correction files based on data derived from actual
test measurements performed by the customer (for
further information, refer to the corresponding
manual or contact SCANLAB).
47
X
Y
X
Y
X
Y
“pillow-shaped“
field distortion caused by
the arrangement of the
mirrors
“barrel-shaped“
field distortion caused by
the F-Theta objective
resulting,
“barrel-pillow-shaped“
field distortion
Field distortion caused by the arrangement of the mirrors and
by the F-Theta objective
