RTC
®
4 PC Interface Board
Rev. 1.3 e
4 Principle Of Operation
29
innovators for industry
4.4 Image Field Size
shows the reference system for the image
field which is used by the RTC
®
4. The Y-axis points
in the
reverse
direction of the input laser beam, the
X-axis points to the right of the Y-axis. X-axis, Y-axis
and Z-axis
(
optional) form a right-handed reference
system. The origin of the reference system, i.e. the
point (0|0), is in the center of the image field.
The size of the usable image field is determined by
the maximum scan angle and the focal length of the
objective or the working distance (i.e. the distance
between the input laser beam axis and the image
field).
All X and Y vector coordinates must be specified as
signed 16-bit numbers (i.e. as numbers between
– 32768 and +32767).
The ratio of a point coordinate in
bits
(1)
and the
actual position of the point in
millimeters
is defined
by the calibration factor
K
.
Let
a
0
denote the side length of the image field given
by the maximum scan angle. The theoretical cali-
bration factor is then
K
0
= 2
16
/
a
0
[bits per mm
SCANLAB provides a rounded value for the cali-
bration factor
K
. This value is slightly larger than, but
close to, the theoretical value.
The actual calibration factor
K
can be found in the
README file that is included in the software package
folder containing the correction file.
(2)
Given the calibration factor
K,
the side length
a
of the
usable image field can be calculated:
a
= 2
16
/
K
Typical Image Field
In general, the objective and the optical configu-
ration of the scan head further reduce the size of the
usable image field.
The scan head has a "typical image field". If the laser
focus moves outside this typical image field, some
vignetting
of the laser beam can occur. The interior
of the scan head can be damaged due to excessive
absorption of laser power. Please refer to your scan
head operating manual’s section on objectives.
Compare the calculated side length
a
(as
described above) with the side length
b
of the
typical image field given in the technical specifi-
cations of your scan head manual.
If the laser focus shall be restricted to points
within the typical image field, the absolute values
of the X and Y coordinates (in bits) must be
smaller than the maximum value
M
, where
M
is the calibration factor
K
multiplied by
half
the
side length of the typical image field:
M
=
K
·
b/2
(1) The expression “bits” is here synonymous with “digital control
value” (see footnote on
(2) The calibration factor is the same for the X and Z direction and
– for 3D scan systems – also for the Z direction.
X
Y
Z
input
laser beam
12
Reference system for the RTC
®
4 coordinates
(0
|
0)
(–32768
|
0)
(0
|
–32768)
(0
|
+32767)
(+32767
|
0)
X
Y
a
theoretical image field
usable image field
13
Size of the usable image field
