RTC
®
4 PC Interface Board
Rev. 1.3 e
5 Advanced Programming
50
innovators for industry
Laser Control
Black-And-White Images
For black-and-white images, the pulse width
will be set either to zero ("black" pixel) or to a
suitable constant value ("white" pixel).
To enhance the contrast, several pulses (instead of
only one) per pixel can be set. To do this, the pulse
width of the LASER1 signal (Q-Switch) has to be set
accordingly – see set_laser_timing (page 108).
Note: If several pulses are used for one pixel,
scanning mode 0 should be used (see page 49).
Greyscale Images
The laser energy discharged at each pixel position can
be varied in three different ways:
• by varying the laser pulse width (A),
• by varying the laser power at each pixel (B) or
• by varying the number of laser pulses per pixel
(C).
The most suitable method depends greatly on the
type of laser employed.
(A) Variation Of Laser Pulse Width
Some laser types allow varying the pulse width
(duration) of each laser pulse. The command
set_pixel defines the pulse width of the LASERON
signal for each pixel in units of 1/8 µs. (The pulse
width of the LASERON signal is equal to the pulse
width of the laser pulse, if pulse width and output
period of the LASER1 / LASER2 signals are larger then
the pixel step period T.)
• It is recommended that some experiments be
performed to determine the appropriate pulse width
range for producing a smooth greyscale. The resulting
pixel "colors" (greyscale values) strongly depend on the
employed material and on the laser.
(B) Variation Of Laser Power
Q-switched lasers in particular produce very short
laser pulses which cannot be easily varied in length.
For such lasers, it is more suitable to create greyscale
values by modulating the laser pulse energy. One
possibility is to vary the laser power with a suitable
modulating device (e.g. an acousto-optical modu-
lator).
The command set_pixel allows specification of a
10-bit output value for each pixel. The value is trans-
ferred to the ANALOG OUT2 port of the RTC
®
4
synchronously to the pixel output – also see the
section "Timing", page 48.
The ANALOG OUT2 port is optionally available
at the 9-pin D-SUB Laser Connector. The output
range is 0 V … 10 V. Please refer to the section "Laser
/ Analog Output Ports (9-Pin Laser Connector)",
page 59.
(C) Variation Of The Number Of Laser Pulses
Per Pixel
Another way to modulate the laser pulse energy of
Q-switched lasers with short laser pulses is to vary the
number of laser pulses per pixel. For this purpose one
can vary either the length (pulse width) of the
LASERON signal, which serves as a gate for the indi-
vidual laser pulses (see figure 20 on page 48) or (with
set_laser_timing) the pulse width and the output
period of the individual laser pulses.
Note: For this method, scanning mode 0 should be
used (see page 49).
5.8 Timed Jump And Mark
Commands
The normal vector commands (jump commands and
mark commands) are processed by the RTC
®
4 in such
a way that the laser focus moves along the surface of
the image field with a defined speed, the jump_speed
or the mark_speed. This is fine for most laser marking
and laser material processing applications.
However, some applications require that each jump
or mark vector consumes exactly the same amount of
time, regardless of its length. In this case, it is prac-
tical to specify the duration of the jump, rather than
the jump speed.
The commands (see page 120)
• timed_jump_abs
• timed_jump_rel
• timed_mark_abs
• timed_mark_rel
allow specification of the duration of the jump (mark)
command with an accuracy of 10 µs (the output
period of the microvectors) and in the range from
10 µs to 655350 µs (≈ 0.6 s).
The jump (mark) vector will be split up into a number
of microsteps that correspond exactly to the specified
time. (Also see the section "Microsteps" on page 15.)
Of course, this means that the jump (mark) speed, i.e.
the velocity of the laser focus (and the angular
velocity of the movement of the mirrors) will depend
on the length of the vector.
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