|
|
Getting to
Know Your Projector
By
Patrick Murphy, Pangolin Laser Systems
(This
is an excerpt of a more detailed article available at: http://www.pangolin.com/resguide01.htm)
The first requirement for a laser show is, of course, a laser.
There are three general types of light show graphics lasers:
low power helium-neon (red), and medium and high power argon
(green-blue) or mixed gas argon/krypton (red-yellow-green-blue).
The power you need depends on whether you're concentrating on
beams or graphics, and on the scope of your shows -- for small
or large audiences.
Low power HeNe lasers are fine for studio work and small-scale
indoor graphics. Medium power lasers use standard 110 volt AC
power and are air-cooled by fans. They can handle many indoor
graphics, and small-scale indoor beam effects. High power lasers
need 220 or 440 volts and require water cooling using hoses or
chillers. They are for large-scale indoor graphics, outdoor graphics
and beam effects.
Usually, a laser projector will work with just about any laser.
Therefore, if your emphasis is on graphics, spend less on the
laser initially and more on the projector and computer system.
This is because you can later rent or purchase a more powerful
laser for your existing projector. If your emphasis is on beam
effects, spend more on the laser to get the maximum possible
brightness. You want as much power as possible so beams show
up even under adverse conditions, such as outdoor shows in high-brightness
areas.
A popular laser for beams is based on frequency-doubled YAG technology.
YAGs are great for beams, but are poorly suited for graphics
as they have larger diameters and usually are not continuous.
(Scanned graphics may have fat, dotted lines.) If you are buying
only one laser for both graphics and beams, stick with continuous-wave
gas laser technology such as argon/krypton.
You can purchase new or used lasers. Probably the best value
is a used laser which has been re-gassed or re-tubed by a company
specializing in this work. Warranties are available from reputable
used laser dealers.
Laser
projectors
T he laser projector includes a pair of galvanometer scanners,
a blanking or color system, and related electronics. You can
either buy a fully assembled laser projector, or put together
your own projector.
Fully assembled projectors are usually made on a semi-custom
basis. The manufacturer will design a system with components
matched for your particular requirements. Full-color (PCAOM-based)
graphics projectors start at around $7500. Adding beam tables
and lumia effects quickly raises the cost. If you are technically
minded, you can buy the projector components and assemble them
into a system. This is an especially attractive approach for
those on a budget with relatively simple needs, or for those
who have previous experience with laser projectors.
Before going into detail on the projector parts, let's look at
the final components of a laser system: the computer and software.
Computer
and software
Laser show software is available for almost every personal computer,
from the Amiga and Macintosh to IBM PC compatibles. There are
even programs with their own custom computers. Look for the software
first; this will help narrow down your choice of computer.
There are three general levels of software: hobbyist, intermediate
and professional. Roughly speaking, price ranges are under $1000
for the hobbyist level, $1000-5000 for intermediate, and above
$5000 for professional. This may seem costly in comparison with
spreadsheets or word processors. But remember that the market
for laser show programs is much more limited.
As with all software, the more expensive programs give you more
features, a friendlier user interface and better service and
support. Be sure to choose software that meets your needs, both
for now and for the future.
The
laser projector is the heart of a graphics system. It contains
the galvanometer scanners and scanner amplifiers, which together
move the beam fast enough to create graphics. It also contains
an intensity device that blanks and/or colors the laser beam.
This section discusses these two vital parts of a projector.
There can be other parts, such as beam tables and lumia special
effect devices. However, this section will concentrate on the
graphics-specific parts, since graphics are vital to today's
laser shows.
Here's some essential information on what's necessary to put
together a good graphics projector:
X-Y
scanners
Scanners move tiny (3 x 5 mm) mirrors that deflect the beam.
Two are needed; one for horizontal motion (X) and one for vertical
(Y). Together, they can position the beam anywhere on the display
screen.
For the most accurate graphics reproduction, use closed-loop
galvanometer scanners (they're also called "position-detecting"
scanners).
The mirrors and mirror mounts for the scanners may be included
with the scanners, or may be priced separately. You also need
a positioning mount. This holds the scanners at right angles
to each other, in the best orientation for X-Y scanning. You
can purchase this or machine your own.
Scanner
amp
Each closed-loop galvo scanner requires a closed-loop scanner
amplifier to condition and amplify the computer's laser control
signals. You can obtain the amplifiers either as circuit boards
or as fully assembled units. Circuit board amps use trimpots
for control adjustments, and require a power supply, usually
+/-15 VDC or +/- 24 VDC at 2 to 4 amps. Fully assembled amps
come in a case with 110 volt AC power supply and knobs to adjust
controls. An advantage of boards is that you save money and you
can mount them inside your projector chassis, avoiding another
box to carry around.
A cable connects the boards to the scanners. You can purchase
this or make your own. The cable is usually short, to provide
the most accurate feedback signals between the scanners and the
amp.
Before
you buy
Before you buy scanners, be sure you know exactly what you're
getting. For example, you may see ads for "abstract graphics"
projectors. This term means that the projector is not intended
for accurate computer control. It usually uses open-loop scanners
with simple amps. Also avoid resonant scanners; these scan only
simple patterns at fixed frequencies.
Incidentally, used scanners of any type are often a wise buy
-- if you know the scanners are in good condition. Should you
be purchasing new scanners, note that there is often a backlog.
Lead times can be as much as ten weeks.
Single-color
control
Most graphics projectors use some type of blanking device, to
turn off the laser's beam as it moves between the visible lines
of a drawing. Blanking in a single-color projector can be done
with a galvo scanner or with an acousto-optic modulator (AOM)
crystal.
The advantage of galvo blanking scanners is their compatibility
with galvo mirror scanners. Their on/off response time inherently
synchronizes with the galvo mirrors' movements. The disadvantage
is that this response time is relatively slow. It is not enough
to simply use an arm which rotates in and out of the beam. Complex,
multi-mirror optical paths are required, so that a small movement
of the blanking galvo can instantly cut off the laser beam.
AOMs operate much faster than galvos, automatically giving clean
on/off response. Many people feel they are easier to work with,
since they do not require the optical path lengthening of galvo
blanking. However, timing adjustments are necessary in the laser
software or in the AOM driver circuit so that the AOM is synchronized
with the much slower X-Y scanners.
Full-color
projectors
A
white-light laser beam enters the PCAOM crystal from the right,
while a radio-frequency signal is injected from the top. The
RF signal sets up a diffraction pattern which acts as a prism.
The result is two beams exiting on the left: the desired color
and a "waste" beam with all colors except the desired
color. Since 1992, most full-color projectors use polychromatic
AOMs (PCAOMs). These control both blanking and color selection
in one device. They are usually used with so-called "white-light"
beams to do full-color graphics.
You can create a white-light beam on a budget by using an air-cooled
argon and a helium-neon laser. The two beams are mixed using
a dichroic filter into a single white-light beam containing red
(HeNe) plus green and blue (argon). For more power and convenience,
select a mixed-gas laser. This will provide white light (either
red-green-blue or red-yellow-green-blue) in a single beam.
Color balance is important, so be sure your beam has a good mix
of colors to make a balanced white. Because the eye is more sensitive
to green, there should be more red and blue light than green.
In a PCAOM system, a white-light beam is directed into a crystal.
Signals from the computer go into the PCAOM driver, which causes
the crystal to diffract different wavelengths (color) away from
the original beam. The diffracted, colored beam continues on
to the X-Y scanners.
PCAOM driver electronics come in 4-, 6- or 8-channel models.
This refers to how many wavelengths (colors) can be simultaneously
controlled. Usually, the more channels the better. For example,
Pangolin's LD Pro controls up to six channels. This lets you
create unique colors such as a pure violet. It is not a mixture
of red and blue, but it is the argon's natural violet wavelength
coming through.
PCAOMs have become the de facto method of color control. This
is because they are simple to align, relatively inexpensive,
and provide extremely fast control. PCAOMs are so fast that each
point in an image can have a different color. (One benefit of
this speed is that TV-like raster images can be created.)
Even if you don't have a white-light laser beam, you may still
want to consider a PCAOM. It can give higher throughput since
it will modulate all available wavelengths (such as the greens
and blues in an argon laser). And, you can move up to full color
at any time, simply by using a white-light beam as input to your
projector.
Basics
and add-ons
Here are some suggested system configurations. Costs given are
only for the parts in a graphics projector; they do not include
the projector chassis, laser or computer.
If you're a hobbyist on a very tight budget, start with a simple
two-scanner system. One scanner controls the X (horizontal) axis,
the other controls the Y (vertical) axis. At a minimum, you will
need: two scanners with mounted front-surface mirrors, a mount
to position the scanners relative to one another, two single-channel
scanner amps (or one dual-channel amp), a power supply for the
amp(s), and a cable to connect the amps and scanners.
To give an idea of costs, the above is about $2000 using the
fastest available scanners, Cambridge Technology model 6800.
Cambridge provides everything except the power supply.
The system above will not have blanking. Although it is possible
to do clever drawings which hide the retrace line, most people
should also add a blanking or color control device. It is certainly
easier to create images when you can hide the beam. A blanking
projector adds a blanking device (galvo or AOM), a driver (scanner
amp or AOM driver), a bracket to hold the device in position,
and a power supply for the driver. Expect to pay roughly $1000
for a monochrome blanking device and driver. A PCAOM can be the
best way to go. At a cost of $1000 - $3000, it serves not only
to blank the beam, but to control color of all but the highest
power laser beams. Having a PCAOM gives you great flexibility
to move up in power and capabilities without buying any new equipment.
The costs given above do not include a chassis. One important
reason to buy a fully assembled projector is that the manufacturer
provides the chassis plus additional convenience and safety features,
such as a shutter. In the United States, the manufacturer should
also provide CDRH certification.
This
last item means that the projector has been built to the standards
of the U.S. laser safety agency, the Center for Devices and Radiological
Health. If you perform public shows in the U.S., you must certify
both your equipment and show sites to the CDRH. Most manufacturers
of complete, turnkey projectors will include equipment certification
as part of their service.
Patrick Murphy is
president of Pangolin Laser Systems, past president of ILDA,
and currently services as ILDA's Air Space Issues Coordinator.
http://www.pangolin.com
Additional
Show Basics Links
|
X-Y laser
scanners by Cambridge Technology Inc.
Ion
lasers and power supply by Spectra Physics
|