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Color smoothing and delay filters for use with PCAOMs and Solid-State lasers |
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Historical perspective |
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In the mid 1970s, when laser lightshows were
first getting started, most companies used galvanometer scanners,
not only for the X-Y beam positioning, but also for blanking and
color modulation. Sophisticated optical networks with mirrors and
prisms were used along with the galvanometer scanners to perform
blanking and color modulation. Enter the PCAOM |
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In 1991 and 1992, Pangolin worked with Manhar
Shah of MVM Electronics (the person credited as having invented
the PCAOM) as well as Bob Belfatto and Eddie Young of NEOS
Technologies, to create the Poly-Chromatic Ocousto-Optic Modulator
(PCAOM). The PCAOM is a single device that can select and modulate
individual wavelengths in a multi-wavelength laser beam. Early color filters used for laser displays |
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In 1993, ILDA's Technical Committee began considering the creation
of a standardized analog circuit that ILDA would recommend to be
used along with the PCAOM. The idea was to use an analog low-pass
filter to slow down the color signals being input to the PCAOM --
the "slowness" being approximately equivalent to that of
galvanometer scanners. ILDA's then Technical Committee Chairman,
Casey Stack, proposed the circuit seen below.
Pangolin's William Benner proposed a simpler and more stable
circuit seen below.
Note that the circuit diagrams show only
a single channel. Three such circuits would be used for an RGB
projector -- one for red, one for green. and one for blue. Also
note that the circuit is adjustable. This allows for precise
matching of the color response to that of the X-Y scanning, and it
also provides for channel-to-channel response matching, which is
important to make sure there are no undesirable color "tails" as
lines go from white to black. The filter circuits shown here were
used by a variety of companies during the mid 1990s. Filters for use with RGB solid-state lasers |
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While most of our clients do not use any kind
of color filter and simply use the "color shift" and "faded line
endings" features of our software, a need has arisen for which a
color filter may once again be handy. Note that the delaying action is a side effect of the low-pass filter. The low pass filter has a group-delay characteristic that will tend to provide a delaying action, in addition to the filtering. The delaying action is handy to match up the separate red, green and blue channels, but the response action of the filter is also important because basically the fastest lasers must be slowed down to become timing and response-compatible with the slowest lasers. Characterizing the response of the laser |
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Before beginning to solve a problem,
the problem must first be "characterized". That is -- all
relevant parts of the problem should (hopefully) be
understood. The best way to characterize the response time
of a laser is to use a dual-trace oscilloscope in
conjunction with a function generator (to produce
modulation signals) and an optical detector (to observe
the resulting light output). If the laser is a DPSS laser,
Pangolin also recommends you use a prism in between the
laser and the detector, since DPSS lasers often output
multiple wavelengths (such as 880nM, 1064nm and 532nm all
at the same time). In most cases, we have seen each
wavelength have its own rise time and fall time. See the
diagram below: For those who do not have a scope or other test equipment, a specialized test pattern will do. You could use the Standard ILDA Test Pattern and look very closely at the top portion -- the part that shows rise-time and fall time and overall delay. Otherwise a specialized test pattern can be used. Back when ILDA was contemplating the standardization of an analog delay filter, one member proposed an update to the ILDA Test Pattern with separate sections to aid in the adjustment of Red, Green, and Blue, while simultaneously providing the "white" portion of the Rise and Fall times from the Standard ILDA Test Pattern. The proposed newer ILDA test pattern is shown below:
When the ILDA Technical Committee decided that the color system in ILDA Standard Projector should operate at its maximum speed, interest in alternative test patterns waned. Therefore the pattern that you see above was never officially adopted by ILDA. However, it is still available to anyone interested. Please contact Pangolin if you would like to use this pattern. "Ready for anything" filter/amplifier |
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Up until now, Pangolin has
prescribed filter circuits to individual laser
manufacturers. The idea is that once these laser
manufacturers understand how to solve this problem,
they would incorporate the solution directly into
the laser diode driver itself, and thus, all laser
customers would benefit from that point forward.
Plea to laser manufacturer's to incorporate this solution |
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Since Pangolin first delved into
solving the problem with RGB solid-state lasers with
different response times, we have prescribed simple
analog solutions for one large laser manufacturer in
Asia as well as several companies in the US. We
suggest that all manufacturers of RGB solid-state
lasers adopt a method to make sure all laser
response times are the same. This provides the best
looking images for the industry and also the maximum
number of happy customers, both for lasers and for
laser displays. |
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Summary |
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This has been a
review of the history of color smoothing and delay
filters used in the laser show industry. They were
used by some in the earlier years of laser lightshows,
and then pretty much not used at all in the more
recent years. With the introduction of RGB solid-state
lasers that do not have the same response time for
each laser, color smoothing and delay filters may once
again find a use -- that is, until lasers and diode
drivers evolve in such a way that all laser response
times can be made the same. |
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Occasionally,
people will write to us and ask how they can get smoother, and more
flowing color transitions from the QM2000 laser controller. Since
the QM2000 was designed with the ability to create photorealistic
TV-like raster imagery, the color outputs from the QM2000 are
designed to be extremely fast. However, in an effort to provide the
capability of producing raster imagery along with flowing vector
imagery, we also incorporated a "faded line endings" capability in
software.
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