How to use the ILDA Test Pattern
"60K" scanning
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What
is the correct use of the ILDA Test Pattern?
There are a number of parameters measured by the ILDA Test Pattern.
The most critical one for speed-related
issues is a circle that is drawn outside a square. As the scanner speed is turned up, the
scanner can’t accurately track the circle, so the circle gets smaller. When the
circle is just touching the center of the square’s sides, this indicates the
circle’s tuning.
It is impossible to scan both fast and wide.
This is why the ILDA tuning standard also specifies the angle at which the test pattern is
scanned. For 30K-tuned scanners, this angle is "about 8 degrees optical
or less". (At 8 degrees, the width of
the test pattern is roughly 1/7th the scanner-to-screen distance).
Pangolin's specifies TrueK 50
scanners as meeting the ILDA requirement at 7 degrees.
In addition, there are other elements of the
test pattern which are important, such as having the circle be circular and not
elliptical.
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Test pattern parameters The ILDA Test Pattern
measures many parameters. But the circle-in-the-square test is the most important for measuring scanner
speed. The projected circle must be touching the inner square's sides, as
shown at right. (On your computer monitor, the circle will
be outside the square as shown above.)
It is also important to specify
the scan angle. The larger the scan angle at which the scanner correctly
shows the circle-in-the-square test, the better.
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Correct tuning The most important speed-related element is the
blue circle around the outside of the green inner square. The circle is carefully drawn. When the blue circle is just touching
the midpoint of the green square's sides, this is the correct,
ILDA-compliant speed.
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Scanning too fast At too-fast speeds, the blue circle is inside the green square.
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Scanning too slow At
too-slow speeds the blue circle is outside the green square as shown above.
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What does it
mean when a system can show the ILDA Test Pattern? Just that it can be displayed, or that
it is displayed a certain way?
In the laser display industry, the ILDA Test Pattern is generally
recognized as the standard of comparison. A computer might be able to output the points in
the ILDA Test Pattern at 60,000 points per second (or more), and the scanner might show
most of the ILDA Test Pattern acceptably, but this is not sufficient -- some
of it is unacceptable.
If someone says "this scanner shows the
ILDA Test Pattern at such-and-such a speed and angle", laserists reasonably assume
that the scanner correctly shows all parts of the test pattern (unless
explicitly stated otherwise).
What is the difference between "60K pps" and "ILDA 60K
pps"?
Saying "my scanner runs at 60K pps" means that it can show
something, at some scan angle, when a computer is outputting points at
60,000 pts/sec.
This may be a true statement,
but it is meaningless. To be useful to laserists, statements about scanner
performance must
also indicate what images are being shown, at what scan
angle, and how well they are being reproduced.
Saying "my scanner
runs at ILDA 60K pps" tells the listener that "this
scanner can accurately reproduce all parts of the ILDA Test Pattern, including the circle
just touching the square’s sides, when the test pattern is run at 60,000 points per
second at a scan angle of 8 degrees or less."
Is the
ILDA Test Pattern the best available?
The ILDA Test Pattern is an industry standard, and is the
most-agreed upon measure of scanner performance. So yes, it is the best commonly-available
test pattern.
Does the ILDA
Test Pattern accurately characterize scanner performance?
It has done so in the past. But one problem with the
circle-in-the-square test is that it measures scanner performance at only one point in the
scanner’s performance regime. This is like measuring a loudspeaker’s performance
by the accuracy at which it plays middle C. While helpful, it does not tell the whole
story.
Measuring at one point has worked
so far because in the past the scanner performance curve has been similar at 12K, 30K and even
36K. If performance is known at one point, it can be inferred at most other points.
But new scanners or scanner amp tunings may
come out which change the shape of the curve. For this reason, it may be necessary for the
laser display industry to develop a consensus on new test patterns that give additional
data.
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