This page
Maximum ILDA compliant size
Large angle performance
Testing for ringing and overshoot
Related pages
Scanner tests: Creation photos
Scanner tests: Scanner-by-scanner
Scanner tests: About the tests
Index to Special Report on Faster Scanning
he photos below show how different scanners perform when displaying test patterns. To see enlarged views of any photo button, just click on the button.
        A separate page has photos showing how these scanners perform when displaying sample images taken from the Lightspeed "Creation" show. Click here to see the Creation photos.

Maximum ILDA compliant size

T hese photos show the maximum angle at which a scanner can correctly reproduce the ILDA Test Pattern, at a given speed. The larger the angle, the better.
        For the three ILDA 30K-tuned scanners, there are clear performance differences. The Cambridge 6800 can correctly scan the ILDA Test Pattern at a much larger angle than the GS G-120 with TurboTrack 2 scanner amps. In turn, the GS/TurboTrack combination can scan at a larger angle than the Catweazle.
        For faster scanners, the Cambridge 6800 using Pangolin TrueK 50 and the Cambridge 6210 are roughly equal in performance.


Large angle performance

About power limiting
Scanner amps used with Cambridge 6800 and 6210 scanners must "power limit" when the scanner is in danger of overheating.
     There are three ways of activating power limiting to avoid overstressing the scanner:
     In current Cambridge 6800 amps and in the Pangolin TrueK 50 modifications, if part of the image is too stressful, then the amp power limits the entire image by reducing signal strength. You see the image shrink in size a second or two after the image is first displayed.
     In the Cambridge 6210 amps, size is also reduced, but only for the part of the image which is too stressful. You see parts of the image shrink in size a second or two after the image is first displayed.
     In a few other Cambridge-compatible amps, the scanner performance is temporarily reduced. The image stays the same size, but corners and other details become more rounded.
T hese photos show the ILDA Test Pattern at a 30 degree optical scan angle. In all cases, the circle-in-the-square test will not work -- the circle will be well inside the square. This means that none of these scanners correctly shows the ILDA Test Pattern at 30 degrees. However, these photos do show how well the scanners can show other aspects of the test pattern.
        The Catweazle exhibits slight rounding, compared with the two other 30K-tuned scanners.
        Both of the fast scanners have trouble with the image. It cannot be shown at 30 degrees without overheating the scanner. As a result, the TrueK 50 amp power limits all of the X-axis scanner's signal. This is why the TrueK 50 pattern is narrower. The Cambridge 6210 takes a different approach. It power limits only part of the X-axis scanner's signals. This is why the "X" and "ILDA" parts of the test pattern are reduced towards the center -- these are the parts that are most troublesome.

Testing for ringing and overshoot

About square wave tests
Square waves are important to test how fast a scanner can move from point to point. The square wave shown here is a large-angle test.
     This capability is important for images where the scanner must jump large distances as fast as possible. Examples are abstracts, rasters, and star fields.
     It is possible to eliminate overshoot by adding steps to large jumps, essentially converting them to a series of small jumps. But this works by slowing down the scanner, which can defeat the purpose of jumping far and fast.
     The Pangolin TrueK 50 and Cambridge 6210 both speed up small steps, while leaving large steps essentially unchanged.
     A key question is whether the small-step speedup balances out the extra steps added to slow down large jumps.
T hese final pictures show a square wave test pattern at around 30 degrees. The scanner is jumping as fast as possible, over a very large angle. Ideally, you should see a dot at each corner. There should be little or no overshoot.
        This is achieved by all the scanner tunings here, except for the Cambridge 6210. It exhibits overshoot beginning at 22 degrees. By 30 degrees the overshoot is severe -- about 3 degrees, or 10%, in the X axis.


This page last updated: Tuesday, June 19, 2007 08:54:17 PM

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