Vol. 16, No. 3, Oct-December 2005 

Tech Tips for Practicing Laserists

Laserists have never before had so many choices available to them when it comes to scanner technology. But with the proliferation of scanners also comes confusion—what benchmarks can the average laserist use to compare scanners and make an informed decision? The ILDA test pattern, first developed by the ILDA Technical Committee in 1992, is an important evaluation tool. The pattern, however, was never intended as a comprehensive test of scanner performance. Instead, it was designed to calibrate scanners so that images could be played back reliably on different projectors. This is still one of ILDA’s overriding goals—to develop a standard projector that allows laser artwork produced on different platforms to be played back with little or no loss of quality.

When it comes to scanner hardware, the Technical Committee is discussing changes to the test pattern that may be needed to keep pace with advances in technology. To help understand the issues evolved in evaluating scanners, we present the following lexicon of basic terms essential to scanner performance. —David Lytle, Editor, The Laserist

Accuracy: The ability of a scanner to consistently position the laser beam at a designated point in space. Accuracy is important when targeting remote mirrors used for beam effects. It is also crucial for polarized 3D images that require dual laser images to be precisely registered. Judging accuracy is subjective when looking at the ILDA test pattern: does it look good to the viewer?
Bandwidth: A measurement of a scanner’s speed capacity. It is almost always measured in Hertz at the -3db point (where the scanner can only track the input signal to 70% of its original size). The equivalent ILDA pps rate can be found by multiplying the -3db rate by 12.

Driver Electronics: The electronics that condition signals for the scanners are a major factor in determining scanner performance. Third-party driver electronics and/or modifications can sometimes boost the performance of scanners far beyond the results obtainable with the manufacturer’s original driver.
ILDA Test Pattern: The circle-in-the-square pattern shows scanner performance at one specific point in the performance curve: the -3db small step bandwidth. In evaluating test pattern reproduction, it is important to know the scan-angle at which the pattern was projected. To comply with the ILDA 30K tuning standard, the pattern should be projected at eight degrees or more. See the diagrams below for more information.

ILDA 12K, ILDA 30K: 12K and 30K refer to the points-per-second speed when reproducing the ILDA Test Pattern. These two speeds were adopted as tuning standards for ILDA-compatible projectors. Higher ILDA tunings are under consideration.

Image Defects: Scanning fast is important, but speed is only part of the equation. Speed usually means less image flicker, but laserists are also concerned with reproducing stable images that do not drift back and forth, images where lines do not wobble or wave, and images where the scanners do not overshoot or undershoot endpoints.

Large Steps: The distance between sequential points in a scanned image is considered a large step when the distance is a significant percentage of the scanner’s maximum scan angle (also called the scanfield). Such large steps across the scanfield are the toughest challenge for scanners. See also: Small Steps.

Scan Angle: A measurement of the size at which scanners can project images. The optical scan angle is determined by the size of the projected image and the distance between the image and the scanners. A scan angle of 53 degrees represents a one-to-one ratio, where the size of the image equals the distance of the scanners to the projection surface. The mechancial scan angle, in contrast, is the excursion angle of a single scanner, or one-half the optical scan angle.

Small Steps: When adjacent points in a scanned image are relatively close together, they are called small steps. The ILDA test pattern is largely a test of small-step performance. Highly detailed laser images typically consist of small steps between closely spaced points, with the distance between points increasing as less image detail is needed. See also: Large Steps.

Points-per-second (pps): The number of points-per-second that a scanner displays. Most laser images are designed around a specific number of vector points, and the scanner must cycle through these points rapidly enough so that the human eye perceives a solid line of light rather than a series of discrete points. If an image contains 1,000 points and it is cycled through 12 times in one second, the resulting scanner speed is 12,000 pps.

Power Limiting: Driver electronics may have power limiting circuits that protect scanners from overheating. Overheating is more likely if the frame contains many large jumps, is scanned at a large angle, and is scanned fast. Power limiting shrinks the overall image size or reduces the scanning speed.

Software Optimization: To maximize image quality, some software products employ “smoothing” algorithms that reposition image points to get better looking results. Not all software includes this feature and not all programs are capable of supporting ultra-high scanning speeds.

Tuning: Scanner driver electronics are calibrated, or tuned, so that scanners can reproduce a specific image at a specific points-per-second rate. Most laser artwork is designed to be played back at a specific pps tuning. A show created for playback at 30K, for example, won’t look as good if the scanners are tuned for 12K.