Blame it on Princess Leia. Ever since Luke Skywalker watched R2D2 project an image of the Princess in mid-air during the first Star Wars movie, people have been asking laser companies to do duplicate the effect. Although laser show companies are skilled at many things, breaking the fundamental laws of physics is not one of them. This doesn't mean, however, that they cant' try …
To begin with, let's tackle the basic question: why can't Princess Leia float in mid-air? The problem comes down to the nature of light itself. When we see a laser beam slice through the air, we aren't seeing the beam-we're seeing light reflecting from dust in the air. If the beam has nothing to reflect off, it's invisible to our eye. Of course, there is another way to see laser light. You simply look directly into the laser cavity itself (please don't do this if you value your eyesight!). What you'll see is light emitted from the source. These two examples cover the spectrum when it comes to how the human eye perceives light. To see Princess Leia, her image must either be reflected from a surface (such a projection screen) or emitted from a source (such as a CRT). In either case, a true free space projection is impossible.
So much for the bad news. The good news is that lasers offer a host of ways to create pseudo-3D images that amaze audiences. Some of these ways are old tricks familiar to lighting designers, others are so high-tech that they have yet to be introduced into the marketplace.
The closest thing approximating a free-floating Princess Leia was a prototype volumetric projector created by Neos Technologies of Melbourne, Fla. Volumetric projection refers to any technique that creates an image within an enclosed space. The Neos system worked by using a spinning helix as the projection surface. Spin the helix vast enough and it seems disappears, much like the blades of a high-speed fan become almost transparent when moving. Neos used lasers to project wire-frame images on the surface of the spinning helix, with a computer controlling the precise position of the image as it traveled up, down and around the helix. The spinning corkscrew-shaped helix was enclosed in a one-meter tall box (don't want to loose fingers reaching for the laser image), which made the effect somewhat less than free-floating. This approach is also extremely expensive to implement, so don't expect to see it soon in entertainment applications.
Laser Magic of Playa del Ray, Calif. also uses the volumetric approach for three dimensional laser artwork, but does so using materials that are far easier to work with. Its Laser Tank, for example, uses special microscopic particles suspended in liquid. "The tank acts like a room filled with smoke," said Laser Magic's Barney Kaelin. But unlike room-sized displays, images in the tank appear extremely bright even with low-powered lasers and high ambient light levels. Text, logos and line art seem to mysteriously materialize within the tank, complete with three-dimensional depth as the laser beam passes through the liquid.
One of the oldest laser tricks, but still one of the most effective, is using scrims as projection surfaces. On a dark set, a hanging scrim will not be visible to the audience. When a laser image is projected onto it, the image will appear to magically float in free space. This is much like projecting a movie or video scene-the image is still flat, but good computer animation will make it look three-dimensional. Unlike video, however, the bright, intense laser beam is quite visible on an open-weave scrim. To heighten the effect, I've seen fog added to the set, so that bright shafts of laser light appear in space, with the scanned image floating within the beam pattern.
A variation of the scrim trick is to use a glass surface as projection screen. Viewers will see through the glass, but the glass also reflects images projected onto it (usually from a 45-degree angle off-stage). With the right lighting, the glass is not visible to the audience, and images appear to float in space. This type of illusion dates back to the "Pepper's Ghost" stage trick first shown in 1863.
In his effort to put on new twist on the century-old trick, Kaelin of Laser Magic developed a special projection screen that creates the illusion of floating objects when it's used in conjunction with video or laser projectors. The company's TransScreen consists of two sheets of acrylic plastic that encase a proprietary film that simultaneously diffracts, reflects and transmits all wavelengths of light. With the right lighting, the screen is invisible to the audience, and projected images will appear to float in space.
Although the TransScreen works with any high-quality video projector, Kaelin says it really shines when used with a laser video projector. Unlike conventional video, laser video projectors use modulated beams of laser light that are raster-scanned to create television images. The lasers, however, only put light on the image itself-they don't need to scan a full background frame, as do conventional projectors. That's a crucial factor when it comes to creating convincing images. Standard video projectors must show a full frame image, and they use so-called "video black" to fill in the blank portions of the frame. Unfortunately, video back is more of a hazy gray color, ruining the illusion that Princess Leia is floating in the void of space.
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Once upon a time 30 lines of resolution was considered cutting edge!