All-dome imaging fits together seamlessly

Often projected onto planetarium domes and set to music, laser light shows usually display moving lines, figures, or other patterns.

Apr 1st, 2003
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Often projected onto planetarium domes and set to music, laser light shows usually display moving lines, figures, or other patterns. Examples of movies projected by laser light-show equipment have been rare—but this situation may change, based on developments in seamless imaging and improved lasers. Today, all-solid-state lasers can supply red, green, and blue (RGB) laser radiation with reasonable intensity and efficiency. The resulting colors cover a much wider portion of the color triangle than ordinary television, thus enabling pictures of saturated and brilliant colors.

A system suitable for practical use has been demonstrated by Carl Zeiss Jena (Jena, Germany), with a dome at the Carl Zeiss Jena factory used as the projection screen. In the so-called all-dome mode, the inner cupola surface is divided into six sections, one of which fills the area around the zenith and the other five of which are trapezoidal and spaced equally below the first. Each section is supplied by one RGB laser-projection head positioned diametrically opposite from its projection section (the projection head responsible for the area at the zenith is also placed at the periphery). All projectors are synchronously controlled. In this way, large and bright images can be penciled onto the hemisphere line by line in a raster fashion, forming a panoramic view.

Image information is supplied digitally from hard disks or generated in real time by a graphic processor. The software controls the six projectors so that edge distortions are corrected and seams between the individual sections are no longer visible. Video input data can be supplied to a single projector according to video norms such as PAL, NTSC, HDTV, XGA, VGA, and so on, with frame rates of 50 or 60 Hz.

An all-dome laser-projection system depicts an image of an atomic structure.
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Conventional TV frames are not optimal for projection onto large portions of a dome. Instead, dome projection offers the possibility to present much more than just a rectangular section of a scene. Dome projection impressively displays "virtual" worlds, educational scenes with scientific content (for example, biological or medical), or enlarged views of microscopic sceneries (see figure). In the case of a planetarium, laser projection can be combined with the already installed sky projection.

Red, green, and blue lasers

The fundamentals of the laser system were developed by scientists at the University of Kaiserslautern (Kaiserslautern, Germany) and by Jenoptik Laser Display Technologies (Gera, Germany), and have been further developed in cooperation with research institutes. Carl Zeiss Jena realized the special optics for the dome projection and the control of the different projection heads. In the system, pulsed diode-pumped solid-state lasers, developed by Jenoptik Laser Display Technologies, emit light at 629-, 532-, and 446-nm wavelengths, reaching outputs of 3.5 W in the red and green and 3 W in the blue. The 10-ps pulses—which are emitted at an 80-MHz rate, resulting in a 1:2000 duty cycle—are acousto-optically modulated.

To create the RGB output, modelocked pulses with a wavelength of 1064 nm are amplified. Part of the light is converted to green light (532 nm), while the remaining power at the fundamental wavelength is partly used to synchronously pump an optical parametric oscillator (OPO) and partly used for sum-frequency mixing with the 1535-nm OPO output. The generated red light (628 nm) is either mixed with the unconverted signal radiation to generate blue light at a wavelength of 446 nm or transmitted to yield red output. The radiation of the three lasers is combined into a single beam and fed via fiberoptics to the scanning and projection head; fiber lengths up to 30 m can be used so that the laser units can be placed outside the visitor hall. The scanners are a polygonal rotating mirror and a galvanometer mirror, allowing a 48-kHz scan rate in the horizontal direction and a 60-Hz vertical scan rate. A shutter provides laser safety.

It can be foreseen that large-screen laser displays will compete with large light-emitting-diode (LED) and organic LED screens. Laser displays have several major advantages. There is almost no limitation to the depth of focus, so that objects of huge spatial depth can be projected, providing a geometrically induced three-dimensional impression to the spectator; curved projection surfaces do not limit the sharpness of the picture. Somewhat surprisingly, about 10 W of RGB laser power is sufficient for a picture filling about 100 m2. In addition, laser projection provides the darkest black available; this high contrast works with the high color fidelity so that finely graduated colors as well as shiny and translucent objects such as glass can be imaged. The challenge is to develop laser projection that is affordable for home video.

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