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TECHNICAL DESCRIPTION OF A 3D VOLUMETRIC LASER DISPLAY SYSTEM
R. V. Belfatto - NEOS Technologies Inc.

EDITORS NOTE: Many readers have asked us for an article on the emerging technology of 3D volumetric displays. We have requested Robert (Bob) Belfatto, Vice President of NEOS Technologies (Melbourne, Florida) and one of the speakers at Laser F/X '95 to explain the NEOS helix-based volumetric display. Volumetric displays create true 3D images that appear in a volume of space. You can see different parts of the image by moving your observation point as the image has vertical and horizontal parallax. No 3D glasses or special viewing equipment is required to observe the image.

Introduction

The 3D volumetric display system allows true three-dimensional visualization of computer-generated images. The three colour beam from the system's lasers are guided by the acousto-optic deflectors to a display medium.
The display medium allows the laser beam to create discrete visible points of light, called "voxels", at any point within its imaging volume. Arrays of voxels are used to create images that are perceived by an observer from a perspective relative to their position.

Helical Display System

The volumetric display system is comprised of three major subsystems: 1) a laser optics system; 2) a computer based controller, and 3) a helical display assembly (Figure 1).

The laser optics system creates, modulates and projects laser beams onto the display medium. The computer-based controller processes instructions and other data. It generates the electronic modulation and deflection signals that control the laser scanner and converts the beam into imaging pulses. The helical display is a volumetric medium that uses simple optical and mechanical principles to transform the scanned laser pulses into visible three-dimensional images.
The display medium used in the system is a rotating helically-curved screen, referred to as the "helix". Its operation is illustrated in Figure 2. A short duration laser pulse striking the screen is diffused, creating a momentary visible spot (voxel), at the point in space where they intersect.
Images created by arrays of voxels can be generated anywhere within the volume swept by the helix. The light-scattering characteristics of the screen makes the images visible to all observers within a large viewing angle.

The configuration of this surface allows its interaction with a vertical laser beam to be easily expressed in simple mathematical terms. Referring to the Cartesian co-ordinate system in figure 1, any planar position (x, y) assumed by the vertical laser beam in concert with any angle of rotation (q) of the helix, is sufficient to predetermine the vertical position (z) of the resulting voxel.
The three-dimensional location of a voxel may thus be controlled by manipulating only the planar position of the beam and the rotational angle of the helix. By synchronising the display of each laser pulse with the appropriate angle of a constantly rotating helix, the simple transformation of an array of X- and Y- scanned laser pulses into an assemblage of voxels is achieved, creating a true three dimensional image. The helical shape possess the novel characteristic of allowing voxels to be created anywhere within the volume bounded by its overall height and diameter.
Placement of the voxels is directed by a 2D acousto-optic (AO) scanner and controlled by a computer-based electronic driver card. Planar placement (X-Y co-ordinates) of the vertical laser beam is regulated by AO beam deflectors within the scanner. The beam is folded by a system of mirrors that increases its length sufficiently to amplify the small angular variations at the deflectors into large lateral deflections and place it in a vertical attitude.
The scanner also contains AO modulators that regulate the timing and duration of each laser pulse, thus controlling the depth (Z co-ordinate) of each voxel. Input data to the modulator drivers is generated by a central processor, using angular position and rotational speed data gathered by an encoder directly from the helix.
A schematic diagram of a single channel AO deflection system is shown in figure 3.

Basically, it consists of an AO intensity modulator followed by two AO deflectors for independent X and Y deflection. This system has an access time of 10 µsec for the acoustic wave to fill the aperture of each deflector and achieve the desired deflection of the laser beam to the new location.
This implies a voxel rate of 1/(10 msec), or a maximum of 100,000 voxels per second. After allowing for a 20 Hz refresh rate, we are left with a 5,000 voxels in a flicker-free image. The usual voxel rate is somewhat less, as an image has a throughput efficiency of about 20% and represents the practical upper limit in off-the-shelf acousto-optic scanning technology. Attempting to go beyond this limit leads to decreased efficiencies and more complicated optical design.
The most straightforward approach to increasing the number of voxels is to couple several scanners together in parallel. To achieve the target of 40,000 voxels, a four-channel system was designed and built by NEOS Technologies Inc. It consists of four of the single-channel scanners described above packed into a single box (Figure 4). By cutting the access time from 10 µsec to 5 µsec the number of voxels in each channel was doubled to 10,000 for a total of 40,000 voxels per frame with a 20 Hz refresh rate.

Applications

The best types of applications for this type of display are air traffic control, medical imaging and entertainment. It is not suited for video at this time. The volumetric display will improve the safety of air traffic by allowing the controller to see a picture of the controlled area and the relationships of the planes in three dimensions without any aberrations. CAT scan and NMR images displayed with a volumetric system will show the actual shapes of organs or tumours for the doctor making disease processes easier to visualise.
The volumetric display system allows the viewer to use their full optical senses.

NEOS wishes to acknowledge Mr. Pavris Soltan and NRAD Labs which provided the display technology to NEOS under a technology transfer license agreement.

R. V. Belfatto - NEOS Technologies Inc.

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