05 May 2004
Advances in LCD and microlens technology mean that 3D displays no longer require special goggles. Rob van den Berg reports on how the latest products are finding their way into mobile phones, laptops and medical institutes.
From Opto & Laser Europe May 2004.
Last year more than 120 companies from every corner of the electronics and computer industries joined forces to push 3D display technology from the laboratory to the consumer. The creation of this "3D consortium" underlines how quickly the market for stereoscopic displays is coming of age, now that the need to wear special spectacles has been overcome. Within a few years all sorts of electronic devices such as mobile phones, PDAs and computers may be equipped with 3D screens, creating a multibillion-dollar market for applications including industrial design, medicine, education, entertainment and possibly even broadcasting.
Following this success, last November Sharp released a notebook computer with a 15" LCD screen that can also switch between 2D and 3D viewing modes (see Further information).
Choice of view Ian Thompson of Sharp Laboratories Europe explains how the company's switchable display is based on the clever use of a pair of LCD panels. The front panel is a TFT-LCD containing a set of image pixels for each eye and the rear LCD acts as a "parallax barrier" which can be turned on and off to switch between 3D and 2D viewing.
"The second [rear] panel is between the backlight and the TFT [thin-film transistor] panel," says Thompson. "It consists of a liquid-crystal layer and a special polarizer called a retardation film. It creates the parallax barrier, a pattern of transparent and opaque vertical stripes, which are precision-aligned to the underlying pixels."
Activating the parallax barrier directs two slightly different images (a stereoscopic pair) to the user's left and right eyes and creates the illusion that the image is 3D. When at the flick of a button the mode is switched to 2D, the barrier becomes transparent and the user's left and right eyes receive an identical image, destroying the 3D effect.
In order to fully realize the 3D effect, it is necessary for the viewer to be within a few centimetres of the "sweet spot", especially in a horizontal direction.
"If the cross-talk becomes too big, and the eyes start seeing the other image, viewing becomes very uncomfortable," says Thompson. "This happens when the viewer's head moves more than half the distance between the eyes, which for different people varies between 6 and 7 cm. To aid the viewer an indicator at the bottom of the screen shows whether their head is in the right position."
According to Thompson, one of the key advantages of the display is that the parallax barrier can be switched off - making the screen suitable for viewing normal 2D pictures - and with no loss in image brightness.
"Moreover, we do not need to apply a separate set of optics such as ridges or micro-lenses on an existing panel," he told OLE. "In our case the two layers can be accurately aligned in the factory."
With respect to applications Thompson believes that the greatest demand is likely to be gaming, but he also identifies promising opportunities in medical imaging where a 3D display could help visualize complex information. "Doctors may be able to simulate a medical procedure before they actually do it," he says.
It uses an array of polarization-activated microlenses to deliver a pair of stereoscopic images to the user's eyes. The array consists of a birefringent material with a lenticular surface relief structure and a second layer comprising an isotropic material with a refractive index matching that of the lens.
The polarization of the LCD determines whether the image will be viewed in 2D or 3D. In the 2D mode, the polarization is such that there is no index step at the microlens surface. Therefore the output wavefront is undistorted, and the same 2D image is displayed to each eye at full brightness.
When the polarization of the LCD switches to the orthogonal state, however, the microlens array becomes active and directs a pair of distinct images to each eye. Typically each microlens covers two image pixel columns of the display - the odd column is directed to the right eye and the even column to the left. By using an array of microlenses a full-brightness 3D image results.
"Typically each lens has a diameter of 200 µm, but there is no real restriction on the size of the display, as long as we design the microlens array to match the pitch of the panel," explains co-inventor Jonathan Harrold. "It is manufactured from well known materials and processes, so once we have developed the architecture - the way the optical elements are put together - our customers are able to mass-produce the displays themselves."
Just like the Sharp display, the sweet spot is only a few centimetres and the technology can switch from 3D to full-resolution 2D.
It is this restriction on head movement that led StereoGraphics of San Rafael, California, to come up with a multiview 3D display: the SynthaGram. Rather than using just a left- and right-eye pair, the company divides the image into nine views. These represent what a camera would see from nine positions along a series of straight lines looking at a scene.
A lens array determines where the light from each pixel is directed. The result is that any pair of views can serve as a stereo pair as if they were left and right images.
The 3D effect is seen in a particular region called the viewing zone, and the 18 inch SynthaGram monitor has five of them. Software designates how the pixels are mapped and in principle any image can be converted to 3D, although it takes around 30 minutes.
Several SynthaGram panels are available, from an 18 inch 1280 x 1024 resolution monitor to a 22 inch 3840 x 2400 version. They are especially useful for wide-angle electronic 3D signs and exhibits in public spaces, but they may also be used for visualizing CAD designs.
Whereas most 3D technologies are based on a flat screen from which the image "jumps out", Actuality Systems of Burlington, Massachusetts, US, offers a completely different way of 3D viewing. The image is displayed in a transparent dome and can be viewed from any angle. Perspecta is the brain child of Gregg Favalora, who has been working on 3D displays since 1988 and is currently Actuality's chief technology officer.
The Perspecta Spatial 3D System exploits the fact that the human brain is capable of integrating a series of 2D cross-section images into a volume-filling 3D image.
The system creates a series of 2D images, each of which resembles a vertical slice through the centre of a 3D object taken at a different angle. The 2D image slices are projected onto a rotating screen by a stationary projector using several relay mirrors. "To obtain a refresh rate of 30 Hz, some 6000 frames are projected each second onto a diffuse projection screen which rotates at 900 rpm," explains Favalora. "A group of three mirrors relays the imagery to the rotating screen in a manner that ensures accurate focus regardless of the screen's angle."
The display, which has a resolution of 768 x 768 x 198 volume-filling pixels or voxels, can be addressed from its own software or using a recently released medical visualization toolkit for data from MRI, PET and CT scans. It allows medical researchers to interpret image data more easily and therefore be more precise when planning surgery and image-guided therapies. Since the Perspecta system has yet to receive approval from the US Food and Drug Administration, it is being offered to qualified medical institutions as a research tool.
A wealth of applications However, Favalora does not want to limit the market. "Studies have shown that people have faster and more accurate reactions to spatial 3D images than all other types of display," he told OLE. "Our technology can be used for any application that relies on high-speed rendering of such 3D images. These include computer tomography of luggage for security installations, and military applications such as LIDAR and SONAR."
As for the future, Favalora believes that Perspecta could enable military planners to monitor flight patterns over unfamiliar terrain, pharmaceutical designers to quickly identify important regions in drug targets, and inspectors to detect and identify suspect items in cargo containers.
"There are literally terabytes of data waiting to be seen," comments Favalora. "We have already shipped 18 systems, but there is room for many thousands more."
The principle of a stereoscopic 3D display
However, the accommodating nature of the eye may prove to be an extra stumbling block for the breakthrough of 3D displays. When viewing a real scene, our eye muscles tend to focus on objects at different distances. But on a 3D display the information is always at a fixed distance, although our brain tells us differently. This may cause eye fatigue and headaches.