Front view of the Wedge display system. The projector injects light into the expansion area (lower part), which is then reflected from the Wedge display (upper part).

Thin, flat screen televisions are all the rage these days, but many consumers prefer the big screen experience provided by rear projection televisions. Technologically, the two concepts are at odds with each other, but that is about to change, thanks to a technology developed by Cambridge Flat Projection Display, Ltd. (CamFPD), a high-tech start-up company spun-off from the Department of Engineering at Cambridge University in the U.K.

CamFPD’s core capability is the generation of licensable optics-related intellectual property for 2D and 3D flat panel displays. The company’s mission is to develop and license these technologies to major players in the large and rapidly growing FPD industry.

Last year the company announced the successful fabrication of its first 50-in. prototype display utilizing its novel, patented Wedge technology. The Wedge technology comprises a set of inventions that allows an image to be projected from a normal digital projector into the edge of a thin, plastic panel, and emerge from the front of the same panel. Normally, images have to be projected from the front, as they normally are in meeting rooms and in cinemas, or from the back of the screen, as with an increasing number of large-screen household rear-projection televisions.

Rear-projection televisions produce good quality images and cost less than LCD or plasma televisions, but they are also very bulky. The Wedge combines the best of both worlds by permitting such televisions to be made as thin as an LCD or plasma television, but at less than half the cost. In addition, the Wedge will ultimately allow much larger screen sizes than either LCD or plasma technology can provide. CamFPD predicts it will be able to produce a 100-in. diagonal Wedge display in less than two years.

CamFPD initially proved the image quality of the Wedge display with a 14-in. demonstrator, but the 50-in. demonstrator signals its ultimate potential.

“This large screen demonstrator convincingly shows that Wedge technology is scalable to large displays and can be a real contender in the huge market for home cinema hardware,” says Adrian Travis, the inventor of Wedge technology and co-founder of the company. The company says that it has been deluged with customer requests for both 14-in. and 50-in. demonstrator units, as well as specially-tailored units for specific applications.

“Two exciting aspects of this technology are the number of prospective customers showing strong interest in using the technology for the original home cinema target and the number of inquiries we are receiving on new ways for applying Wedge optics,” says Quintus Travis, CamFPD co-founder. “We now have strong relationships in automobile, avionic, defense, waste management, and security industries, as well as with various well-known names in consumer electronics.”

CamFPD expects licensees to begin mass production of the technology in late 2005 or early 2006, and is currently arranging manufacturing partners for some of the key components of the system.

The Wedge optics technology is also reversible, allowing the concept to be used for imaging as well as display. The company has already demonstrated that Wedge optics can act as a camera and a display simultaneously by having both a camera and projector at the edge of the plastic panel.

This bidirectional concept permits touch screens, scanners and readers to be easily incorporated into display systems. The bidirectional device has a wide range of applications that could include security systems, ATM machines, and high-volume consumer and commercial applications such as interactive televisions and telephone displays.

How it works

Fig. 1. When a ray of light is injected into the thick end of the wedge, repeated internal reflection reduces the angle until the critical angle is reached, where the ray then emerges. The angle of entry for a ray determines the number of bounces before it exits, so the angle of entry determines the exit location.

The Wedge display is a two-dimensional flat panel display that is about 3 mm thick. Wedge optics represent a projection technology that breaks the link between image size and economics, permitting the cost-effective fabrication of large-format screens.

The theory behind Wedge optics is relatively simple. One begins with a wedge-shaped pane of glass or plastic, where the wedge pane is flat on one side and sloped on the other side. When a ray of light is injected into the thick end of the wedge, it will propagate by internal reflection toward the thin end of the wedge.

Each time the ray reflects off one face of the wedge, its angle relative to the opposite face is reduced. These repeated bounces will lead to the angle becoming increasing smaller until the critical angle is reached. At this point, the ray of light will emerge from the face of the wedge, with the help of an appropriate anti-reflection coating.

When a ray enters the wedge, the larger the angle is between it and the normal, the greater the number of bounces will be required before it emerges. Since the number of required bounces is directly proportional to the distance the ray will travel up the wedge before emerging, the angle of entry can be used to control the vertical position of emergence. (See Fig. 1.) So, by modulating rays injected at a variety of angles, an image from the edge of the Wedge display is projected onto its side.

When the rays emerge from the Wedge display, they travel almost vertically, so a diffusive screen is used to make the projected image visible.

Fig. 2. Horizontal magnification of the image, as shown at left, takes place by fan-out in the expansion region, the flat plate, lower part of the system. To produce a conventional-looking display, the flat plate portion, the expansion region, can be folded behind the Wedge display, as shown at right.

The image to be displayed originates from a video projector and is focused at a shallow angle onto the edge of a flat plate of glass that serves as an expansion region. (See left side of Fig. 2.) Once within the flat plate, rays from the far left and far right columns of the video projector diverge to either side of the sheet, at which distance the sheet terminates and the rays pass into the bottom of the Wedge display itself.

In order to produce a conventional looking display, the flat plate portion, the expansion region, can be folded behind the Wedge display through the use of prisms. (See right side of Fig. 2.) This technique provides all the benefits of a flat panel display and the great flexibility of a projection display. The advantages of this concept include:

Form factor. The Wedge technology can create ultra-thin screens, less than 3 mm thick. The screen can be free of margins or frames, because the picture can reach right to the top and sides of the display and the projector can be tucked away at the base. Aspect ratios are not limited to the conventional 4 x 3 or 16 x 9, and the screen does not even have to be rectangular. The thin and light form factor will allow Wedge displays to be employed where existing large displays cannot, for example in partition walls in office cubicles.

Daylight visibility: The Wedge display concentrates all the projected light into the screen, giving a significant advantage over front projection, which also suffers from poor contrast and shadowing.

Transparency: Moving images attract attention. CamFPD is often asked by showroom designers if it can show a picture on a transparent screen. The answer is yes, and without dazzle from the projector.

Robustness: In certain applications, such as in bus shelter advertising, vandal resistance is desirable. Wedge screens can be pure plastic and they don't contain delicate electronic components. The projector can easily be shielded in a vandal proof box.

Future directions

With the advent of small and affordable laser projectors, and using mechanical scanning of the laser beam into the corner of the panel, it should be possible to eliminate the need for the expansion slab portion of the Wedge system.

Using narrow wavelength (laser) illumination allows the Wedge display to be very thin, which could ultimately allow a display that can be rolled up. The use of a graded index film would prevent the interface from being scratched.

Wedge optics might also be used in the reverse direction in conjunction with a sensor array (such as the CCD arrays used in digital cameras). This technique could allow the system to recognize fingers touching the display or even to recognize gestures. In order to simultaneously project an image in the forward direction without the projection source blinding the sensor, laser illumination could be employed in combination with filters, which prevent the laser wavelengths from reaching the sensor.