Maximizing Video Display Contrast in a Lighted Room

Business Problem

Group videoconferencing and home-theater systems require large-area displays to be maximally effective, but it is difficult to maintain high image contrast in a lighted room. This is a problem for videoconferencing, because the cameras required need ambient illumination to produce good signals, and for home theater, because most viewers do not like to watch television in the dark.

Rear-projection and liquid-crystal screens do trap ambient light fairly well, providing reasonable contrast, but the depth of rear-projection systems may be objectionable, and large LCDs are not available yet. Direct-view cathode-ray tubes as large as 45-inches in diagonal have been made and could provide sufficient contrast, but they are even deeper than rear-projection systems of the same screen size. Flat-panel color plasma displays are beginning to be available at around 40-inches in diagonal, but they are still quite expensive. Incidentally, both CRTs and color plasma panels use reflective phosphor screens, which results in a difficult design tradeoff between brightness and contrast. Front-screen projection systems require no space at all behind the screen, but they have been considered ineffective in a lighted room because they are inherently reflective.


According to a widely quoted study by the Gartner Group, the videoconferencing market is growing at 48% per year from a base of approximately $1.1B in 1995. However, the strong growth is in the desktop market, thanks to the timely emergence of ITU standards for audio/video compression and increasingly powerful personal computers. The market revenue for group videoconferencing may actually be shrinking, according to a study in December of 1995 by Forward Concepts of Tempe Arizona. Some industry observers believe, however, that the improving video and audio quality of desktop systems will eventually re-ignite demand for conference-room systems. If that happens, a less expensive projection display solution will probably be needed to match the rapidly falling costs of the other components. Portable front-screen (FS) projection systems using liquid-crystal light valves (LCLCVs) and new, long-lived lamps are just beginning to be available for less than $6K. This is already significantly less expensive than high-end rear-projection CRT solutions that use a separate projection room behind the screen. Furthermore, the cost of FSLCLV projectors should come down rapidly as the cumulative production volume begins to catch up.

The problem remains: how can FSLCV systems achieve high image contrast in rooms lit brightly enough to allow videoconferencing cameras to operate?

Bellcore Solution

FSLCLV projectors use polarized light internally, and a Bellcore patent [1] teaches modifications that exploit that fact to increase the contrast of the projected image by a factor of 2. This is accomplished with half-wave plates that align the polarization of the 3 color components of the projected light with each other and with a horizontally polarized screen. The Bellcore patent also teaches a preferred method of polarizing the room lighting to increase the contrast of the projected image by more than 2. This system uses the horizontally polarized projector and screen and special multilayer polarizing filters for the ceiling light fixtures. The multilayer polarizer works by partially reflecting horizontally polarized light back into the light fixture except when the light is traveling straight down. Therefore most of the room light that travels directly from the ceiling fixture to the screen is vertically polarized and is absorbed by the horizontal polarizer there.

All of the components in the Bellcore system are currently available, and polarized projectors with long-life lamps and high output currently cost less than $6K. This may be an attractive price point for corporate users because the projectors are portable and the cost of retrofitting a conference room is greatly reduced. Conventional slide and vu-graph projectors would continue to work in the converted conference rooms. Furthermore, the polarized projectors mentioned above can display personal computer screens as well as video sources. Thus, the polarized projector could be used for conventional presentations when not in use for videoconferencing, and, as a bonus, the room lighting would not have to be dimmed.

Although portable LCD projectors have been available for several years, they have not become commonplace in conference rooms, much less in home theaters. There are probably three reasons for this. One, the metal-halide lamps used in these projectors were unreliable compared to CRT projection tubes. Two, the lamp arc length was excessive for efficient coupling to LCD light valves at economically small sizes. New lamp technology from Philips has made impressive gains in both these areas, and this technology is being used in at least one projector on the market. The third reason is, of course, the belief that front-projection washes out in room light, but the Bellcore configuration alleviates this problem. There is therefore an opportunity for LCD projectors to redefine the group videoconferencing market. In addition a significant impediment to FSLCV technologies for home-theater use has been removed.

Technical Background

The Bellcore configuration described above has achieved a contrast ratio of 15:1 in experiments carried out in a conference room in 1991 with the components commercially available at that time. Based on this result and the increased output of state-of-the-art projectors, a contrast of at least 50:1 can be achieved, which is sufficient even for high-definition television. Furthermore, polarizing screens have also become available (a separate sheet polarizer was used in the experiment).

In the 1991 experiment, the projector was modified by inserting a half-wave plate in one of the internal beams in the projector. The projected image size was about 3 feet by 4 feet, and its peak brightness was 18 foot-Lamberts. The screen had a gain of 2.5 and a vertically corrugated structure that increased the horizontal viewing-angle range to about 70 degrees. The screen was polarization maintaining but not polarized, so a separate sheet polarizer had to be used. Adding the polarizer increased the contrast by a factor of 2.4, which showed some of the effect expected from the multilayer polarizers on the ceiling light fixtures.

The multilayer polarizer story is rather interesting and even slightly controversial. The vertically polarized light is more effective for tasks like reading because the horizontal polarization is responsible for glare. Therefore the claim is made that the total electrical power consumed in the electrical lighting system can be reduced with the use of multilayer polarizers. The Federal Energy Management Program of the Department of Energy currently plans to demonstrate this effect at federal facilities [2]. The energy-saving, commercially available multilayer polarizers are not very strongly polarized. In the 1991 experiment, the light falling on the screen was measured to be only 25% polarized, which should have increased the contrast by 30%. On the other hand, if the lighting power could be reduced by 50% or more, as claimed, then at least another factor of 2 increase in contrast would be achievable.

Company Background

Bellcore is a leading provider of innovative, state-of-the-art software, training and consulting services for the telecommunications industry, as well as other information-intensive industries and organizations. The company has over 5800 employees and $1B in annual revenue.

Bellcore holds over 600 patents in diverse areas including multimedia-network delivery techniques, information and network integrity, fraud management, lithium-ion batteries, information-retrieval algorithms, and database innovations geared towards large-scale systems and privacy issues.

To Follow Up

Bellcore is interested in licensing this patented solution to interested parties. To follow up, please contact Lanny S. Smoot at Bellcore, 445 South St., Morristown, NJ 07960 (973) 829-4476 (fax -5963) or send him email at


1. David A. Braun, Terence J. Nelson and Lanny S. Smoot, United States Patent 5,335,022 “High-Contrast Front Projection Video Display System,” August 2, 1994.

2. “Upcoming FEMP Initiatives Unveiled,” Energy Conservation News, Vol. 19, No. 2, September, 1996.

(last updated January 18, 1998)

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Copyright 2003, Terence J. Nelson