Take the Same Care in Choosing Video Screens as You Do with Projectors
Picking the right screen material to use with a large-screen projector is no simple task. The options are numerous, the cost differences enormous, and the impact on your viewing experience significant. Selection and maintenance of a projection screen should therefore be done with as much care as selection of your video projector.
Example of rear-screen projection: Digital Projection utilized 24 Stewart FilmScreen rear-projection screens at its booth at this year's Infocomm.
You should carefully consider screen materials, shapes, and sizes based upon the application. Ultimately, it is the projector/screen combination that makes the difference between killer images and frustrated viewers. In this column, we will review basic considerations when choosing between front- or rear-screen configurations, and variations of screen materials for special applications.
Front Screen or Rear Screen?
Rear screen requires a dedicated space behind the screen and is therefore more costly. The benefits are obvious: better contrast, brighter images, and no obstructions if someone walks in front of the screen. If you have lots of ambient light in the room, rear screen is usually an excellent choice. Rear-screen material is designed so that light from the viewing side of the screen does not directly compete with light passing from the projector.
The screen materials are usually gray, and they enhance black levels in the projected image. Rigid screens can be made of glass or plastic. Glass screens gain their reflective properties based on the coating applied to the glass. Plastic screens are often etched using diamond-cutting tools. Etched screens act like a lens and redirect both the light entering the screen and the light projected on it.
A lenticular screen actually increases the intensity of the light by reflecting more of the projected light toward the viewing axis while redirecting incoming light away from the viewing axis. Think of it as the difference between a floodlight and a spotlight. By changing the width of the grooves etched into the screen, you can change the viewing angle of the image.
Rigid rear screens also allow you to isolate any noise from the projector. With many of today's projectors utilizing brighter lamps, and that equipment therefore generating greater heat, system noise can be significant. Multiple projector arrays are also best accomplished using rear screens because any image masking or projector stacking can be done behind the screen, keeping clutter out of view.
The negative aspects of the rear-screen approach basically involve space and cost. First, consider the space needed behind the screen for the projector. Short-throw lenses for projection systems vary from .8:1 to 1.5:1. The .8:1 lenses are very large and can be expensive. Plan on spending twice as much for ultra-short throw lenses (<1:1) as compared to standard projection lenses.
To determine the throw distance, multiply the first number in the lens ratio (.8) by the screen width. For example, a 10ft. wide image with a .8:1 lens would require 8ft. of throw distance from the screen to the projector lens. That means 8ft. of dead space behind the screen. You can reduce this space by bending light off of a mirror. If we bent the projected image halfway through the throw distance, we could reduce the depth to 4ft. (plus half the length of the projector). In this case, we would have 4ft. in front of the mirror and the other 4ft. from the mirror to the screen (for a total of 8ft.). This would require a mirror greater than 5ft. wide because the image will be 5ft. wide at the end of a 4ft. throw. This again adds complexity and cost. With a standard 1.5:1 lens, the throw distance required would be 50% greater, or 15ft.
The other consideration with rear screens is off-axis viewing. Rear screen images tend to look great if you are standing perpendicular to the screen. As you move from one side to the other, you will start to see a falloff in both brightness and resolution. Every benefit has its tradeoffs, but if you have the space and budget, rear-screen installations can make a clean integrated solution.
Front Screen Material
You can select matte screens, gain screens, silver screens, gray screens, even perforated screens for improved audio performance — the choices are endless. But the starting point is the same: budget and application. Place your emphasis on application. If the system doesn't perform up to expectations, it's not a good investment at any price.
A white matte screen provides unity gain (1) and the best overall color rendition. This is the least expensive and most popular screen material. If you have a good projector with adequate light output and good black levels, a matte screen is a great choice.
Gain screens can effectively provide an increase of image brightness ranging from 20% to about 50% (a gain of 1.2 to 1.5). Introducing reflective material to the screen surface generates screen gain. This results in a narrower viewing angle, but a brighter image within that area. Care must be taken that ambient light is not reflecting on the gain screen, as this will dramatically reduce contrast ratio or black level.
Many of today's projectors are extremely bright, and choosing a gain screen unnecessarily can result in reduced image quality. Light reflected off the screen is measured in foot-lamberts (Ft.L). This measurement takes into account screen gain, and SMPTE recommends measurements from 14 to 20 Ft.L for cinema projections, and 30 to 45 Ft.L for presentations. To estimate your system's Ft.L, take your projector's lumen rating (ANSI) and divide it by the square feet of your screen. Multiply this number by your screen gain, and you should have your calculated Ft.L. Measured results will most likely be lower because projector lumen ratings are typically aggressive and are based on new lamps or CRT's.
Gray screens and silver screens are popular with LCD and DLP projection systems. The reason for this is that the gray base color improves the perceived black level of these projection technologies. It's the black level that creates the perceived contrast, and slight changes in low lights can have dramatic effects on contrast. Gray screens actually have a negative gain (.9). DLP and LCD projectors often have more than adequate light output to generate required Ft.L, but require some help in low light.
Newer gray screens combine absorbing material and reflecting material. These silver screens have a small gain while still enhancing low-level blacks. Careful attention must be used with this type of screen material. If the projector is too bright or the throw distance too short, hot spots or distortion patterns may occur. This material should be used when improved black levels for DLP or LCD projectors are needed, and gain is needed to achieve the minimal Ft.L desired. If you don't need the gain, don't use it.
Another popular screen choice is perforated material, used when audio speakers are placed behind the screen. For home-theater applications, this creates the effect of unified sound and images. This creates dynamic results with center-channel speakers and surround-sound, while effectively hiding speakers behind the screen.
Proceed with caution, however, when using perforated screens with fixed panel displays like LCD or DLP. Perforation of the screen can often create a “beat” pattern with the pixel matrix of the display. This results in a kind of moiré pattern on the screen. Typically, you can remove this problem by adjusting image size or position. New micro-perf screens are much less susceptible to this problem, and the perfs are small enough that they don't impact image resolution.
George Walter is a display industry veteran with more than 18 years experience in sales and marketing of display products. His work with professional displays has encompassed the range of broadcast, cable and satellite, live presentation, and home-cinema applications, using light valve, CRT, LCD, and now DLP technology. Over his career, Walter has worked for General Electric, Barco, and currently Digital Projection.
Email him at [email protected].