Articles and whitepapers

Projection Screens for Home Cinema (3/3/2005)

By Paul Lawrence, Anders+Kern UK

Consumers have many exciting choices of home cinema screen, including plasmas, LCDs, rear projection TVs and two-piece projection systems. Just a few years ago, a 28-inch TV hooked up to your two-channel stereo was considered to be a hot system! Well not anymore. Today's consumers want and expect their home cinema experience to sound and look just as good as the local theatre - and just like the local theatre, a two-piece projection system consisting of a separate projector and screen, is the best choice to deliver just that.

The importance of the screen

Depending on the equipment involved, there can be a great difference between the picture quality of video displays and projectors. Both can look poor if set up wrongly, but when set up correctly, a screen and projector can offer a much larger and clearer image than a plasma or LCD.

There are many different types of screen available, from simple manual pull-down screens and fixed-frame screens to electro-masked screens and trapdoors. Whatever your budget, remember that the screen is as important as the projector. Most people replace their projector more often than their screen, so it is worth getting the best screen you can afford.

Screen terminology

As with any technology, screen technology has a language of it own. The following is a basic explanation of some commonly-used terms:

Gain

Gain is the industry term used to measure the amount of light reflecting from a screen surface. No actual light 'gain' takes place. Gain measures the screen brightness and directional characteristics. A matte white or unity screen will have a gain of 1.0. For a home cinema, a screen with a gain of 0.92 to 1.3 will fit most applications.

The gain of a screen also affects the viewing angle of the screen. A screen is a passive device - it cannot create energy. As the gain of the screen is increased, more energy and thus more brightness is focussed towards the centre of the screen, so the outer edges of the screen lose their brightness. This is a trade off. The brighter the screen, the smaller the viewing angle, so, using Stewart Filmscreens as an example, a screen with a gain of 1.3 will have a viewing angle of around 160 degrees, whereas a screen with a gain of 1.0, will have a viewing angle of about 180 degrees, and a screen with a gain of 0.92 will have a viewing angle of 160 degrees.

Aspect ratio

Aspect ratio defines the shape of the rectangular picture in a TV set. It is the width of the picture relative to the height. Our standard TV picture, in terminology used by the A/V industry, is 4 units wide by 3 units high, or 4:3, and can be measured using the following formulae:
Diagonal x 0.8 = Width
Width x 0.75 = Height

Another common aspect ratio is 16:9 used for HDTV. This is measured as:
Diagonal x 0.87275 = Width
Width x 0.5625 = Height

Obviously, the native aspect ratio of the projector used, will dictate the aspect ratio of the screen.

Screen size

The general rule for screen sizes is as follows:
Minimum Viewing Position = 2x Screen Height
Maximum Viewing Position = 8x Screen Height.

For HDTV it was found that for high quality images, the optimum viewing position is 3.3x screen height.

Screen and projector characteristics

Although a screen is a passive device, it can add a great deal to the quality of an image. Contrast, brightness, and perceived black levels will all be affected by the screen material used. Contrast is simply the difference between light and dark areas. As you add ambient light, you will lose contrast, so some screen materials are designed to reduce the effects of ambient light, allowing them to be used in daylight conditions. The brightness of an image can also be enhanced by adding gain to the material.

The effect of ambient light (image courtesy of Stewart Filmscreen)

Blacks are never really true black but dark grey because projectors do not produce the colour black. Black is perceived in the area of the image in which the projector produces no light output, and these perceived black levels can be enhanced by using a screen material that adds contrast to the image.

Choosing a screen

When deciding on which type screen to use in your home cinema, you need to ask yourself a few questions. Firstly, in order to determine the aspect ratio, you need to know in which format your source material will be, such as DVD, laserdisk, or terrestrial, satellite or cable TV. If you plan on watching in more then one format, then a masking screen is a good option. Masking screens use retractable panels to 'mask' the image to the desired format, providing a suitable black frame around the image at all times, with no grey bars.

The next question to ask is what the light conditions during the main viewing hours are. Ambient and direct sunlight can affect image quality and must be controlled. Black out blinds may be me needed if ambient light is high during main viewing since most home cinema projectors are not that bright.

The size of the room and whether the screen will go on the short (most common) or the long wall, will help determine the best material and gain your screen should have. The seating should be arranged so that it is in the viewing cone for the screen, as different surfaces have different viewing cones. Also, the location of windows and other ambient light need to be taken in to consideration.

Next, decide if the screen can be fixed to the wall permanently or if it should be out of the way when not in use. If the latter is the case, then a manually retractable screen, or better still, a motorized screen, can be a solution. Whatever the case, aesthetics are also an important consideration. How will the screen and/or its frame look? Screen frames and housings can come in a wide range of styles, and as far as the audio is concerned, speakers do not necessarily have to be placed adjacent to the screen, but can be positioned behind it if an acoustically transparent screen material is used. It is also worth noting that many screen manufactures are now developing what are called 'media screens'. These have the appearance of a plasma TV and have the added benefit or working in high ambient light.

Installation and maintenance

In many home cinema installations, it is more than just the screen that is being setup, in which case installation is best done by the trained professional - especially if the screen is being integrated with third party automation and control systems. Where screens are mechanical/electrical devices, a service contact is recommended.

Summary

The choice of screen is important in ensuring that you maximise the potential of your projector and get the optimum image quality. A good screen will greatly improve your projector's image - even a low-end projector can produce a great image with the help of a good screen.

Paul Lawrence is the Internal Sales Manager for Anders+Kern UK Ltd, distributor and integrator of leading displays, projectors and screens.

www.anders-kern.co.uk