Screen selection

A lot has changed in the way A/V and worship have come to interact over the years. Audio systems have enjoyed a long and successful history in this area but video is still a relatively new addition. My father was using a fairly sophisticated sound reinforcement system and a fairly simple overhead projector to augment his sermons for years before we installed a projector, pulled in some wires and entered the 20th century.

Now that it is the 21st century, the gear we installed back then has been replaced, piece-by-piece, as better equipment has been developed. For all the significant improvements that modern, high-definition projection systems provide, however, they still need a place to put the image where everyone can see it. Of course, that’s where the projection screen comes in.

As you might suspect, ‘where everyone can see it’ is not quite as simple as it may seem. A variety of factors determine what will be suitably visible and there are two basic criteria we can use to specify the right screen.

The first is whether or not the data being projected on the screen is going to be clearly visible or if it is going to be ‘washed out’ by the other lights in the room. Control of ambient light, that is any of those other lights, is an important consideration to make since it has the potential to render an image completely invisible.

If we were to project white text on a black background in a darkened room, the brightness of the white text compared to the relative darkness of the background will give the appearance of black and white. No matter how efficient the projector is at blocking light to create the dark areas of the image, however, ambient light that illuminates those portions will cause the contrast between black and white to become less pronounced. Should the contrast collapse too far, the image will be completely lost.

Fortunately, it is not always necessary to eliminate these competing sources of light, but it is recommended that they be directed in such a way that they do not illuminate the surface of the screen. If these lights are adequately controlled, it should be possible to install a matte white surface. This kind of screen provides excellent brightness uniformity and should have the same perceived brightness whether viewed from directly in front or off to the sides, either vertically or horizontally.

Where ambient light is not adequately controlled, it may be necessary to specify a high-gain screen. Gain is listed as a number, which represent how many times brighter than the reference-standard that material will be. Matte white is listed as 1.0, so it is just as bright as the reference. A glass beaded screen might have a gain of 2.5, meaning that it is two and a half times as bright. The trade-off inherent in a high-gain screen is that it will not be as uniform as matte white and it can be possible to see a dimmer image when seated far enough away from the centre of the screen.

Just how far off centre one needs to be before the brightness drops off appreciably is what we refer to as the half gain angle. Sometimes called the viewing angle, this is the other significant number listed in screen specifications. It gives the number of degrees away from perpendicular to the screen’s centre where we measured half the brightness that was available at perpendicular.

The benefit of high gain is not only in that it creates a brighter image, but also for its ability to direct ambient light away from the audience. If light streams in from the windows beside the screen, a large portion of that light will be reflected towards the opposite wall and a minority of the light will actually be directed towards the audience. This will minimise the impact of that ambient light and aid in maintaining the contrast ratio.

Even with excellent contrast, the second important criterion – whether or not the screen is the proper size and positioned correctly – will have a significant impact on visibility. Obviously, the size of the room, especially the height of the ceiling, is going to play a significant role in determining the maximum size possible, but what size is correct?

In general, the height of the screen should be approximately one sixth of the distance from its surface to the person farthest from it. This ratio is based on research on human visual acuity and represents where the average person will be able to easily read appropriately formatted text and comfortably recognise video images. If the screen is any larger than this, it becomes possible to overwhelm the viewers in the front row. If it is made smaller, it will become increasingly difficult for the people in the back row to comprehend what is being shown.

Regarding the position of the screen, it is recommended that it be at least 1.2m above the floor. Different seating arrangements and stage heights will influence this number, but the essential consideration is that the bottom of the screen is elevated above whatever would obscure it, including the heads of the people in the next row forward. Besides elevation, the horizontal position of the screen is also important. As rooms get wider and wider, the angles between the viewers and the screen increase as well. When that angle exceeds 45° it can become very difficult to read text reliably as some amount of distortion results. For very wide rooms, the best approach may be to install multiple screens, each positioned to be visible to a different section of the audience. This will keep the message free from distortion and will also ensure that the benefits of gain, where applicable, will be used.

There are a few more criteria involved beyond the scope of this basic overview, which are important for conveying the message.

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Published in Worship AVL Asia newsletter