SCREEN TOWERS

From SMPTE Data

Drive-in theatres have used all available building materials in the construction of screen towers over the years. Materials such as plywood, masonite, concrete, gunite, plaster, sheet metal, corrugated aluminum and even asbestos have reflected images to audiences nationwide.

A drive-in theatre faces more rigorous problems concerning the screen surface than an indoor theatre. First, because the screen surface is exposed to the weather, it deteriorates more rapidly. But more important is the fact that a drive-in theatre uses more screen area and, therefore, there is less light per square foot of screen available to form the "picture".

Further complicating the drive-in theatre's problem is the fact that the ambient (extraneous) light cannot be controlled. This ambient light appears light on the screen where the picture is supposed to be dark and, therefore, by "washing out" the picture makes the problem quite severe.

The obvious thought at this point is that more light is needed. This, of course, is true -- the more light that can be generated from the lamp, through the film and onto the screen, the smaller is the problem. Unfortunately, there is a rather severe constraint on how much can be put through a film, and on the economics of generating more light.

SCREEN TILT, CURVE AND SURFACE CONFIGURATION
If the screen is tilted and/or if the screen is curved, there is a bad effect caused by first surface reflection when the screen surface is wet. The wetness of the screen surface can be caused either by dew or rain. When this happens the shiny smooth surface of the water acts as a mirror (specular) and, especially along the centerline of the screen in line with the projection beam, the reflection becomes so intense that the picture is completely destroyed.

If the screen is not tilted, then the first-surface reflection is directed over the heads of the audience. In this case, whether the screen is curved or not makes little difference.

When the screen is curved and tilted a rather small area along the centerline of the screen is affected, but it is affected more intensely and over more of the screen area simultaneously. To eliminate this effect almost totally, it has been found desirable to use other than a flat surface on the screen.

A screen constructed of corrugated steel sheeting or corrugated aluminum, with the corrugation running from top to bottom, has been found to eliminate this effect.

A further advantage in the corrugated surface screen is that light from the sides of the screen (if only from one side especially) is illuminating only half of the surface of the screen, and, therefore, the picture is not "washed out" as much. If the screen surface is constructed of corrugated material and also painted the first coat of bright aluminum paint with the second "diffusing" coat, then further advantages are observed.

The appearance of the picture is less affected by ambient light because extraneous light upon the screen tends to be reflected back toward its source. Also, the corrugations shield part of the area of the screen from some of the extraneous light. These, combined, substantially improve contrast ratio, and the apparent luminance of the picture to the audience is high.

The normal distances involved in viewing a drive-in picture screen are such that 1-in to 2-in corrugations, approximately, are not noticeable or detectable in most areas of the theatre. Actually, knowing that the corrugations were there and observing the picture from in front of the first ramp of the theatre made it possible to detect the corrugations. However, at any distance greater than that from the screen the corrugation absolutely could not be detected.

The combination of corrugation and bright aluminum undercoat paint with a finish coat of thin white paint appears to give excellent results even in rather wide-angle theatres. Surface "shininess" while the screen was wet with rain or dew appears to be almost totally eliminated by the fact that the corrugations created an unlimited number of reflection angles and the first surface reflection appears to be negligible when the screen is wet.

It is possible to buy the corrugted material in almost any length, and lengths of 50ft can be readily obtained. In case only shorter lengths are available in your area then it is permissible to overlap the material with the following precaution:

It is normally presumed to be proper to overlap the bottom sheet by the sheet of material above. Unfortunately, all the rain collected off the back of the screen washes all the rust and the dirt down through the crack onto the surface and front of the screen. It is suggested that just the opposite construction be used in theat the lower sheets overlap the upper sheets. In this case the rain washing down the face of the screen will wash the dirt, rust or paint into the crack and onto the back of the screen where it will not affect the picture.

Please note that you must ask the manufacturer or the supplier of the sheeting for the approved type of nail. Ordinary nails or other fasteners may not be suitable because they may tear their way through the metal when there are strong winds. Most approved types of nails have a plastic or rubber-like grommet underneath the head of the nail to protect the material.

Screens built straight up and down and without tilt or curve, and of moderate size with a corrugated metal surface that has been painted first by bright aluminum and then by a thin coat of white have generally done the best all around job under difficult weather and ambient light conditions.

"DAYLIGHT" DRIVE-INS
A great deal of research was conducted in the development of a pre-dusk drive-in theatre screen, also known as the infamous "daylight" or "containment" screen. The principles involved lenticulation of an extruded aluminum surface or a highly reflective vinyl plastic surface, which would enable showing of pictures during daylight hours. The screens, however, proved too expensive and fragile to be feasible. Problems such as excess cross transference of light, inability to maintain uniform striation of lenticulation, reflection of too much light or developing a mirrored surface, the necessity to design and use custom optical lens and prism systems and the necessity to revise the angle at which the screen tower is erected made it impossible to implement these screens.

TOWER STRUCTURES
Materials used for drive-in screen towers have included wood, concrete and structural steel. The only real choice today is steel. The decision by the industry to use structural steel has resulted in fairly standard-sized tower structures. In design, these generally include a series of typical vertical trusses, diagonal knee braces and an optional skin or enclosure at the base of the picture area. The knee braces can be eliminated by using a vertical cantilever. Screen tower foundations, of course, must be designed for the particular soil bearing value of each tower site.

Experience dictates that, generally, a series of grade beams, the tops of whcih aere flush with adjacent natural grades, is the most practical approach to the ultimate possibility of pouring a concrete slab within the tower area for future storage. Caissons can very simply be connected to the perimeter grade beams (when required because of lack of soil bearing value). A typical structural steel tower, in the normal two-to-one width-to-height ratio can deliver a creditable picture without an ornamental enclosure at sides or bottom. But certain theatre operators prefer enclosure so that the base may be developed for storage and the outside tower can provide suitable background for a theatre or marquee sign.