Fig. 7 (a) Front protection; (b) rear projection.

limits of acceptability. The projection screen is a major component in determining visual comfort.

A variety of screen types are available for both front and rear projection. They differ significantly in their characteristics, affecting both the appropriate size of viewing area and the tolerable level of ambient lighting.

Projectors The reader is cautioned that any discussion of projection devices can only report on the existing ranges and characteristics of commercially available equipment. Technological developments can render much of today's equipment obsolete; at the time equipment is selected, the newest models should be investigated for improvements in optical systems, lumen output, remote capability, ease of operation, and cost,

Overhead Projector. The overhead projector currently is one of the most popular projection devices in classrooms. Ease and speed of transparency-making, high lumen output, elimination of the need for room darkening, and ease of operation are among its good characteristics. The only special requirement for this projector is a tilted screen in order to prevent keystoning of the imago. Projectors range from fanless desktop models to those which include the projection of slides and filmstrip through the projector's optical system.

35 mm Slide Projector. The classroom use of 35 mm color slides has substantially increased with the production of inexpensive, foolproof 35 mm cameras and remotely controlled projectors. With the low cost of slido production and the space savings in storage, the 35mm slide is being used more and more extensively than the 3%- by 4-ln. slide. For efficient use in classrooms, the projectors should be capable of remote on-off, forward-reverse, and focus. Ideally, the fan should have a thermal device to allow cooling of the projector after the lamp is turned off. Highly desirable characteristics tor a projector will be ease of loading, low coat of slide trays that accept all sorts of mountings, and freedom from jamming. Lenses ere available that will allow projection from as short a distance as t W (1 width of the screen) for rear projection to 6 W for front projection in larger auditoriums. With improvements in lumen output, mirrors can bo used to reduce the space required for rear screen projection. Projectors ere available with xenon light sources, random access, digital readout, and audio projector programming.

3 hy 4-in. Slide Projector. Most 3/*-by 4-in, slide projectors manufactured today are of the manually operated type, For the purpose of this report and where remote control of the projection device is necessary. the few remotely operated projectors currently available are discussed. Some of the most desirable characteristics of these projectors are high lumen output, the capability with an adapter of projecting 35 mm slides, the ability to handle polaroid slides, and short-throw lenses for rear screen projection.

Motion Picture Projectors. Currently the mm projector has a monopoly on motion picture projection in the classroom. However, with the introduction of the new, large-frame 8 mm film and sound cameras, there may be a marked changeover to 8 mm for small-group use, and it will be introduced for independent study, Film projection using 35 mm film has never found extensive educational application.

Film Strip Projectors and Prevtewers The low cost and availability of film strips on almost every subject make this form of visual aid attractive to many teachers. Projectors with remote control are required for rear-screen projection. Simple film strip previewers may adequately serve the student studying independently. In between are projectors appropriate tor front projection with small groups

Television Projectors. Television projection has its greatest implications for large-group instruction. Its use as a method of displaying and magnifying gross images is excellent. Its promise as a first-rate teaching tool is dependent not only on its ability to have good contrast, brightness, and definition but also simple maintenance. Projectors are available for closed circuit or broadcast in either black and white or color and black and white. In general, the more expensive the projector, the more acceptable the image and the higher the lumen output. Projected television requires slightly more than a 2 W throw distance. Prices vary tremendously from $2,800 to $50,000 or so.

More Information. For more comprehensive information on projection equipment the reader is directed to the Audio-visual Equipment Directory, National Audio-Visual Association, Inc.. 1201 Spring Street, Fairfax, Virginia.

Space for Rear Projection. In designing for rear projection, one of the problems the architect faces is the allowance of the correct amount of space for the location of the projection equipment. Figure 8a shows a projector located at a 1 W throw distance and indicates the maximum bend angle for seat A as over 75 This is unsatisfactory for this seat; the allowable bend angle is established by the screen characteristics, and at present, the maximum bend angle is 60, Figure 86 shows a 2 W throw distance and a maximum bend angle at seat A of about 60. which is satisfactory. Figure 8c shows a total depth of rear projection area as 1 W. but by using a mirror, it still permits a 2 W throw distance and a 60° bend angle,

Mirrors Reduce Light. The use of mirrors, however, has its drawback in that about a 10 percent loss of image brightness occurs. One must also be careful of reflections of ambient light of other projectors or classroom light passing through other screens and affecting either the mirror or the screen. This can be combatted by locating black drapes to mask the projectors from this stroy light.

Some General Rules. A few general rules are helpful in locating projectors and establishing space for rear projection equipment;

• The larger the screen, the longer tl»e throw distance.

• Conversely, the smaller the screen, the shorter the throw distance* Mirrors may be used to fold the projection beam for space saving with smaller screens or with projectors with high lumen output on larger screens.

• For initial schematic design a 2 W depth behind ell the screens should be allocated for the rear projection area,

• The use of extra closeup lenses decreases the viewing area, and may result in some distortion around the edge of projected images.

The Viewing Area Viewing Area Not Critical in Most Classrooms. Before projected materials were introduced, the objects to be viewed in the usual schoolroom were the instructor, the chalkboards, nnd sometimes maps and charts. The instructor was free to move about the room, and the other objects of visual attention were usually distributed over several wall areas. All of them received their illumination by Ihe general lighting of the room itself With no fixed area ot attention, sightlines and viewing were not critical as long as the general lighting was adequate.

Projected Images Restrict Viewing Area For the effective use of visual aids, however, the requirements for good viewing are much more demanding. The projected image necessarily occupies a fixed position, and, except on the TV receiver, is in a flat plane. Whereas a three-dimensional object may well be viewed from the side, a flat picture can be seen intelligibly only within the limils of a "cone of view." To see the image properly, the viewer must be within the limits ot this cone, and neither too near the image nor too far from it. The area defined by these limits is referred to as the viewing area. Its importance in the planning of spaces for image viewing is fundamental, whether the space be a small informal conference area or a large lormal lecture hall.

Shape of the Viewing Area. The shape of the viewing area, then, is approximately as shown. Its size is always based on the size of the image to be viewed. The human eye comprehends detail only within a limited cone angle (about 2% min of arc), and the length of chord subtending this arc. e.g. the image width, varies with its distance from the observer, Thus an object 20 ft away and 6 ft long appears ihe same as a similar object 10 ft away and 3 ft long. The size of the viewing area is determined by three dimensions, as shown in Fig 9

* The minimum distance (I), which is the distance from the nearest part of the image to the eye of the closest viewer

* The maximum distance (2), which is the

distance from the furthermost part of the image tothe most distant viewer

* The maximum viewing angle (3), which is the angle between the projection axis and the line of sight of a person located as far from this axis as he can be and still see all image detail in proper brilliance

Two Ways of Establishing the Viewing Angle Whether ihe apex of the maximum viewing angle should be located at the screen or at some other point on the projection axis is a moot point. There is some disagreement among authorities, too, as to how it should govern the side limits of the viewing area Some prefer the use of the edge angle," while others use the angle at the center of the screen. By either approach, the limits defined are essentially similar. In this study, an edge angle of 40 has been used in laying out viewing areas for rear projection, since it is felt this best represents average screen characteristics. With front projection, the use of the 'center angle" is probably more common practice, and its values range from 20 to possibly as high as 50 . The maximum value of the angle used in determining the viewing area tor receiver TV is 45

Minimum and Maximum TV Viewing Distances

Size ot

Mm viewinu

Max viewing

TV tube

distance, 4 W

distance, 12 W

17 in

4t1-11 in.

14 h-B in

IS in

5 1t-l it».

15 ft-2 in.

21 in

B ft-4 in

19 ft-0 in

23 in.

6 ft-B itt

19 lt-4 in.

24 in.

7 ft-6 nt

21 ft-5 in

27 in

9 It-B in.

24 ft-5 tn.

Defining Minimum and Mmtimum Viewing Distances. Practical minimum and maximum distances are both expressed as multiples of the image width (W). They vary both with the medium being used and with the type and quality of material being protected, and may be affected also, in some degree, by personal preferences. They have not yet been precisely determined by scientific methods, and it is doubtful that such data would have much practical value anyway, The generally accepted values, resulting from numerous studies, are those [See Fig 10 )

Minimum distance Maximum distance film, slides and pro/ected TV

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