teaching aids W. ROBERT BARNARD The Ohio State University Columbus. ohio
Maintaining consistent levels of illumination in the cl:rssroom with adequate contrast on the projection scrren measurably assists the student in his tasks. The problem is to obtain sufficient screen brightness t o obtain a picture with adequate contrast in a welllighted classroom or laboratory. The selection of a projection screen as well as the selection of a projector should not be made independently, but should be coordinated wit,h seating, room lighting requirements, and specific uses projected for the classroom. There are 7 factors t o be considered in selecting a projection screen: 1. Screen brightness required 2. Ambient light problems 3. Necesssrv rontrast of ~roiectedimam 4. Screen reflection and gain factors :i. Pieburo sire 6. Projection lens focal length 7. Projection light output,
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Items G and 7 are discussed in ACBpublication no. 18, "Modern Teaching Aids for College Chemistry," and will not be covered in this article. The rcquircd screen brightness should be about 12ftLamberts for motion pictures, calculated as follows: Screen brightrmss = (screen gain X projector lumen output)/sq. ft. of picture men
Contrast is defincd as the ratio of screen brightness t o
Projection Screens and Chalk Boards in the Modern Chemistry Classroom
non-image brightness, and non-image brightness is the product of ambient lighting and reflective characteristics of the screen and can be one of the most import,ant parametcrs in selecting a location, size, and type of screen (see Fig. 1). Screen Surfaces
The "standard" screen material for years has been the matte surface which is a smoot,h non-gloss white or aluminum surface t,hat reflects light evenly over a wide area (50' from the projection axis). Picture brightness is less than that from other surfaces, but it is free of texture or "hot" spots and is a popular choice for use with the overhead projectors which usually provide ample picture brightness. Beaded screens have white surfaces with imbedded or attached small glass beads or silica chips. Most of the light reaching the heads is reflected back towards its source. A beaded screen provides a very bright image for viewers seated near the projection beam, but as a viewer moves away from the beam the image brightness decreases rapidly. At an angle of 20" from the beam axis the image brightness is about the same as that on a matte screen and beyond this, angle brightness is less than the matte screen. Recently, specular screen surfaces have been introduced which offer more control of projected light and help solve ambient lighting problems. Patterns of very small opt,ical shapes molded into the screen fabric act as lenses and direct reflected light into asymmetrical patterns. Light normally directed t,owards the ceiling or floor is attenuated, and the distribution of projected light is effcctively re-directed towards the observer. Light output is the highest for all types of screens to a useful viewing angle of .50°. Lent,iculated surfaces may be "silver," white, aluminum metal, or "pearl" surfaced, which affect their reflect,ive Suppliers o f Front Projection Screens
Figure I. Brightness curve3 for some projector Rnishes and types. Projection screens ore toted or to gain: the rtondord-o freshly m o p e d block of MgCO8-is said to have a goin of one at O 0 viewing ongle. A rcreen with o gain of two would reflect twice or much light os the reference under standord conditions. Brightness curves for some projector flni.hes and types are presented. Obviously, rating 'a projection rcreen by goin is meaningless without ipecifying on ongle relative to the projection axis. Screens are o v d o b l e with published goinr of three at SOD and os high as six at narrow
Bodde Screen Co. 11341 Bradley Ave. P.O. Box 711 Ran Fernando, Calif. Da-Lite Screen Co. Warsaw, Ind. Hurley Screen Co. 26 Sarah Dr. Farmingdale, N. Y. 11% Volume
Knox Manufacturing Co. Schiller Park, Ill.
Radiant Manufacturing Corp. 8220 N. Austin Ave. Morton Grove, Ill. 60033 Universal Screen Ca. 72.i1 N. Tanla Ave. North Hollywood, Cniif. 91603
45, Number 8, August 1968
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MATTE WHITE
BEADED
LENTICULAR
Figure 2. Screens are mode in many types, most of which give non-rymmetrical light reflection. The incorporation of glass bead% silica chips or polterns molded into the screen wtfoce produce distributions which con enhance points 1 through 4 givenin the text.
efficiency and ability to faithfully reflect color. (Metal surfaced screens tcnd to intensify and separate color.) Lenticulated screens should always he mounted in stretch frames. All other types of screens over 4 ft wide should have some mechanism t o pull the fabric tight; otherwise, expect a t least 6 inches of the fabric on each side to curl. The three types of screen surfaces are shown in Figure 2. The wide choice of screen characteristics allows freedom of screen locat,ion and permits improved contrast and brighter pictures in the classroom or laboratory. Rear Projection Screens
Where the front projection screen is designed as a reflector, the rear projection screen serves as a lens, dispersing transmitted light t o create images. Rear projection screens are useful if there are limitations in locating the projector on the viewing side of the screen or if it is necessary to show films or slides in rooms with high light levels such as laboratories. Rear screen projection also affords the instructor access t o the face of the screen; and, since the screen surface facing the teacher is smooth, water soluble markers may be used directly on the screen face t o add details or data to supplement projected information. I t is much easier t o obtain poor results with rear jcreen projection systems, especially in sizes larger than 3 ft wide, due to the number of combinations possible in screen characteristics and projection light paths. Rear projection screens are madc on Plexiglas, glass, or flexible plastic on which one surface has been matted or coated with a pigA mented carrier hav, >( I ing an index of re: : fraction higher than the carrier. Pigments may he dark to minimize reflected room lights and thus enhance image contrast in brightly lighted areas, although reducing overall brightness, ! a.DBSE.VE% or light - colored t o picture Figure 3. Becouro the bend angle for light brightness. transmitted from a prolector a t location B to Unfortunately, obrenersrining atwide anglesfrom therear w e e n is greater, rorolutton m d resolution tends to pmie6lion m o g e brightness ore reduced. suffer in rear screen B
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systems as does hrightness distribution (gain) depending largely on the distance the projector is from the screen (see Fig. 3). The farther away the projector is from the screen, thc smaller the "bend" angle required t o direct light t o the observer, Near the edge a rear projection screen must bend light a t greater angles for a short focal length lens than for a long focal length lens. Sometimes mirrors are used to fold the projected light path to enable the use of long focal length lenses, but refractive errors due to the mirror system tend to degenerate the image. A single mirror must he used t o project motion pictures via rear screen to invert the image. Slides can be reversed in their holders. TV projection works nicely with rear screen systems; images can be reversed electronically, and the optics associated with current models have wide-angle lens characteristics. Rear screen systems employing flexible screens over 4 f t wide should have the capability for darkening rear screen compartments to control stray light which reduces contrast and occasionally projects shadows of the projectionist, as well as sound treatment to minimize equipment noise transmitted through the screen. Portable rear screen systems can be purchased as integral systems with 8mm projectors from Fairchild Camera & Instrument Corp., 221 Fairchild Ave., Plainview, L.I., N.Y. 11803; h4.P.O. Videotronics, Inc., 528 N. Michigan Ave., Chicago, Ill., 60610; or Technicolor Inc., P.O. Box 517, Costa Mesa, Calif. 92627; and with 35mm slide projectors from Alotiva Ltd., 87-73 I h g s t o n PI., JamaicaEstates, N.Y. 11432; or they can he assembled in the department shop. Plans are available from Eastman Iiodak, Publication No. 5-29, "Rear-Projection Cabinets." Cabincts are available from Busch Film and Equipment Co., 214 S. Hamilton St., Saginaw, Mich., Hudson Photographic Industries Inc., Irvington-On-Hudson, New York; Staples-Hoppman Inc., 3240 Duke St., Alexandria, Virginia; and H. Wilson Corp., 546 W. 119th St., Chicago, Ill. 60628. Screen materials are available from Polacoat, Blue Ash, Ohio. Sheldon Equipment Co., Muskegan, Michigan, manufactures a "chalkscreen" projection center (no. R-2980) which contains a 42-in. rear screen with a suitable writing surface, integral support for a projector, and storage space in a 47-in. wide cabinet designed to match their laboratory furniture line. For multiple screen projection, e.g. rear screen, film, slide, and overhead front projection, two screens are required since combination front and rear projection screens are not presently available. A rule of thumb to calculate screen size is that a student should sit no further back than 6 times the screen width (6W). This can he modified t o 5W when projecting in a lighted room (see Fig. 4). The adaptation brightness of the eye is related to comfortable seeit~g;if the eye h m Iwrn r~dnptrdr u rhr surround~ngsI I ~ R hieh level id t>riehrt~ra>. rhcrr rhe nicrure 3 h d d tnnwl~ that brightness to maintain consistant visual acuity
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Chalkboards
Although its usefulness in large auditoriums has been debated, in most classrooms the chalkboard is one
Figure 4. o, Screen locotionr in the clawoom are urudiy dictated b y the r i l e and position of the chalkboard; many room design, preclude rim& toneow use of the board and screen. I f there i s not adequate space above the board, accorionally the rcreen can b e locoted b o n e ride, high enough to minimize obrtructed viewing and tilted to facilitate o perpendicu. lar proiection oxis and minimize reflection~fromroom lighting. b, In the example given here a beoded rcreen would not be a good choice for the room. Room liohtr over the rcreen should b e directed rliahtlv tawmrds the
DEMONSTRATION
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SPARKS FROM A LECTURE HAS BEEN REPORTED BY G. P. HAIGHT AT THE UNIVERSITY OF ILLINOiS.
of the most prominent and used features on the front wall, but it also is one of the most ignored by classroom planners. The effective writing area of the chalkboard is in the 3-61/2 ft range, vertically; and the horizontal dimension is subject to the same criteria as a projection screen: 50' maximum angle for detail perception. The different kinds of boards come in lengths from 6 t o 16 ft, and it is desirable t o have a one-piece board if possible. A maximum of 10 ft is suggested for boards that have moving panels because of their inherent unsteadiness. For this type of board, motor-operated ljanels arc well worth the init,ial investment. Chalkboards are available in a wide variety of materials and colors but the abrasive surface of the board is the most important factor because it determines the
effectiveness (line width, erasing ease, glare control, etc.) and life of the board. Hardboard makes a durable surface but relies on paint to provide the abrasive surface, and under continuous use it has a useful life of several years. Glass boards have the advantages of less chalk dust and easier cleaning and maintenance; however, there is always t,he possibility of their brealiing in a frequently used portion of the hoard. "3Ietal" boards, made of porcelain-enameled st,eel, have an excellent writing surface and erase easily, as well as being handy for the use of magnetic models. Recently a continuous chalkboard of revolving panels constructed on a linen base has been introduced in Canada. I t is called the "Unique" and is available from: 3Ic N e a l Stevenson Ltd., 435 Midwest Rd., Scarborough, Ontario. A good guide for anyone comparing chalkboards is the manufacturer's warranty for a propcrly installed unit. There are many combinations of color for chalk and chalkboard. Besides the standard white-on-black and the now familiar yellow-on-green combinations, white chalk on a blue board, which is supposed to reduce glare, is now in use. Another innovation is the white board which is written on with midc felt-tip markers. Thc manufacturers suggest that this combination increases visual perception because contrast, line width, and color can be controlled better t,han with the chalk-on-darkboard varieties. The white boards, mailable from Eberhard-Faber, Crestwood, Wilkes-Barre, Pennsylvania 18703, and Math-Mastcr Labs, Inc., Box 310, Big Springs, Texas, can also serve as fnmt projection screens. Another approach to improving visual perception is t o improve the chalk used on the dark boards. Large diameter chalk, which allows greater control of line
Figure 5. Above, o chalkboard rhowing portions with and without specid lighting. The most efficient wmy to improve visibility at the chalkboard is to provide a wall bracket employing a high-output or Power Groove fluorescent lamp in a rpecular, parabolic-rhoped reflector; the current recommendation is for 150 footcandles of lighting at the rurfoce. Left, the diogrom shows the conrtruction of such o light. The lamp should be positioned at least 12 in. from the woll and the reflector directed three-quarters of the way downthe woll. Further recommendationr on classroom lighting ore avoliable in the General Electri~publication TP-i 02'"School Lighting."
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width than thin chalk, is available from the American Crayon Co., Sandusky, Ohio. The same company also has developed a fluorescent chalk which produces brilliantly glowing lines when it is illuminated with a uv source. However, the hoards must be cleaned thoroughly or traces of the fluorescent particles on the board ruin the effect. Care also must be taken to illuminate the board without the 3500 A uv radiation falling on the instructor because, while it is not harmful, it will cause fluorescence in the eye which is perceived as an irrititing fog effect and also add a bright glow to clothing laundered in some popular detergents. The chalk, Fluorescent Excello Squares, is available in such colors as Flash orange No. 2143 and Flare red No. 2142. Figure 5 shows the effect of proper lighting on visual perception.
NEW AIDS Teachers who are filming and projecting their own 8mm film mav find that the new film chatters. loses the ~roiector Imrp a d e n d ? rcra~cl~cor projrr!; fir.yermark< Thii w l l h i r w film whirlt 1.na r d t ermd,,nhlrw lo3~.wr~ntrd .&rn that ia c;ts.l) darnnynl, P S ~ C C ~ H I I ~$18 cdr1r:dg~.1md projectors. An inexpensive process has been developed by the Vacuumate Corp., 427 W. 42nd St., New York, N. Y. 10036, which eliminates these problems by giving the film a t,reatment that hardens t,he surface and adds an internal moistener that prevents the film from becoming brittle as well as movidine surface lubrication. The film is sealed against fingermarks, dirt, etc. One treatment lasts the life of the film. Old, brittle films or slides can be "rejuvenated" with the same process.
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General References Eastman Kodak Co., "Audiovisual Projection," Kodak Pamphlet No. S-3. Eastman Kodak Co., "Legibility Standards for Projected Material," Kodak Pamphlet, No. S 4 . Eastman Kodak Co., "Special Issue," Kodak Pamphlet No. T-98.
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VLAHOS,Petro, "Selection and Specification of Rear-Projection Screens,"Journal of the SMPTE, 70, (1961). General Electric. "School Liehtine." 1964. -. TP-102.,~ Ad\,i..ory ('.,until 1111 C d q e Chrrnistr\, ' \ l o d r r r ~'Lhrchiuy Aids fm (.'c,llrgr Ch~rnia'ry," Srrlal hhlir;~riouYo. 18, 1966.
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