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Publication numberUS2238267 A
Publication typeGrant
Publication dateApr 15, 1941
Filing dateDec 19, 1938
Priority dateDec 19, 1938
Publication numberUS 2238267 A, US 2238267A, US-A-2238267, US2238267 A, US2238267A
InventorsDavid Katz
Original AssigneeDavid Katz
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Lecture and projection desk
US 2238267 A
Images(5)
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Description  (OCR text may contain errors)

' April 1,5, 1941. D. KATz LECTURE AJNp PROJECTION DESK 4Fin-1d Dec. 191 195s 5 sheetssheet 1 -INVENT'O R VApril 15, 1941. D. KATz 2,238,267

' LECTURE AND PROJECTION DESK Filed-Dec. 19, 195s A 5 sheets-sheet 2 April 15, D.y K ATZ LECTURE AND PROJECTION DESK FiledDec. 1 9, 1938 5 Sheets-Sheet 5 FIG. a

INVENTOR April 15, 1941. E D. KA'rz 2,238,267

' LECTURE AND `PRoJEGTIoN DESK FledDec'. 19, 1938 5 Sheets-Sheetl pril 15;, 1941. n. Mz 2,238,267

LECTURE AND PROJECTION DESK Patented Apr. 15, 1941 "*UNlTED STATE s PATENT oFFlcE -movements in lecture and projection desks of the type described in my copending applications,

Serial Nos. 192,755 and 241,530 (Patents Nos. 2,lu1,l33 and 2,181,134, issued November 28,

lin my lsaid copending applications, I have described acombination lecture desk and projection table whose principal distinguishing characteristic is that it possesses facilities whereby writing clone on the top of the'desk by a lecturer ,is slmul- -taneously projected through the interior ofthe 1 desk upon a substantially ver-tical screen, facing the audience. Brieiiy speaking, said combination, which will for brevity hereinafter be referred toas a Scriptoscope, comprises the following essential parts, A lecture table having a side and walls of opaque material, forming a substantially inclosed dark chamber, is provided in-the ltcp thereof with a transparent writing panel having a fiat surface, preferably, a plano-convex lens. Above this panel, exterior to the desk chamber, is provided a flood light or other convenient source of illumination, which is adapted to. direct a concentrated beam of light through said transparent panel into the interior of the desk. Within said interior, below the writing panel, and in position 'to interceptthe rays of light coming downwards through the latter, is a projection system comprising a focusing lens and a mirror or a system of mirrors,'adapted `to re' direct, focus and project upon a translucent ver-y ticalscreen, usually a sheet of fabric, any light rays and shadow rays coming from the writing plane. When the lecturer writes in ink on the writing panel, the writing is'thus focussed upon the translucent screen, and goes through said screen to face the audience, which in turn faces the lecturer.

The writing may be done by the lecturer directly on the iiat upper s face 'of the writing panel, but it is preferable to employ a sheet 'of transparentv paper or other light transmitting ofthis invention will become more :apparent from y the following discussion of the problems involved in the practical use of the Scriptoscope.

In my Patent No.2 ,181,133 I have illustrated the Scriptoscope by certain models wherein the projection-screen was directly in front of the lecturer as he was facing his audience, and substantially the entire area was disposed below the level oi the top of the desk., These models instantly introduce several practical objections, as follows:

1. Shape of desk and dimensions-If the distance from projection screen to focussing lens is about 45, and if the latter is vertically below the center of the writing panel which in .turn will in a practical design be about l5" from the edge of the desk nearest to the lecturer, the .total distance from lecturer fto projection screen will be about 60". Therefore the length of the desk, that is its front .to rear dimension asmeasured along the line of sight from lecturer to audience would have to be 6G, a rather inconvenient dimension, wasting a great deal of space and producing an awkward design.

' 2. Height of projection screen.-If the er1--l tire area of the screen is below the level of the top of the desk, and if theaudience is situated in rows of chairs behind each other and obstructing material, which is stretched fiat against the writing panel, and which may be moved out' of the operating region when used up and replaced by a fresh sheet from a convenient supply.

My present invention has for its objectto improve the Scrlptoscope in several important respects, particularly usato general shape and size, as to disposition of the` projection screen for best view ,by the audience, vetnias lto elimination of undesirable images" or reflections from the field of view on the projection screen.` The nature of these improvements and of the general objects 55 each others view, the desk would have to be.

placed on a platform 3 or 4 feet high to he visible to every person in the audience. Such a high platform is easy to achieve in a large hall or auditorium, but would look rather unusual in an ordinary classroom of average height.

3. Undesirable images or "ghosts.-But in'addition .,to' theabove rather obviousproblems, a Vproblem of quite unforeseen nature developed during the course of my experimentation. I

found to my dismay, that while the projection I observed by the audience is very clear and pronounced and can be discerned clearly even in daylight ,'(excludingbof course, direct sunlight) from various corners of an average sized room, the projectionvbecomes in part obliterated by an annoying glare to persons situated squarely in front of the projection screen. This glare comes from the appearance of a very bright spot of light or image at some point behind the screen which filters through the cloth and weakens the pro.- jected script in a clrculararea of substantial size. Thisimage will generally move as the observer moves his head, and' for this property, as

well as for its general annoying quality, I have named it the ghost. l y

The disagreeable action of the ghost is limited to an area spreading out fan-wise from the projection screen to Ithe opposite end of the lecture room, and covers an angle of about 12 to 18 to each side of the middle line. Anybody sitting outside this angular area can see the projection clearly and without interference; but the seats in the very middle of the auditorium, which normally constitute the choice seats for a lecture, are exposed to the interfering action of the ghost.

Now according to my present invention, some of the above difficulties, or all three oi them are successfully eliminated by rearranging the optical system of the Scriptoscope so :as to employ a plurality of internal mirrors instead of a single mirror, and correspondingly redesigning the shape and dimensions of the desk proper. rIhe rearrangement of the optical system is done in such a manner, that the rays of projection shall not travel in a straight line from the focussing system to the projection screen, and that rthe last segment thereof shall approach the projection screen at an angle greater than zero with the line of sight from the projection screen to the average observer in the audience.

For a better understanding of this and the other important features of my invention, reference is made to the accompanying drawings, which form an integral part of this invention. In these drawings- Fig. 1 is a perspective view of a Scriptoscope according to this invention whose chief function is to overcome the first problem (neat and compact design) without particular provision for the other two problems.

Fig. 2 is a perspective view of a modified form of this invention which is designed to solve the first problem while simultaneously assisting in overcoming the other two diiculties aforenoted.

Fig. 3 is a perspective view of the Scriptoscope of Fig. 1 (or Fig. 2) from the side of the audience, the front wall being removed, and parts of the projection screen and desk-top being broken away to expose the interior. Y

Fig. 4 is a perspective view of a different modification of this invention, designed to overcome the first and third diliculties and at least partially assist in solving the second problem.

Fig. l5 is a vertical section of Fig. 4 along the line X-X of Fig. 4, showing the interior arrangement of the optical system.

Fig. 6 is a vertical section of a still different modification of this invention, designed to overcome the first and third difiiculties above noted.

Fig. 7 is a further modification of this invention, showing in section a design which may outwardly follow the style of Fig. 4.

Fig. 8 is a diagrammatic representation of a hall, in vertical section, showing the disposition of lecture desk, platform, and average observer relative to each other, and showing further how the design of Fig. 5 eliminates the ghost from the eyes of the observer.

Fig. 9 is a similar view of a hall wherein the floor of the pit slants downwards toward the platform; in this view the design of Fig. 7 or 8 is preferred for the Scriptoscope.

Fig. 10 is a modified form of this invention, wherein all three of the above noted problems are solved simultaneously. In this modification the projected `area is entirely above the top of the desk, and a platform may be used or omitted altogether.

Figs. 11 and 12 are vertical sections of Fig. 10, along lines X-X and Y-Y, respectively.

lecture hall or class room, showing the relative disposition of the lecture-desk, platform and average observer when using the model of Fig. 10. This figure also explains how this model eliminates the ghost from the field of view of the observer.

Finally, Fig. 14 is a perspective View of one more modified form of this invention which solves simultaneously all the three problems hereinabove noted,

While Fig. 15 is a vertical section of the same along line :r-:r, parts of the housing in the rear being broken away to expose the interior optical system.

Referring now to the figures in detail-In all figures. l designates generally the cabinet of the Scriptoscope, having a top member il, legs l2, outer walls I3, and fioor Il, so as to form a substantially enclosed darkened chamber. In each modification there is a writing panel W in the top thereof, consisting preferably of a plano-convex lens with its plane surface uppermost, fiush with the top of the desk. This writing panel is `adapted to receive and pass into the interior of the desk rays of light coming down from a concentrated source of illumination, such a's a spotlight, in the room above the desk, as already fully explained in my previous applications above referred to.

rEach modification further has provisions for stretching a sheet of Cellophane 6, or other transparent sheeting from a roll 6l, over the surface of thewriting panel W, the necessary tension being applied in any convenient manner, for instance by pushing up roll S2. Finally, each model contains in the interior thereof .a focusing lens L supported on a suitable bracket '1, and on the. exterior thereof a projection screen S, held in place by a frame Has already described in connection with my prior applications above referred to. The modifications shown in Figs. 1 to 3 and 10 to 12 further have inner partitions 8 with round passages therein 8|, for passing along the beam of concentrated light through the interior of the desk while shielding the projection screen S against the rays of light from inner lamp 4, which is needed to provide an illuminated background for the writingpanel, as fully explained in my copending application Ser. No. 241,530.

The modification of Figs. land 3 differs from A my previous designs. in this respect that instead Fig. 13 is a diagrammatic vertical section of a 75 of the single mirror M, following the lens L, this modification has three plane mirrors M1, M2 and M3 arranged so that the rays of light travel successively from one to the other in the order named, and are finally refiected by the mirror M3 onto the screen S. The latter is in this particular model mounted in a vertical plane, in the front wall-of the desk, entirely below the level of the desk-top, but to one side of the line running from the lecturer to the audience. The lecturers position is behind the desk, in the view shown in Fig, 3. Of the three mirrors, M1 is tilted on a horizontal axis, while M2 and Nh have their planes vertical, and preferably parallel to each other.

As will be seen from Fig, l, this design solves very neatly the problem of compactness, but it does not take into consideration the other two problems. The desk would still have to be mounted on a high platform, and the ghost would still be apparent in a limited zone spreading out fanwise from the screen S forwards.

The last dimculty can, nevertheless, be overcome by turning screen S on a vertical axis through an angle of 15 or more counter clockwise, and further turning mirror Ms through half this angle.' The desk is then to be placed on the extreme right of the stage as the audience faces it.V The action of the ghostis then limited to a narrow strip or wedge along the right-hand wall of the lecture room, as will be more readily apparent after the theory of the ghost action has been studied in detail in conjunction with Figs. t and 9 below.

In Fig. 2, the construction of the interior may be the same as in Fig. 3, but the part of the desk containing the projection screen S and mirror Ma is built into the form of a separate section I8 and is raised. so that part of the screen S is above the level of the desk-top. The mirror M1 isthen' tilted sufiiciently to throw the beam of the proiection upwards through a substantial angle to hit the center ofthe screen. f course, the height of the mirror Ms in this case must be increased to correspond. The eilect of this' construction is that the requisite'platform need not be so high, and, provided the lower edge of the screen S is about on the same level with the eyes of the observers, the ghost will not be visible to any of them, as explained more fully in connection with Figs. 8 and 9.

In Figs. 4, 5, 6 and '7,.the idea of the raised projection screen is embodied in a modification havv ing a somewhat different internal arrangement of mirrors. In this modification the screen is di- Y rectly in front of the writing panel, but is inclined on a horizontal axis.. receding either at the top (Figs. 4 and 5) or at the bottom (Figs. 6 and 7). The top of the desk wherein the writing plane is located may also be inclined (as in Figs. 4, 5 and Fig. tv l shows the general appearance of this modification,

7) or may be horizontal (as in Fig. 6).

while Figs. 5, 6 and 'l show several possible arrangements of the internal mirrors and projection screen.

The sections i9 shown in Fig. 4 are not an essential part ofthe Scrlptoscope, but are added merely for artistic balance and to provide room for drawers or' other auxiliary attachments of a lecture desk. It will be noted also, that in the modifications of Figs. 4 to 7, the inner light 4 and the partitions Vtmade necessary by it are omitted. These are superfluous, inasmuch as the projection screen is directly forward of the writing panel and therefore provides an illuminated background as the lecturer looks through said writing panel.

It will be noted that in Fig. 5 the last segment ,mfiiof the beam oi' light as it approaches the screen `has a general upwards direction, while in Figs. 6 and 'l it has a downwards direction, the angle of the median in either case (the axis of the beam) being more than 12 to the horizontal. The principal object of this feature is to eliminate the ghost completely from the field of view'of the audience, as` explained more clearly in Figs. 8 and 9, which are next considered.

4 In Fig. 8, the Scriptoscope of Fig. 5 is mounted on a platform P of sufficient height to bring the lower edge S2 of the screen S to'about the level of the eyes of the contemplated audience'. Consider; ing now the beam of light as it strikes the screen S, it appears from "my researches that the ghostis formed by the convergence of this -beam of light backwards from the" screen S. In other `words, the ghost is the virtual image formed by the beam the ghost is located along the rearward prolongatypical instances.

other hand is particularly suited for auditoriums l with a slanting iioo', or where the audience is is determined by the angle of divergenceV of the reiiected beam. can therefore be determined readily by a geo metricai diagram asshown in Fig. 8, wherein 'th 5 extreme rays B1 and Ba of the beam Bn arejpru-fyA duced backwards until they intersect at G. If 'the mirror Ms be tilted on a horizontal axis, the location of G will rise or fall depending on the direction or the amount of the tilt. w The principle of this modiiication of the inven tion is therefore to give the last mirror, M3. such "l a tilt that the ghost G shall fall below the line E--Sa joining the observers eye E to the lowest portion S2 0f the illuminated eld of the screen S, thereby burying the ghost, so to speak, behind the partition I3 belowSn. The tilt of the mirror .M3 is the sole factor determining this effect. The

inclination of screen S plays no part in this. The

screen- S in inclined' simply to permit the projected beam to strike it as nearly perpendiculariy as possible in order that the'outer regions of the projected field may not find themselves perceptibly out of focus.

In Fig. 9 the same effect is accomplished by giving the projected beam a final downward direction, so as to elevate the ghost G'above theline E'S1 joining the observers eye E to the upper edge S1 of the screen S. `The ghost is thus hidden from View by the upper portion of the wall i3 above pointA S. The Scriptoscope of either Fig. 6

or Fig. 'i may be employed in this layout.

It will be noted that the platform P in Fig. 9 4

need not be so high las in Fig. 8. It will be 'noted further that the layout of Fig. 8 is particularly z suited for auditoruims with a level floor or where the lecturers stage is above the level of the audience. Lecture rooms and small auditoiums are The layout of Fig. 9 on the positioned above the lecturer as for instance in a court room where the judges platform or the jury-box is above the level of the attorney's desk.

`As wm be clear from Figs. 5, 6 and 1 the exact x angle of the last mirror M3 to the horizontal or vertical is not definable, inasmuch as it depends on the particular design and the disposition of the other mirrors in the system. It must therefore be defined functionally. To achieve the hiding of the ghost, the tilt of mirror Ma must be such that the angle formed by the'uter rays of the beam with the line of sight from the observer to the outer regions of the projection screen (angle 0 in Figs. 3, 9 and 13) shall be greater than il. Considering that in the average apparatus the divergence of the beam as it approaches the projectionvscreen (angle BoGB1 in Figures 8, 9 and 13) rwill be about 48 to 18, theabove condition may be restated as requiring that the axisof the projected beam shall approach the midpoint of screen S at an angle not' less than 8 to "18 to the line of sight from the average observer to the midpoint ofthe screen S. As' a matter of safe practice, it will be found advantageous to make 55. this angle between 12 and 25.

The designs of Figs. 2 and 4 to 7 solve the first 2nd third of the above noted problems, but vstill require the useoi an elevated platform. In Figs. 10Yto l5 are shown two models which do not require any platform at all or `require only a low platform of say 6 to 12" in height. In both of these models the audience reads the projection on a member which is totally above the level of the desk top,y and which member maybe inclined to` tion of the axis of renection, and its exact position 751 the vertical at a suitable angie to hide the ghost.

In Fig. 10, whose sections are `shown in Figs. 11 and 12, the projection screen S is located in a horizontal position, substantially flush with the top of the desk. The audience Views the projection, not directly on this screen, but through its reflection S' in a mirror M5 hinged on to the rear edge of the desk as at 2|, -and adjustable to any suitable angle by the aid of brackets 22. If this mirror M5 is given an angle of 50 to 60 to the horizontal, the image S of the projection screen S will be located in a plane at 100 to 120 to the horizontal; that means, with a tilt of to 30 rearwards from the vertical. The ghost G will then become lowered to a point below the line of sight E-Sz from the average observers eye E to the lower part Sz of the reflected prol jection field (see Fig. 13).

To bringA about the desired projection in a4 horizontal plane, four internal mirrors are employed. The mirror M1 which immediately follows the focussing lens L is tilted on an axis which runs diagonally across the beam of light, so as to throw the reflected beam in the direction of the forward, upper, right-hand corner of the desk. Here it is intercepted by mirror Mz which has its face parallel to and facing mirror M1; as a result, the beam of light is reflected in a substantially vertical, downwards direction. Next it strikes mirror M3 whichthrows it rearwards into mirror M4. The latter is supported Y on the backing member l5 and directs the beam of light upwards into the horizontal screen S.

The mirror M5 may be backed by a suitable wood panel 20, hinged on to the desk at 2l; so that after use, this member may be swung into a horizontal posi-tion to cover 1 the projection screen, and convert the apparatus into a fiattopped desk` In Fig. 14 the same effect as in Fig. 10 is obtained by placing the projection screen S entirely above the plane of the desk-top, in a plane inclining rearwards from the vertical through an angle of some 15 to 30 and intersecting the desk-top in a line ush with the forward edge thereof or a few inches to the rear of the same. In this fashion, and also because of the projection is located to one side of the lecturer, the view from the latter to his audience is very little obstructed, and the lecturer can face practically every member of his audience.

The arrangement of the internal optical elements is shown in Fig. 15. Immediately following the writing plane W, is a plane mirror M1, inclined on'a horizontal axis which runs diagonally across the beam of light so as to reflect the latter into the direction of the .lecturers right andi to his rear. Here, at a convenient position, the beam is intercepted by the focussing lens L (which may be supported by any convenient means, omitted from the draw-lng for the sake of clearness), and passes on further to mirror M2. The latter has its face parallel to that of mirror M1, and -therefore redirects the beam downwards in a vertical direction. The beam next strikes mirror M3 which is tilted on a horizontal axis running parallel to the front of the desk and throws the beam upwards and forwards onto screen S. The ghost will be located at some point on the rearwards prolongation of the beam M3-S, and ltherefore will be entirely below the line of sight from audience to projection screen.

The section of the desk containing the planoconvex lens W contains also the inner-illumination lamp il, and is therefore to be separated by Walls I3 or partitions 8 from the section I8 which houses the mirrors M2 and Ma as well as screen S. A circular opening in this partition provides for passage of the beam of light, but is not indicated in the drawing in view of the broken-away portions of the wall behind Cellophane-roll 6l. The section I9 to the left of the desk from the lecturers viewpoint is added merely for artistic balance, and to provide room for drawers or other desk auxiliaries.

The script and printed gures on the writing panels and projection screens of Figs. 1, 2, 10 and 14Y are not part of the construction, but have been inserted merely to illustrate the relative orientations of the writing and projections, which of course must appear normal respectively to the lecturer and to the audience.

The circular arcs on each of the projectionscreens show the limits of the illuminated area, due to the circular outline of the writing plane.

It will be clear that the above specic embodiments are merely illustrative, and that the various features of construction shown in these may be variously rearranged and recombined to produce numerous other embodiments falling within the spirit of this invention as defined by the subjoined claims. In the claims below, the eX- pression the -top of the desk shall be understood as referring to that portion of the top which supports the transparent writing panel.

I claim as my invention:

1. A device of the character described, comprising in combination a lecture desk for use by a lecturer facing an audience, said desk comprising a body of furniture material defining a substantially enclosed chamber within the interior of the desk; a transparent writing panel in the top of said desk adapted to transmit into said interior chamber light-rays coming from sources above said desk; a shadow-transmitting projection screen supported on the body of the desk and replacing part of the furniture material defining said enclosed chamber; means above said desk for directing a concentrated beam of light downwards through said transparent writing panel into said interior chamber; an optical projection system mounted in said interior chamber in optical alignment with said writing panel and said light-beam directing means; said optical system including a focussing lens and an odd plurality of plane mirrors; means for supporting said focussing lens and mirrors in operative relation to each other and with respect to said projection screen whereby to direct said beam of light onto the inner side of the projection screen while simultaneously projecting lthereon shadow-images of opaque figures formed by a lecturer on the upper surface of said writing panel, said supports for the mirrors being further fashioned and adjusted for holding the mirrors in such positions with respect to each other as to give to the final segment of the beam of light as it approaches the projection screen an average inclination of not less than 12 to the line of sight running from the midpoint of said projection screen to the average observer in said audience.

2. A device as in claim l, the projection screen being in a predominantly vertical plane with its active area at least partly above the level of said writing panel in the top of the desk.

3. A device of the character described comprising in combination a lecture desk having a transparent writing panel in the top thereof adapted to transmit into the interior of the desk a concentrated beam of light coming from a source above said desk, an optical projection system including a focussing lens and an odd plurality oi plane mirrors positioned to intercept said beam ci' light and focus it upon a shadow transmitting projection screen `for through projection to an audience facing said screen; screen being in a predominantly vertical plane with its active area at least partly above Ithe level oi the top of the desk and to one side oi said Y transparent writing panel; and said mirrors beinn so arranged with respect to each other and with. respect to the screen as to cause the beam' stantially near the level of the top of the desk said projection y ed in the cabinet in optical alignment with said writing panel and including a focussing lens and an lodd plurality of plane mirrors; and means for supporting said lens and said mirrors within said cabinet in operative optical relation to each other and with respect 4to said Writing panel and Y projection screen, whereby shadow rays originatlill and being viewable by the audience by its refiection in a plane-mirror located above the top of the desk and inclined thereto 'at an angle somewhat greater than 45.

5. A device as in claim v1, the projection screen being in a predominantly vertical plane with its active area entirely below the top of the desk and directly in front of the Alecturer as viewed by 'the audience.

6. A lecture desk of the character described comprising in combination a cabinet, a trans-g parent writing panel in the top of said cabinet, a shadow transmitting projection screen in .a predominantly vertical plane communicating with the interior of said cabinet through a light excluding channel; illuminating means external to the cabinet adapted to direct'a beam of concentrated light through said writing panel into the interiorof the cabinet; an optical system mount-` ing in points of light-obstructing material on said writing panel are directed unto the inner side of said projection screen and are converged to a substantial focus thereon.

7. A lecture desk comprising in combination a cabinet having an opaque top member; a plano- 4convex lens covering an opening in said top member, adapted to serve as a support for a sheet of transparent writing material and further adapted to transmit into the interior of said cab ine-t a concentrated beam of light coming downwards from a source above said top member; a projection screen situated in a predominantly vertical plane with its active area at least partially above said top member and to one side of said plane-convex lens; means including wall members and communicating partitions, forming a continuous enclosed channel leading from said plano-convex lens through the underside of said top member to said projection screen; and an optical system Within the interior of said desk adapted to intercept said beam of light coming downwards through said plano-convex lens and .to project it upon .said projection screen from the inner side thereof; said optical system consisting of afocussing lens and a plurality of three plane mirrors, the last one of said mirrors along the path of the broken light beam being so disposed A andtilted as to cause the final segment of said beam of light to impinge upon lsaid screen in an upwardly direction and to` appear as comingl from a point below the lower edge of the active area of said projection screen.

' DAVID KATZ.

Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US3283652 *Feb 15, 1965Nov 8, 1966Busch EdwinModular multiple screen film projection systems
US3489494 *Jul 11, 1967Jan 13, 1970Girvan N Snider JrProjector
US3818609 *Sep 18, 1972Jun 25, 1974M WoolmanDevice for paired learning and process thereof
US4046437 *Nov 14, 1975Sep 6, 1977Aristide Leon CaronCombination cabinet for audio-visual teaching
US4334742 *Mar 4, 1980Jun 15, 1982Datagraphix, Inc.Microfiche reader assembly and method
US4561740 *Jun 6, 1984Dec 31, 1985Minnesota Mining And Manufacturing CompanyDual-magnification rear projection lectern
US4784382 *Jul 18, 1986Nov 15, 1988Myers Elizabeth AChildren's user-friendly podium
Classifications
U.S. Classification353/44, 434/432, 312/237, 353/64
International ClassificationG03B21/132
Cooperative ClassificationG03B21/132
European ClassificationG03B21/132