CA1103492A - Compact optical viewer for microfiche or cassette - Google Patents

Abstract

ABSTRACT

A compact microfiche reader. Light from a microfiche or cassette microfilm is projected and under-goes several reflections prior to striking a viewing screen. One of the reflecting surfaces is defined by a prism sheet having a multiplicity of prisms. Light is both reflected by a passes through the prism sheet, the prism sheet thus defining a dual-function optical surface.
This makes possible a more compact viewer. Several embodiments employ foldable reflecting surfaces and a foldable viewing screen to further reduce the size of the reader.

Description

This invention relates to a microfiche reader, and more particularly to a compact microfiche reader ` wherein the projected microimages of a microfiche are reflec$ed several times prior to strikiny a ~Jiewing screen. Many of the embodiments of this invention represent an improvement on the device shown in U.S.
Patent 3,941,467 issued to Rapany, et al. As set out , ,;
in that patent, a microfiche reader folds a projected, conical beam of light by causing the beam to be successively ~ 10 reflected against a plurality of reflecting surfaces until .~ it strikes at a viewing screen. The plurality of reflect-ing surfaces is defined by the several exterior faces of .
a solid. The solid is of a shape termed a trapezium.

One of the re1ecting surfaces of the trapezium performs ;:.
a dual function. It forms one of the reflecting surfaces and additionally permits subsequently reflected light to pass throuc~h it for final imaging on a viewing screen.
- The manner chosen by Kapany et al in accomplishing this result resides in the selection of certain angles of the reflecting surfaces so that rays of light initially striking the dual-function surface do so at an angle of incidene greater than a certain critical angle for total internal reflection. The critical angle is, in turn dependent upon the index of refraction . ,..-.~.
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~133~2 of -the materia]. from whicll -the trapezium is fashiolle~d.
Tlle Kapany et al constr~lction also utilizes a ~'resnel lens whicll is no-t esselltial ~or the operation of the :' device, bu-t is apparently u-tilized only to give a brigh-ter '': 5 and more uniforlllly illumina~ted pic-ture, particularly at "`
the edges.

;-~ According to the practice o~ this invention . .
`'. ~ a similar action takes place in that a light cone is .
e-Lfec-tively folcled several times before strilcing a viewing screen, yet, there is no requirement that a , ~ so'lid trapezium or other block o~ light transmitting '` ~ material be employed. Instead, a similar dual-function :
' ~ s~lrface is de~ined by a series of prisms ~vhich define a ~: prism sheet. The prism sheet perfolms the same function as a dual--function surface of Kapany, and additionally ' acts to Pocus the final ima~e on a viewin~ screen. One advantage exhibited by this inven-tion over the construction of Kapany et al is that significant weight ancl cost savings are realized. Thus, in l'ieu of a .. . .
- 20 relatively heavy and expensive solid plastic block (the trapezium), reflectin~ surfaces are deflned by planar mirrors in sheet or panel form which may be inexpensively fashioned and positioned relative to each other.
S'till further, the use of a solid block of ' 25 transparent material requires a longer light cone .
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. - 2 -; throwing distance for the same sized viewing area. Thus, ;~ for a solid block of transparent material of inde~ of refraction n, the throwing distance is _ times the throwing distance required by the practice of this invention. For .
example, if a Kapany et al type of viewer has a throwing :
~; distance of nine inches, using polymethylmethacrylate (n=1.5) as the material for the trapezium, the practice of this invention admits of a throwing distance of about six inches. A lesser throwing distance yields a viewer ~. .
still more compact for a given size viewing screen.
More particularly, this invention relates to an ;` optical projector of the type for projecting a cone of light from an illuminated micro-image, including, a plurality ;;~ of reflecting surfaces for intercepting and folding a cone '` 15 of light, one of said re~lecting surfaces reflecting light incident thereon at angles greater than a critical angle but which transmits light therethrough for any light incident ,: .
~; thereon at angles less than the critical angle, characterized by, the one said reflecting surface defined by a prism sheetl .:
and a viewing screen parallel and adjacent to ~ut spaced ` from the prism sheet.
Additionally, this invention relates to an optical projector of the type of projecting a cone of light from :
an illuminated micro-image, including, first and second - 25 substantially parallel mirrors, said second mirror positioned with an edge adjacent the edge of a third mirror, the first and third mirrors making an obtuse angle with each other, a prism sheet positioned in the plane of the second mirror and adjacent thereto, the prism sheet positioned opposite to the third mirror and adapted to reflect at least a portion ~i -3~

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- of the light received from the first mirror while trans-mitting at least a portion of the light received from the third mirror, a viewing screen parallel, contiguous to, ~, ~ but spaced from the prism sheet.
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IN TEE DRAWINGS:
FIG. 1 is a schematic view of a simple folded ` mirror configuration according to the practice of this invention.
~, `~ FIG. 2 is a partially schematic view of the - 10 device of Fig. 1 in its folded condition.
FIG. 3 is a perspective view of the viewer of Figs. 1 and 2 when in the open position.
~ ~ FIG. 4 is a view similar to Fig. 1 and ; ~ illustrates a modification.
FIG. 5 is a view illustrating the device .~ shown in U.S. Patent 3,941,467 issued to Kapany et al.
~ FIG. 6 is a view similar to Fig. 1 but -- illustrates an embodiment employing a prism sheet which performs a dual function.
FIG. 7 is an enlarged view taken at the indicated , . .

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portioll oL ~ig. 6.
FIG. 8 is a view ill~ls-trating a modification oL` a prism sheet ~`IC. 9a is a view simi:Lar -to Fig. 7 and - 5 ill~lstrates the use o~ two prism sheets if a mirror immediately follows in the op-tical path of the first prism sheet.
FIG. 9b is a vie~v~similar to Fig. 9a and illustra-tes the use of a second pair oE prism shee-ts.
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~ 10 FIG. 10 is a view similar to Yig. 2 and - illustrates a comple-te viewer constructed according to the principles shown at Fig. 6.
~ ~'IG. ll is a partially schematic sectlonal view ;, ~, o~ ~ig. 10 taken along--section 1I-ll and illustrates~
t~vo lenses for projecting microimages based on -two parallel tracks loca-ted on a Philips cassette film strip.
FIG. 12 is a partially schematic view similar to Fig. 10 and sho~vs a reader exhibiting compatability for both a Philips cassette and a standard four inch~by ~.-. : .
six inch microfiche.
FXG. 13 is a partially schematic view illustrat-ing another embodiment o~ the invention.
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FIG. 1~ is a view illustrating the open pos~i-tion of the device shown at Fig. 1~, ~IG, 15 is a view illustrating still another ~' "

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mO~3iliC~tiol~ O:r t~le illvelltiOn.
FIG. 1~ is a vie~ of the device o~ Fig. ]~ when in the roidecl position.
~ IG. 17 sho~vs ~ vie~ver o~ the same type as ill-lstratecl at Fig. L5, e~cept that the :~olding o~ the viewing screen and a rnirror is different.
FIG. 18 represents still another embodiment.
:. -FIG. 19 is a viesv o~ the reader sho~lin at - ~~ig. 18 ~ th the addition of a mirror and ~olding vie~ing screen, Re:;erring no~v -to Fig. 1 o~ the Ara~vings, a micro~iche ~iewer or reader constructed according to . .
~rone t!mbodiment o~ tile invention is schema-tically illus--~. : - , : ~ , trated. The nurneral 10 denotes a projec-tion lens ~or ~: .
projecting a microimage ~rom a micro~iche or other photographic reduction, the microfiche not being , , .
~ ~illustrated. The principal ray of the light cone -.
projec-ted -,'rom lens or lens sys-tern 10 is denoted by the nurneral 12 ~nd ~irst s-tril;es mirror 14. The ray then strikes second mlrror 16 and thence third mirror 18 for final imaging and viewing on vie~ving screen 20. Pivot elements 22 and 2 are positioneA as indicated rel~tive to the final mirror 18 and the viewing screen 20. As sho~vn at Fig. 2 o~ -the dra~vings, -the device is ~olde~ by first pivo~ting screen 20 abou-t pivot 24 and -then pivoting - 5 - .

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; abo~l-t pivo-l: 2~. The n~nn~ral 2~ design.ltes generally the ` indica-ted elemellts of t;he vie~Yer ~ Fig. 3 is a perspective `~ vie~/ indicating 1;he general shape and proportion of the vie~Yer when in the e~tended or vie~Ying position.
; - 5 Fig. 4 illustrates ano-ther embodiment similar to that shown at Figs. 1-3 except tha-t the vie~ing screen j, s in front of the projected light cone ~hose principal axis is denoted by the numeral 12. The ... .
reader will no-te the similarity to the embodiments of `- 10 Figs. 1-~ to a foldable viewer sho~Yrl in U.S. Patent 3,667 ~39 issued to Artaud. In that vie~ver a foldable arrangement is illustratecl wherein projected light striles two reflecting surfaces for subsequent imaging ; on a viewing screen. The device illustrated at Fig~.
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1-4 O:r this application however displays the advantag~
` of an adclitional reflecting sur-ace -to thereby o~tain greater magnification. Further the folding action in ; order to mal;e the device more compact is different Referring nO~lJ to Fig. 5 of the drawin~s the numeral 30 denotes generally the principal optical elements of the Kap~ny device earlier mentioned. The numeral 31 denotes a solid block of tran5parent material through which a cone of li~ht ~Yhose apex is denoted by t}le numeral 32 is projected. The cone of li~h-t represents ; 25 light frorn a microima~e. The principal ray is denoted by :.
the numeral 33 alld strikes First rerlectint s~lrrace ~

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.. ' , 34~2 ~ Irom thellce it strikes, at more thaII a critical angle ~ : wi-th resl)ect to the perpendicular -to -the surface 35 ol : the solid and is thence re~lected to reI:lecting surface -~ 37. ~rom here the light passes normally out ~hrough sur~
~ace 35 iOI' viewing. The ~eader ~vill observe the extended - dash lines ad3acent -the cone apex 32. They illustrate : the berldillg action su~fered by the light when it passes ;. from air into transparent material 31. ~ -~eferring no~Y to Figs. 6 and 7 o~ the ~rawings, a compact optical vie~ver is illustra-ted which also employs a dual-~unction surface, but which is superic~r from the ViewpOillt o:E~both size~and cost. :At Fig. 6 the numeral ~.
. 12 again ind~cates the cen-tral ray from a projected cone .
- of li~ht ~hich carrles.a microimage which is to be viewed.
The x^a~ first s-trikes reflecting sur-face 14 and then .~ strikos prisrn sheet 4~. I-t is -then totally internally .. reflected at the upper surface of the prism shee-t, as is the case ~Yith the Kapan~ surface 35, and -thence strikes second mirror 16. From thence it~exits normally -to ~he~
: 20 prism shee-t and thence to viewing screen ~6. lYhile .: shown as pass Lng straight through prism sheet 44, the reacler will understand thdt light -from mirror 16 is .~ re~racted as it passes through the prism sheet, as shown ~t Fig. 7.
~e~erring now -to Fig. 7 o~ the dra~Yings, a magnified portion of the screen and prism sheet of ., ., : - 7 -. - . , :

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; Fig 6 is shown. The numexal 44 denotes the prisrn sheet, the numeral ~6, the viewing screen, and the numeral ~8 :, ;~ an air gap be-tween the two. The prism sheet 44 is defined by a plurality of redges which are on one surface of a , ~ 5 sheet, with the ridges and the sheet being transparent.
Each ridge has two faces, each face being generally straight in transverse cross-section although one of .~-,- :
t~e two faces of any ridge may be curved. The ridges in ~- all of the prism shee-ts illustrated ~re straight and are :., parallel to each other. However, ridyes having an elliptical contour, with the axis of the light cone defining one of the two foci of nested ellipses may be employed Referring now to Fig. 8 of the drawings, an embodiment of a prism sheet is illustrated. The prism sheet 44 is not to be conEused with a Fresnel lens, to which it bears a superficial resemblance. The individual ridges which make up the prism sheet are elliptical in contourj although in transverse cross-section they ~are simlLar to the previously described embodiment. The ridges are denoted b~ the numeral 50, while the numeral 52 ; indicates the intersection of the principal ray 12 o~
the cone of light with the prism sheet. The rcader will observe that point 52 is one foci of the nested elliptical ; ~ 25 ridges.
; Referring now to Figs. 9a and 9b o~ the drawings, two , ~
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ancl fo~lr prism sheets arc illus-trated. In tlle e~ent that , only a single prism sllee-t is employcd Eor the reader, such as shown a-t Flgs. 6 and 7, a viewing screen is placed ~-- adjacent the flat surface o:E the prism sheet. ~Iowever, if light passing throu~h the first prism sheet is to be subsequently reflec-ted, it is necessary to inc:Lutie a second .
prism sheet so as not -to change the direction ol the ligllt rays. However, i-E th~ thickness of the prism sheet is too great, the lmage ;on the rear projection screen becomes baclly distorted because of the crossin~r o-f th~
ligllt rays as shown in I`ig. 9a. B~l-t -the distortion becomes negligible to -the eye if the number oI prisms is large, e.~, 2~00 prisms per inch. The distortion produced will not be resolved by the human eye.
In order to improve the image on the screen~ a - second pair of prism sheets can be placed immediately ; ~ next to and in registry with the -~irst two prism sheets~ ~-thereby undoing the crossing~of rays as is shown in Fig.
,' 9b. It is to be understood -that in any of the embodl-ments shown here~n utiLizlng a pair o-f prlsm sheets, the op-tics can be improved by adding an additional pair as in Fig. 9b.
In Fi~. 9a the first prism sheet is denoted by -the number 5~ and includes surfaces 5~ and 58 on a typical ridge. ~ir ~ap 60 is interpo5ed between the :Elat surface oE the prism slleet 5~ and the flat sur~ace o-f a second ,' .

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34~2 :
~ prism sheet 62. Faces 64 and 66 on a typical ridge of ":
prism sheet 62 correspond with surfaces 56 and 58. The ~.
reader will note that surface 58 is parallel to surface 64, while surface 56 is parallel to sur~ace 66. Further, 5 any one or both of homologous surfaces 58, 5a~ may be curved, to effectively define a lens. Similarly, any one or both of homologous surfaces 56, 66 may also be curved to similarly effectively define a lens.
;~ Figs. 10 and 11 of the drawings illustrate 10 another embodiment employing two projection lenses. At Fig. 11 the numeral 12 and 12' indicate the principal axes of two projected light cones, each light cone correspondin~ to a respective part of two parallel tracks carried by an optical film on a cassette. The 15 reader is denoted generally by the numeral 2g' while the numeral 26 again represents, generally, the indicated auxiliary devices in a typical reader.
ig. 12 illustrates another embodiment, similar . . ~
-~ to those previously described, wherein the compact 20 reader can accommodate either a microfiche, such as a standard f~ur inch by six inch microfiche or a cassette. The microfiche is denoted by the numeral 70 ;~- and employs projection lens 10, while the cassette is ,. .
denoted by the numeral 72 and employs projection lens 10'.
25 The reader will understand that mirror 11 is pivoted out of the optical path when cassette 72 is employed.

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Th-ls, eit~lel~ -the microfiche Ol the cassette may be uscd ~` 1'ol~ projec tion.
~ eferrillg no~Y t;o Fi~s, 13 al3d 1~, another modification of a compact viewer is illus-trated. The 5 numeral 7G denotes a first mirror ~or receiving a cone oi light projected from a c~ssette or other carrier of , . . .
optical inEorma~tion on a reduced scale. The numeral 78 indicates a second mirror while the numeral 80 indicates .. .. .
a third mirror. ~s be-fore, the numeral ~ clenotes a prism sheet while the numer~l 46 indicates a viewinD screen.
Tlle pl'illC ipal light ra~ is indicated b~ the numeral 12 The action of the device is similar to that previously described ~vith respect -to other embodiments. Mere, howe~er, the reflecting surfaces, particularly the first and seconcl, are substantial:Ly parallel. As before, light striking prism sheet 44 at more than the critical an~le is reflected, ~vhile light rays substantially normal to the pl'iS~ sheet are transmitted throu~h ~or vîewing .
on screen ~G.
The embodiment of ~ig. 1~ di~fers from that o~
~ig. 13 in that the former includes pivots ~6 and 88 as well as additional mirror 82 coupled to viewing screen &4.
-- The screen when no-t in use, is folded cloc~vise about pivot 88 until i-t attains parallelism with mirror 82, then these two elemen-ts are lolded clocliwise about pivot 86.
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.. ~lso, a p,~ir of palalle:l. and spaced prisrn sheets denoted ~: by the numerals 5~ and ~2 are cmploye~, -their ac-tion and clisposition be:i.ng thc same as that descli~ed ~Y:i tll respect .,. to Fig. 9 of the drawings.
Referring now to Fi~s. 15 and 1~ another - . ~
;. modil'ication is illustrated. The numeral 90 indicates a ',. ~irst mirror, the numeral 92 a second mirror, and the numeral 94 a third mirrol. The numeral 96 i,ndica-tes a..
-- vlewing screen, while again -the nurneral 12 indi.cates the ~, ~ ., .
principal ray of a cone of light projec-ted from a casset-t~ ox other information storage device~ The vie~Y-ing screen is provided~w~th a pivot 9S connec-tin~ it to :, : .
:,.,'~ the -top of mirror 94, wIlile the bot-tom of mirrol 94 ~ carries pivot 100 which is also a-t -the left portion~of '',.~ 15 mirror 92 in Fi.g. 15. Numel~al 102 indicates a t,ransparent wheet such as a ~lass plateD The opera-tion ol the device ~' : ; wil:l be apparent from a consldera-tion of Fi~. 15, In .,,, order to Iold~the device ~vhen Dot in viewing usage, screen ,'.~ 96 is plvoted about axis 98 in a cloc~wise direction until .,~ 20 it is parallel to mirror 94.~ These two elements are .' -then folded about pivot lOO in a counter-cloc~wise ',. direction until -they are parallel to mirror 92. Glass ', plate 102 is for the purpose of pro-tec-ting the interior of the device when it is in the folded position. ~ig, 17 indicates a sirnilar construction, the main difference . . .

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being in -the mallnC~l of folding :Cor storage and in the ; usc Or t~vo dual f~mctloll prism sheets. ~lirror g~ is the ;~'. next to the last reflecting surIace, while mirror 9~ is -the las-t re:Electing surface. Again, the nume.ral 96 ~ .5 indicates -the viewin~ screen while numeral 12 the principal '~ ray. In order to fold the devic:e of Fig. 17, mirror 9 is moved to the right so tlla-t its bo-ttom portion slicles alollg the top of the viewer, vie~ving screen 96 thus swingin~ about a~is 100 in a clockwise manner.
; :lO ~e~erring now to Fi~ , another embodiment .. . is illus-trated o:E a compact reader according to the . practice of this .invention. The numeral 110 deno-tes a :
first re:t'lecting sur:Eace, (neglectin~ any small mirrors .` near the apex of -the projected light colle) the numeral .~ 15 112 a second reLlecting surI'ace, while numeral ~ again .: represents a prism sheet with associa~ed spaced vie~Ying ~- screen ~. The numeral 113 indicates a slot for the ~' ~ rec~ption of a microfiche, although it will be unders-tood ; ~ ~ that elements 26 may be modified so as to include a . . .
:` 20 Philips cassette which carries optical information instead 'v of the micro:Eiche. A light cone projected from the . .
''. stored micro information has a principal ray denoted b~
the numeral 12, as before, and strikes surfaces 110 and ' 112 as indicated. ~eflecting sur~ace 114 is .. ~ 25 positioned as sho~vn, extending up~vard to the left as ~ 13 -,, . ~ .
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:, viewed at Fig. 18 until adjacent the left end of prism -- sheet 44. The dual-function action of prism 44 is the same as that previously described.
Fig. 1~ illustrates a modification of the device shown at Fig. 18, the modification differing in the addition of another mirror 11~ and viewing screen 116 which is attached to the top of mirror 115 by means of a pivot 120, mirror 115 in turn carrying a lower pivot 122.
~- The optical action of the embodiment of Fig. 19 is believed -~ 10 evident. To fold the device, scxeen 116 is pivoted about axis 120 in a clockwise direction until it is in parallelism .- . .
with mirror 115l the two elements are then folded in a clockwise direction about pivot 122 to the top of the device.
The prism sheets such as 44 of Fig. 7 and 54, 62 of Fig. 9a, which are employed in most of the embodiment described above, may be formed in the manner similar to .~
that shown and described in U~S. Patent 3,728,422. A stack ~
of generally rectangular shims of metal, are distorted so that the shims slide relative to each other. One set of edges of the shims thus assumes a staggered relation except , ~ that the edges are not biased, but are still rectangular.
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The edges are distant from each other and displaced an amount to produce an edge cross-section the same as that of prism sheet 44 of Fig. 7 of this application. Again, s the shim edges are mirror polished. The resultant shim assembly may now be employed either as a mold or as a ma-ter for electro-forming. The mirror polished shim ends yield the desired ' :
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optically flat surfaces of the individual ridges of -the prism shee-t, such as flat surfaces or faces 56, 58 of prism sheet 54 of Fig. 9a of this application.
'l'he invention can also be used ln connection with microfiches and cassettes carrying distributed - sub-images of each original information and/or having lensettes being integrally with the microfiche. For further details of this technology, see, e.g., U.S.
; Patent Nos~ 3,950,769, 3j995,288, 3,967,2%9, 3,806,708 and 3,704,068. The invention can be used for still pictures and for moving pictures. Protection for these ; usages is being claimed herewith.
It should be noted that the prism sheet or prism sheets should be as thin as possible. It also should be taken care of that the prism sheets should be supported or arranged in a way that they will not ,-urve ;
or bend with variation of environmental conditions.

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Claims (12)

The embodiments of the invention in which an exclusive property or privilege is claimed as defined as follows:

1. An optical projector of the type for pro-jecting a cone of light from an illuminated micro-image, including, a plurality of reflecting surfaces for inter-cepting and folding a cone of light, one of said reflecting surfaces reflecting light incident thereon at angles greater than a critical angle but which transmits light therethrough for any light incident thereon at angles less than the critical angle, characterized by, the one said reflecting surface defined by a prism sheet, and a viewing screen parallel and adjacent to but spaced from the prism sheet.

2. The projector of claim 1, wherein prisms of said prism sheet define one surface of a transparent sheet.

3. The projector of claim 1, wherein the optical path of a cone of light striking the said re-flecting surfaces passes through air, except when passing through the prisms or prism sheets.

4. An optical projector of the type of pro-jecting a cone of light from an illuminated micro-image, including, first and second substantially parallel mirrors, said second mirror positioned with an edge adjacent the edge of a third mirror, the first and third mirrors making an obtuse angle with each other, a prism sheet positioned in the plane of the second mirror and adjacent thereto, the prism sheet positioned opposite to the third mirror and adapted to reflect at least a portion of the light received from the first mirror while transmitting at least a portion of the light received from the third mirror, a viewing screen parallel, contiguous to, but spaced from the prism sheet.

5. An optical projector of the type for pro-jecting a cone of light from an illuminated micro-image, including first and second substantially parallel mirrors, said first mirror positioned with an edge adjacent the edge of a third mirror, the first and third mirrors making an obtuse angle with each other, a first prism sheet positioned in the plane of the second mirror and adjacent thereto, the prism sheet positioned opposite to the third mirror and adapted to reflect at least a portion of the light received from the first mirror while transmitting at least a portion of the light received from the third mirror, a second prism sheet adjacent and parallel to but spaced from the first prism sheet, a fourth mirror, said fourth mirror being pivoted at one edge thereof about an axis perpendicular to the axis of the light cone, the fourth mirror carrying a viewing screen pivoted to an opposite edge of the fourth mirror, the viewing screen being pivotable to a position parallel and adjacent the fourth mirror, the axes of the two said pivots being parallel to each other.

6. An optical projector of the type for pro-jecting a cone of light from an illuminated micro-image, including a plurality of reflecting surfaces for inter-cepting and folding a cone of light, one of said reflecting surfaces reflecting light incident thereon at angles greater than a critical angle but which transmits light therethrough for any light incident thereon at angles less than the critical angle, characterized by, the one said reflecting surface defined by a prism sheet, a second prism sheet adjacent and parallel to but spaced from the first prism sheet, a final reflecting surface, said final reflecting surface being pivoted at one edge thereof about an axis perpendicular to the axis of the light cone, the final reflecting surface carrying a viewing screen pivoted to an opposite edge of the final reflecting sur-face, the viewing screen being pivotable to a position parallel and adjacent to the final reflecting surface, the axes of the two said pivots being parallel to each other.

7. An optical projector according to claim 6, characterized in that a third and a fourth prism sheet identical to the first and second prism sheets and arranged in the same way, are positioned between the second prism sheet and the final reflecting surface.

8. Optical projector according to claim 1, characterized in that there are means to simultaneously project cones of light from different illuminated micro-images.

9. Optical projector according to claim 8, characterized in that there is a plurality of nests of elliptical prisms, the common focus of each nest being coincident with the respective main ray of incident light cone.

10. An optical element according to claim 1, comprising a sheet of transparent material, the sheet having a plurality of prisms formed on a first surface of the sheet and having a substantially flat second surface which is substantially parallel to the plane of the first surface, the sheet being formed of a material having an index of refraction such that a ray of light impinging on the surface of a prism from a first direction is substantially totally reflected with the second surface of the sheet while the ray of light impinging on the surface of a prism from a second direction is sub-stantially transmitted through the second surface.

11. An optical element according to claim 10, in which the first direction is a direction substantially perpendicular to a plane of the surface of the prism and in which the second direction is a direction substan-tially perpendicular to the plane of the second surface of the sheet.

12. An optical element according to claims 10 or 11 and further including means for directing the first ray of light in the second direction.