CA2110807A1

CA2110807A1 - Retroreflecting polarizer

Info

Publication number: CA2110807A1
Application number: CA002110807A
Authority: CA
Inventors: Michael F. Weber
Original assignee: Individual
Current assignee: 3M Co
Priority date: 1991-06-13
Filing date: 1992-05-20
Publication date: 1992-12-23
Also published as: EP0588937B1; MX9202841A; DE69213234T2; US5559634A; JP2004078234A; KR940701548A; WO1992022838A1; JPH06508449A; EP0588937A1; JP2002196148A; DE69213234D1; BR9206133A

Abstract

A retroreflecting polarizer (10), comprising optical thin films coated on a structured material (12, 14) divides an incident beam of light into polarized components (18-s, 18-p) transmitting one component (18-p) through the polarizer and reflecting the other (18-s) back to the source.

Description

W~92/22~38 2 ~ 1 0 ~, ~ 7 PCT/U~92/0427~

RETROREFLECTING POhARIZER

5 Technical Field This invention relates to polarizing thin film stacks coated onto substrates having structured surfaces~

lo Back~round A MacNeille polarizer ~omprises alternating repeatiny layers of a pair of thin film materials deposited on a bulk substr~te material. The pair of thin film materials comprises one low refractive index I5 material and one high refractive index material. The indices, called a MacNeille pair, are chosen such that, for a given angle of incidence of a light beam, the reflection coefficient for p-polarized light (rp) is essentially zero at each thin film inter~ace. The 20 angle at which rp is zero is called the Brewster angle, and the formula relating the Brewster angle to the numerical values of the indices is called the MacNeille condition. The reflection coefficient for s-polarized ; light (r9) is non-zero at each thin film interface.
25 ~Therefore,~ as~mo~e~thin film layers are added, the total reflectivity~ for ~-polarized light increases while the~reflectivi~ for p-polarized light remains essentially~zero. ;Thus, an unpolarized keam of light, inciden~ upon th~thin film stack, has some or all of 30 the~s-polariæed compQnents reflected while essentially all of the p-polarized component is ransmitted.
; Such a~thin~film~stack is deposited on two general ; types~of substrates, which~then classifies the type of polarizer~ produced as either immersed or non-immersed.
35~For~example, if the thin films are deposited on a flat face which~forms~the hypotenuse side of a right angle : `
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Wi~92/2283~ 7 PCT/US92/04271 (Porro) prism, and bonded to the similar side of an identical prism, the polarizer is an immersed polarizer. If the thin films are bonded between two planar slabs of transparent media, the polarizer is a 5 non-immersed polarizer. In general, a polarizer is non-immersed if the geometry of the bulk encapsulant does not affect the immersion constant nj*sin(~;) of the light beam in a thin film material ~.
For either immersed or non-immersed polarizers, lO the p-polarization component of an incident light beam is transmitted, while the s-polarization component is reflected from the thin film stack at a~--angle equal to : the angle of incidence. The total change in direction of the s-polarization component from the incident 15 direction is goo for cube polarizers and usually about 60 for slab polarlzers. Thus, the s-polarization component is typically unavailable for further use, leading to a decrease in overall intensity of light ava~ilable, un:less ~dditional optics are employed to 20 :~re:direct: the s-polarization compon~nt. For example, U~.S. Patent 4,9I~3,529 (Goldenberg et al.) discloses a liquid crystal display (LCD) television projection system~using two~re:flectors, a polarization rotator and ,, , ~
a prism~to recombine:both components.
25~ Such~systems are~ undesirably large for use in many common:visua:l:display~systems, such as overhead proje~ctors, and~especially;:in portab~e or laptop computer displays~where~a ~hin profile is desired.

30 ~Disclosure o~ Inventlon~
I The invention is a retrore~lecting polarizer, compr~lsing~

(a)~a first~materia~l having a structured surface 35 consisti~ng of a linear array of substantially right angled isosceles pri~sms arranged side by side and ~, W~92/22838 ` 2 1 ~ ~ 3 ~ 7 PCT/US92/04271 having perpendicular sides which make an angle of approximately 45 with respect to the tangent to a smooth surface opposite the structured surface, (b) a second material essentially like the first 5 materi~l, (c3 on the structured surface of at least one material, at least one optical stack of alternating layers of high and low refractive index materials of se1ected optical thicknesses; the first and second ~0 materials all opti~ally cemented to form a single unit in which the refractive index of the first and second materials, and the refractive indices and optical : thicknesses of the layers of the optical stack, are all :~ chosen to produce seIective reflection of polarized 15 light, such that:
(d~ wit~in one portion of the optical stack, an incident light beam of mixed polari~ation is separated into an s-polarized component and a p-polarized component, : ~ (e) the s-polarized:component is reflected onto another portion of the optical stack and there reflected paral:lel to~the incident beam but proceeding ~ :
in an opposite ~irection, and (f~ the p-polarized component is transmitted 25 parallel to:the inciden~beam.

Brier Descript1on_of~ehe DrLw-In~
Figure ;l~is~a cross~sectional view of a portion of one~preferr;ed~e~m~odiment~of: ~he :~invPntion .
~ Figure 2 is:an~:enlarged sectional view of a port;ion oflthe embodiment of Figure l.
Figure 3;is~a~;:schematic side view of an optical :system employing the invention.
: Figure 4 is a graph~of the transmissivity and :35 :reflectivity of light incident upon one embodiment of : the invention.:

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W092/22838 ~ 1 1 0 ~ 0 7 4 _ PCT/US92/04271 Detailed Description of the Invention ~ igures l and 2 show an inventive retroreflecting polarizer lO, comprising two pieces of transparent substrate material 12 and 14, between which is is a 5 composite optical stack 16.
The pieces l2,l4 each have structured surfaces (which face each other), and non-structured surfaces.
As shown, piece 12 is a top layer and piece 14 is a ~ubstrate, but the entire assembly may be inverted with l0 no loss of functionality, essentially interchanging the :~ roles of the two pieces.
In the embodiment shown, the composite optical stack 16 is deposited upon the structured surface of ~: the upper piece 12, and the structured surface of the lower piece l4 is:optically cemented (i.e., adhered by a very thin layer of transparent adhesive) to the composite optical stack 16 by an adhesive 24 to form a single unit. However, the composite optical stack : :could comprise two~:sub-stacks, one sub-stack deposited 20 on the~top layer and the other deposited on the substrate, with adhesive 24 betw~en the two sub-stacks.
The composite~optical:stack compri~es at least one se~ of~:~pairs of alternating layers of materials having low~and~high~indices~:of~refra~ction compared to each 25~other.~ The~thicknesses of the layers are chosen such that the~quarterwave~criterion:is met for the wavel~ngth o:f~the incident collimated light beam 18 by each;o~ layers~20~;and 22. ~ The~shape of the structured surfaces,~thé~optlcal properties of the substrate ::30:;material,: and~the~:properties of the composite optical stack,~all,:combine to~di~ide~the incident light beam into~wo polarization components. One co~ponent, 18-s, is refl:ected:;twice~in:such a manner as to be retrorefle~ted, i~.~e.~,~ directed back toward the source 35 of ligh~beam 18. ~The other component, 18-p, is - transmitted paral~lel to lncident beam 18.

211~ 7 W092/22838 PCT/U~g2/~271 (In Figure 2, the division of incident light 18 into components 18-s and 18-p is shown as occurring at the first interface between the substrate and the composite optical stack, but this is illustrative only.
5 Actually, some division occurs at each interface between thin films, with the net result being as shown.) In the embodiment shown, the composite optical stack comprises a repeating stack of a pair of : ~ 10 materials. One of the materials is a relatively low refractive index (nL) material 20, and the other is a relatively high index (nH) material 22. The construction of such a stack 16 is abbreviated (HL) 2.
In general, more layers are u~ed, such as a (HL3s sta~k, ~:~ 15 and generally the average optical thickness of each material is a~quarterwave thick, with reference to a chosen wavelength::of interest (typically but not necessarily in~the~visible spectrum). However, to optimiz:e performan'ce, the:individual thic~nesses of all Z0;~thin~fi~lm layers are~varied 'slightly from the average ~ thick~ess, in~accordance with known principles, using ";~ :commercially available~:software:to calculate the desired values.~
Also,~more~:than~two~pa:irs of materials or average ,2~5~thicknesses;mày~:~be~used,;such~as a (H~LI)5~(H2~)5. This would~be~:~one to~extend:~;the useful optical bandwidth of the~invention~'or~`the~range:of-angles over which the vention~reflects,~essentially~all s-polarized light.
~3~ Each~;of~substrate~pieces~12~and 1:4 comprises a : : 30 ~transparent,:~:p~e~féra~ly integral ~(i.e., a single dontinuous~lpiece~as opposed to an assembly or a laminate)~:material~having a~structured surface which :consists of:~a;~linear~array~of~substantially right ,angled~ sosceles~:prisms~arranged side by side. The 35~;~perpendicular;:sides~of~each prism make an angle of - approxim:ately:'45 with~ respect t~ the smooth surface W092/22838 2 i ~ 7 PCT/~S9~/042?l -- 6 -- .
opposite the structured surface (or~ in the most general case of a flexible substrate, with respect to the tang~nt to the structured surface). Angles other than 45 are useful for other applications, but angles 5 near 45 (e.g., ~0 to 50) are preferred in this invention. This places a constraint on the design of the optical stack: only two of the three indices of refrartion (nL and nH for the optical stack, nO for the :~ ~ubstrate pieces) can be chosen independently. (An 10 additional implication is that nL must always be less than nO if high transmission of p-polarized light is desired at all wavelengths.) These values are ;~ determined by the MacNeille condition relating the Brewster angles of each material interface to the 15 numerical values of the indices of the materials forming the interface: ;

tan(~L) = -(nHlnL~
or, : tan ( ~H) = (nL/n~) ; al~ong~with Snell~'s;~law~relating~0 to ~L and ~H.

: 25 ~ In;theory, an~infinite set of values of nH and nL
exist~f~or~a given ~nO~ :~but in practics, the available choi~es~:~of mater:ials~;~for the substrate pieces and thin films~are~`~limited~ and~design of the invention reduces to~:choosing~whi~ch o~the limi~ed set of values of nH
30 ~and nL~:~around~th~at::`value::of nO will produce the desired results.~ IThe~gre~ater the dif:ference between nL and in the wider~the~ optical~bandwidth over which the invent~ion will divide incident light into separate polari~zations. :;:
35~ ~ A:suitab~le thickness of the substrate is 0.36 millimeters, ~éa~sured from the smooth surface to the :: ~

W092J22~38 ~ U 3 ~ 7 PCT/US92/04271 lowest point of the grooves. Suitable groove heights (measured perpendicularly) are 0.18 mm. For such a film, about 28 peaks per centimeter is preferred, but there is wide latitude in the dimensions.
Preferred substrate materials are flexible, homogeneous, and isotropic. Suitable materials include commercially available acrylics and polycarbonates ha~ing nominal indices of refraction of 1.49 and 1.59, respectively. Other possible materials, selected to 10 provide the required functionality, include ; polypropylenes, polyurethanes, polystyrenes, and polyvinylchlorides. Generally, polycarbonates are preferred for their relatively high indices of refraction, clarity, and physical properties.
Higher index materials include polysulphone (and variations such as polyethersulphone and polyarylsulphone), polyethylene teraphthalate ~PET), and polyethylene napthalate (PEN). The sulphone~
reguire high processing temperatures, but in turn can 20 withstand higher ambient~temperatures in use. PET and PEN may~rystallize or exhibit~bir~fringence depending on the process~parameters~. All these materials ha~e indices ~in~the range~ of 1~63 to 1.65, and as such~
allow~th;e use~of~the~film pair~SiOJTiO2 while retaining 2~9~ high~transmission~0f~p-polari~zed~light.
A suitable material is taugh~ in U.S. Patent 4~805~ 984~(Cobb,~ 3r.3~,~ but in~this invention the total internal~ reflection~property of~that material is not rel~vant,~because~he~optical properties o~ the 30;~ material;~are~signiflcàntly ~hanged when it is employed in this~;invention. ~
Suitable materials for the~thin films 20 and 22 include any~materials~which are trans~arent (exhibit 19w~ absorption~in the~spectrum~of interest. For - 35~ bro~dband visible 1ight, suitabl~e thin film materials are silicon dioxide;~SiO2) (n=1.45); amorphous W092/22838 2 1 1 ~ ~ iJ 7 PCT/US92/~271 hydrogenated silicon nitride (a-SiN:~) (n=1.68-2.0~;
titanium dioxide (TiO2) (n=2 . 2-2 . 5); magnesium fluoride ~MgF2) (n=1.38); cryolite (Na3AlF6) (n=1~35); zinc sulphide (ZnS) (n-201-2.4); zirconium oxide (ZrO2) (n=2.05); hafnium oxide (n=2.0); and aluminum nitride (n=2.23. Silicon ~itride (Si~N43 is suitable, but has not been furmed successfully on the preferred polycarbonate:substrate.
Several thin film deposition techniques can be 10 used t~ deposit the composite optical stack on the substrate. Thermal and electron beam evaporation, and ion beam sputtering are the methods of choice for precision optical coatings, the latter method producing superior films in terms of adhesion to the substrate, :15 hardness, and~environmental stability. Magnetron i sputtering is also used extensively for broadband coatings such as anti-reflective coatings on glass, and especially ~or~large area applications such as architectural:glass. However,~on the whole, thermal : 20 and electron beam evaporation should provide good thin film ~ualities and~suf~iciently high deposition rates for~`acceptable manuf~acturing rates. More importantly, low index:films such~:as magnesium fluoride and cryolite can~be~deposit~d~by;this method.~ Electron beam 2s deposition is~;regular~y~used~in the coatings industry or hi;gh~index~materials such as titanium dioxide, zirconium oxide~,~:hafnium oxide:, ~nd aluminum nitride.
The~pro~èss~used~:in the reduction to practice of the~invèntion~was~plasma assisted chemical vapor ::: 30 deposition (PAC~D).~ Using this PACVD, the following proceduresj~and resultant products~,are possible.
SiO2~may~be~deposi*ed by reacting silane (SiH43 or almost`any:organosilane;~in the~PAVCD process with oxygen~or nitrous oxide at between 50 and 250 35~ milliTorr, using~l:ow power RF plasmas of about 50-100 watt/ft2 of e~lectrodè area. Nitrous oxide is somewhat :::`: :` :` ~

'r~ C~ ,X.~ ;r~ ;rr~ s;r~ 5~ r~,-r~,r"~ ~,".~ ",~; ;"-5-~;--q .

wo g2/22838 2 1 :I Q 8 ~ 7 PCr/USg~ 271 .;' g preferred because it generally results in less powder formations in the gas phase.
Tio2 may be formed by reacting titanium tetrachloride (TiCl4) with oxygen and nitrous oxide at 5 the same power levels. By varying both the rela~ive and absolute flow rates of the 02 and ~2 for a given flow of TiCl4 vapor, the index of refraction of the film is easily varied/ from 2.0 to 2.4. Residual chlorine in the film can result in poor adhesion to 10 polycarbonate. An oxygen flow of several times in excess of the reactant gas is preferred.
The visibly transparent a-SiN:H material has an index of refraction which varies mainly as a function of deposition temperature, with the higher indices 15 requiring temperatures of 250 Celsius or mo~e. The films may be deposited from mixtures of silane, ammonia,~and nitrogen. Films formed at lower temperatures from conditions suitabl~ for high index films:(i.e., silane, starved nitrogen, no ammonia) 20 produce undesirably high absorption of blue light. It is possible to form films having indices between 1.68 and 1.8 on polycarbonate below 100 C, with low optical absorpt~ion, although `the lower index films are somewhat brit~le. ~ ~ ~
~ . ~
The PACVD~process was carried out usiny a deposition~:system~according to:the teachings of U.S.
Patents 4,841,908~and:~4,874,63~1 (Both Jaco~son, et al.~3. ~Briefly, this; multi-chamber deposition system employs~:a~;~large:volume vacuum chamber within which are 30:plurality:0f deposition chamb rs for different composition layers, each chamber having separate seals to minimize back diffuslon of~any dopant gases from adjacent deposition chambers. A:continuous roll of : substrate proceeds from a supply roll through each of 35 the deposition chambers and onto:a finished take-up roll. The direction of web travel is reversed ~: :

w~g~/2~38 ~ 3~ a7 - lo - PCT/US~ ,271 repeatedly to produce the multiple layers of repeating re~racti~e index materials.
The index of refrackion (nA) of the adhesive 24 should match that of the upper and lower pieces 12 and 5 14 as closely as possible. When the index of the adhesive is less than that of the adjoining piece, the non-zero thickness of the adhesive leads to some refraction of light away from the original beam ~: direction. Adhesives of nA = 1.56 are available from :: 10 the Norlund Company. Suitable adhesives are Norlund numbers 61 and 81 optical cements (nA = 1.56~. Another :~ ultraviolet curable resin (nA = 1.50) can be made from : Union Carbide number ERL 4221 epoxy resin with 1% (by : ~ weight) Minnesota Mining:and Manufacturing Company ~; 15 number 41-4201-91185 sulphonium salt initiator. The initiator is dissolved in methelene chloride which must : be evaporated~ off before mixing with the epoxy. Other W curable mixtures, not as preferred, may be made from urethane acrylate ba~Se resins, diacrylate diluents, and 20 suitable photoinitiators. ~W :::curable adhesives may cause~slight:~absorption,~ mainly~in the blue end of the spect~um, in the completed~polarizer of about 1-2~.
: Any~the~mosetting~adhesive or epoxy will work also provided~It~ha- Iow~optica1 absorption and high index.

Example~
A1ternating~thin~fi1m layer~s of matched quar~erwave~optica~ thickness were coated on the :structured:side~:of~a~14:mi1 thick polycarbonate version 30~o:f the~preferred~substrate material;:described in U.S.
Patent 4,805j98~4~(;Cobb, Jr.) In Example 1, ~oating wa~s:~done~by::;the~plasma~assiste~ chemical vapor deposition (P~VD)~process~desoribed above, using a 5 .inch~wide and 8 inch~long gas "showerhead" type 35 electrode. To:form~the~retroreflective polarizer, an ~ ~:

W092/22838 PCT/US92/0~271 , . . .

uncoated piece of the TIR material was adhered to the optical stack with an optical adhesive.
In Example 1, the polarizer had three optical stacks ! each having twelve layers, either silicon 5 dioxide (SiO2) or titanium dioxide ~Tio2). The unusually high number of layers was required because the PACVD technique as described above did not produce a uniform film thickness near the prism peaks as opposed to the bottoms of the grooves. The first stack 10 had a quarterwave thickness centered at 400 nm, the next centered at 550 nm, the third centered at 700nm.
The polarizer performance is shown in Figure 4.
Transmissivity of the s-polarization component, T(s), was at or near zero throughout nearly all tha visible 15 spectrum~ while reflectivity of that component, R~s), approached the 95% level typical of the most efficient ;~ common~reflectars. Transmissivity of the p-polarization:component, T(p), was very acceptable, nearly 80% or more throughout the~visible spectrum.
It is useful to provide a few details of the angular dependence of the~retroreflecting polarizer.
, ~ :: : : ~ :
The:first feature :is the~ angular dependence of t~ansmission~for~p-polarized light, through one prism facet.~The;ang1e~theta~is~measured in air from the : 25 unit vector normal to:the outside surface of the retroréflecting~ polarizer.~ The~assumed ~ilm stack is a combination~-of;three stacks~designed to cover the visi~le~spe~trum:at~all~angl~s of incidence. The transmis~sion~speotrum~vs. ~angle:~is broader at lvnger 30 wa~elengths (+45 at 650 nm).~ This stack comprises twenty-eight layers::;an eight layer stack centered at 6:00 nm~and;45 (immersed)~, along. with a double stack, of ten:layers each,:designed fo~ 15, with center waYelengths of 450:~and~600 nm.
; 35 ~ The computer: calculated angular dependence of transmission, ~for a wavelength of 450 nm, shows an assymmetry of p-polarized transmission for positive and :: :

W0~2/Z2838 PCr/US92/0~271 2i1 f~ J7 - 12 -negative values of theta. This arises from the inclination of the prism facets at 45 from the substrate surface, wheraas the angle theta is measured in air from the norm,al to the outside surface. Total 5 transmission through the polarizer is the sum of two transmissions, at com~limentary angles, through ~wo opposing facets. When both terms are taken into account, the transmission curve is symmetrical.
Tertiary and higher order reflections from light 10 transmitted laterally at the second prism can be accounted for as well, but do not have a great impact on the shape of the curve.

Applications The invention is suitable for applications requiring polarized light that would benefit from increasing the ~intensity of the polaxized light available from an unpolarized source, and especi~lly those requiring polarized light over relatively large 20 areas and/or in~ relatively compact ~especially thin~
applications~
For example, the inventive retroreflecti~g polarizer can be~ combined in a very simple manner with a ~uarterwave~re~ardation;~plate and a refleGtor to 25 ~recombine~the~two~components o~an incident light b'eam into ;a sing1e~polarized component~of light. Such an ; arrangement is~ hown~ln;~igure 3. A combined reflector and~sour~ce~;of;~incident~light 118 is illustrated schematical1y~as; 130~ Incident~liyht 118, having mixed 30~polarizat;ion,~is~not~affected~by~quarterwave retardati~n~plate 120,~but is split into components 118-p~and~118~-s~by~retroreflecting polarizer 100.
Component 118-p~is transmitted~directly to display device llO.~Component~ s is~retroreflected back 35~through a quarterwave~rétardation plate 120 as shown by 119, and ref~lected ~(and displaced transversely upward ~:
~ for clarity as~component 1213~back through the :

2 1 ! ~' 3 ~ 7 W092/22838 PCT/U~92/~4~71 . ., .~

quarterwa~e retardation plate again as shown by 121.
The two passes through the quarterwave retardation plate represent a total rotation of ~0, i e., component 118-s now has the same polarization dirPction 5 as component 118-p, and is also directed toward display device 110, thus nearly all of the intensity of incident unpolarized light ~18 is available in polarized form at display de~ice 110.
The great advantage of the invention in this ~: ~ 10 system is that because all components may be relatively ~ thin and large in area, and lie on essentially the same : optic axis, the profile of the system can be greatly reduced. Where reduction in profile is not as much a ~: concern, or where convenient for other reasons, the 15 QptiC axis can be red:irected without loss of generality.
Reflecting source 130 may be the light source of a backlit computer display, or an overhead proje tor such as models~widely available from the Minnesota Mining 20 and~Manufacturing Company. :Display:deviGe 110 may be a qroup ~of one :or more birefringent LCD panels, employed in monochrome or color applications~ such as those disclosed~in;~U.~S. Pa~tents 4,917,465 (Conner et al.) and 4,966~,441 (~Conner)~
;25 ~For~this application:, assuming a polycarbonate : substrate~o~ index::~nO = 1.~586,~:the~ideal thin ~ilm : indices~are;~nH~ 2~ 0:and nL~=~;1.35. With this pair of indices;~, the:theoretical minimum~:composite optical :stack~for~a~;photoptic~ e.:,~ covering the entire : 30 visible~spectrum)~retroreflec*ing polarizer is two sets f~eight layers~ e~ HL)4+~(H~'L~)4. One set has a bandwidth centered:on 425~nm~and~the other has a b~andwidth~center~ed~on~650 nm. A1though cryolite has the:most desired~low index (nL =~l.35), it is soft and 35 slightly hygroscopic,~so magnesium fluoride ~nL = 1.38) is~preferred. :Z~irconlum oxide (nH = 2.05) is one W092/22838 ~l~a8~7 PCI/V!i92/0427 pref erred high index material, although several other materials arP suitable.

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Claims

I claim:

1. A retroreflecting polarizer, comprising:
(a) a first material having a structured surface consisting of a linear array of substantially right angled isosceles prisms arranged side by side and having perpendicular sides which make an angle of approximately 45° with respect to the tangent to a smooth surface opposite the structured surface, (b) a second material essentially like the first material, (c) on the structured surface of at least one material, at least one optical stack of alternating layers of high and low refractive index materials of selected optical thicknesses;
the first and second materials all optically cemented to form a single unit in which the refractive index of the first and second materials, and the refractive indices and optical thicknesses of the layers of the optical stack, are all chosen to produce selective reflection of polarized light, such that (d) within one portion of the optical stack, an incident light beam of mixed polarization is separated into an s-polarized component and a p-polarized component, (e) the s-polarized component is reflected onto another portion of the optical stack and there reflected parallel to the incident beam but proceeding in an opposite direction, and (f) the p-polarized component is transmitted parallel to the incident beam.

2. An optical system comprising, along a common optic axis:
(a) a source of incident light of mixed polarization;
(b) a reflector;
(c) a quarterwave retardation plate;
(d) the retroreflecting polarizer of claim 1;
(e) a display device employing polarized light;
in which the p-polarized component is transmitted to the display device, and the s-polarized component passes through the quarterwave retardation plate to the reflector, returning through the quarterwave retardation plate to become a second p-polarized component before proceeding to the display device.