High efficiency retroreflecting polarizer

A retroreflecting polarizer comprises a pair of first and second substrates, each essentially like the other, each having a planar first surface and a structured second surface, whereby the structured second surface comprises a linear array of isosceles prisms with faces formed at an angle of approximately tan.sup.-1 (1/n) with respect to the planar first surface. The respective structured second surfaces of the first and second substrates are mated with one another through one or more of substantially uniformly thick layers, all of the same optical material, that are parallel to the structured surfaces. The structured surfaces and layers are separated from one another by associated air gaps. The interfaces formed within the retroreflecting polarizer satisfy Brewster's condition for a light beam normally incident upon the retroreflecting polarizer which is split into a .sigma.-polarized retroreflected light beam and a substantially .pi.-polarized transmitted light beam.

TECHNICAL ART 
The instant invention generally relates to retroreflecting polarizers. 
BACKGROUND OF THE INVENTION 
Polarized light is created from most lamp assemblies using a flat window 
which absorbs one linear polarization state while passing the other. As an 
alternative, many device concepts are known within the art which do not 
absorb but actually reflect the unwanted polarization state using 
repeating prism structures. This reflected light can then be rotated or 
otherwise converted within the lamp assembly so that it passes through the 
prismatic device on future attempts, thereby delivering fully polarized 
light without the absorption losses of conventional polarizers. 
However, such prismatic device concepts are difficult to manufacture 
because they require acute prism apex angles (90 degrees or less) and 
further require materials having a high index of refraction. Many such 
concepts further rely substantially on the conventional polarization 
properties of light incident at a 45 degree angle to a refractive 
boundary. These polarization properties are optimum at the Brewster angle 
which is dependent upon the indices of refraction of the adjoining media. 
For light passing from air into a material with an index of refraction of 
about 1.6, the associated Brewster angle is approximately 58 degrees, so 
that the above described prismatic device with a 45 degree angle to the 
refractive boundary would not achieve suitable efficiency unless expensive 
coatings are applied. 
In accordance with Maxwell's Equations, at the Brewster angle, reflected 
light is polarized perpendicular to the plane of incidence, i.e. 
.sigma.-polarization, and the transmitted/refracted light is mostly 
polarized parallel to the plane of incidence, i.e. .pi.-polarization. The 
Brewster angle .theta..sub.B is given by .theta..sub.B =tan.sup.-1 
(n.sub.T /n.sub.l) where n.sub.T is the index of refraction of the medium 
of transmittance/refraction and n.sub.l is the index of refraction of the 
medium of incidence/reflection. 
U.S. Pat. No. 5,559,634 to Weber discloses and claims a retroreflecting 
polarizer comprising a plurality of thin film stacks coated onto 
substrates having structured surfaces comprising a linear array of 
isosceles prisms having sides which make an angle in the range of 40-50 
degrees, which is relatively expensive and difficult to produce because of 
the relatively large angle of the associated structured surface, and the 
need for a plurality of different coatings necessary to satisfy the 
MacNeille conditions whereby the reflection coefficient for 
.pi.-polarization is essentially zero at each film interface. 
SUMMARY OF THE INVENTION 
The instant invention overcomes the above-noted problems by providing a 
retroreflecting polarizer comprising a pair of first and second 
substrates, each essentially like the other, each having a planar first 
surface and a structured second surface, whereby the structured second 
surface comprises a linear array of isosceles prisms with faces formed at 
an angle of approximately tan.sup.-1 (1/n) with respect to the planar 
first surface. The respective structured second surfaces of the first and 
second substrates are mated with one another through one or more of 
substantially uniformly thick layers that are parallel to the structured 
surfaces. The structured surfaces are separated from the proximally 
adjacent layers by associated air gaps, and the respective layers are also 
separated from one another by associated air gaps. 
In operation, light normally directed upon the planar first surface of the 
first substrate is transmitted through the first substrate onto the 
structured second surface thereof. The angle of the structured surface 
satisfies the Brewster condition whereby upon reflection therefrom the 
reflected portion of the light beam is polarized normal to the plane of 
incidence. The remaining transmitted portion of the light beam refracts at 
the interface formed between the structured surface and the adjacent air 
gap so that the refracted light is polarized primarily within the plane of 
polarization. The instant invention is therefore adapted to polarized 
light in accordance with the Brewster condition whereby the polarization 
process occurs at an interface for which the light beam propagates from a 
high index of refraction to a low index of refraction, for which the 
associated structured surface can be constructed with relatively shallow 
angles and is therefore easier to manufacture. 
After exiting the structured second surface of the first substrate, the 
light beam propagates through a plurality of layers, each essentially 
parallel to the structured second surface of the first substrate, and each 
constructed from the same optical material as the first substrate. The 
reflection and refraction processes at the interfaces created by each 
layer further separate the .sigma. and .pi. polarization components so as 
to improve the efficiency of the polarizer. The remaining .pi. polarized 
light beam then exits the planar first surface of the second substrate in 
essentially the same direction as the incident beam. 
In accordance with another aspect of the instant invention, a device is 
provided for transmitting one polarization state while reflecting an 
orthogonal state comprising an optical substrate of index of refraction n 
having a first planer surface, a second surface and a region between said 
surfaces comprising a plurality of air gaps, each layer alternating at an 
angle of plus and minus a slant angle from the first planer surface given 
by tan.sup.-1 (1/n). 
In accordance with yet another aspect of the instant invention, a method 
for fabricating a polarizing device comprises the steps of positioning a 
top and bottom optical element each having an alternating prismatic 
structure so that the prismatic structures are facing each other, 
producing a plurality of uniformly thick windows, alternating in 
accordance with the above slant angle, and assembling them in a nested 
arrangement. 
Accordingly, one object of the instant invention is to provide an apparatus 
for efficiently transmitting one polarization state while reflecting the 
orthogonal state. 
A further object of the instant invention is to provide an apparatus for 
efficiently transmitting one polarization state while reflecting the 
orthogonal state that can be manufactured relatively easily. 
A yet further object of the instant invention is to provide a method for 
efficiently transmitting one polarization state while reflecting the 
orthogonal state that can be accomplished with a single optical material. 
In accordance with these objectives, one feature of the instant invention 
is a pair of substrates each constructed from a given optical material 
having a planar first surface and a structured second surface, whereby the 
structured second surface comprises a linear array of isosceles prisms 
with faces formed at an angle of approximately tan.sup.-1 (1/n) with 
respect to the planar first surface, whereby n is the index of refraction 
of the optical material. 
Another feature of the instant invention are one or more layers interposed 
between the structured second surfaces of the first and second substrates, 
whereby each layer has an essentially uniform thickness and is essentially 
parallel to the structured second surfaces of the first and second 
substrates, and each layer is constructed from the same optical material 
as the first and second substrates. 
Yet another feature of the instant invention are a plurality of associated 
air gaps adjacent each of the surfaces of each layer. 
Yet another feature of the instant invention is a means of securing the 
substrates and layers to one another. 
Yet another feature of the instant invention is a means of sealing the 
assembly of substrates and layers at the periphery thereof. 
The specific features of the instant invention provide a number of 
associated advantages. One advantage of the instant invention with respect 
to the prior art is that by arranging the structured surfaces so as to 
satisfy the Brewster condition for light traveling from a high index of 
refraction media to a low index of refraction media, the structured 
surfaces can be constructed with shallower angles which provides for a 
more compact assembly, greater polarization efficiency, and reduced 
manufacturing costs. 
Another advantage of the instant invention is that the instant invention 
can be constructed from elements which are each made from the same optical 
material. 
Yet another advantage of the instant invention requires only two distinct 
parts. 
The instant invention will be more fully understood after reading the 
following detailed description of the preferred embodiment with reference 
to the accompanying drawings. While this description will illustrate the 
application of the instant invention as a general retroreflective 
polarizer, it will be understood by one with ordinary skill in the art 
that the instant invention can be applied to lighting systems or to back 
lighting systems.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
Referring to FIG. 1, a retroreflecting polarizer 10 has a planar first 
surface 12 and second surface 14 disposed orthogonal to an incident light 
beam 16. Within the retroreflecting polarizer 10 exists a plurality of air 
gaps 18 alternating at plus and minus the aforementioned slant angle with 
respect to the planar first surface 12. The incident light passing through 
the planar first surface 12 is therefore refracted at the interfaces of 
each air gap at an incidence angle equal to the Brewster polarization 
angle. 
The incident light beam 16 passing through the plurality of air gaps 18 is 
therefore split in accordance with known principles into a reflected light 
beam 22 of one linear polarization state and a transmitted light beam 20 
of primarily the orthogonal state. The reflected light beam 22, making an 
angle of incidence of twice the slant angle with the planar first surface 
12, undergoes total internal reflection off planar first surface 12 to be 
directed back toward the air gaps 18 as is understood to one having 
ordinary skill in the art. Due to the alternating air gap angles, 
reflected light beam 22 is therefore either reflected off the faces of the 
multiple layers to pass orthogonally back through the planar first surface 
12, or it passes between the air gaps, now acting as wave guides until the 
alternate angle of the air gaps is encountered, again reflecting back 
through planar first surface 12 in a substantially normal direction. The 
entirety of the reflected light beam 22 exiting the retroreflecting 
polarizer 10 is therefore polarized orthogonal to the transmitted light 
beam 20 transmitted through the retroreflecting polarizer 10. 
Alternately, referring to FIG. 2, retroreflecting polarizer 10 comprises 
first 26 and second 26 substrates of optical material and one or more 
layers 28 of the same optical material interposed therebetween. Each 
substrate further comprises a planar first surface 28; and a structured 
second surface 30 comprising of a linear array 29 of isosceles prisms 27 
arranged side by side and having faces 31 formed at an angle of 
.theta..sub.B ' with respect to said planar first surface, where 
.theta..sub.B is approximately tan.sup.-1 (1/n) with n being the index of 
refraction of said optical material The layers 28 are separated by air 
gaps 18. 
In operation, an incident light beam 16 normal to the planar first surface 
28 of the first substrate 28 is transmitted therethrough. The incident 
light beam 16 splits at the interface of the structured second surface 30 
of the first substrate 24 into a first reflected light beam 50 and a first 
refracted light beam 60. Because the angle of incidence at the interface 
is .theta..sub.B ' which satisfies the Brewster condition for light 
passing from a high index of refraction medium to a low index of 
refraction medium, the first reflected light beam 50 is .sigma.-polarized 
and the first refracted light beam 60 is substantially .pi.-polarized. If 
the index of refraction n of the optical material is greater than about 
1.272 (i.e. greater than the value of n for which sin.sup.2 (.theta..sub.B 
')=cos(.theta..sub.B ')), the first reflected light beam 50 will be 
totally internally reflected at the planar first surface 28. This first 
reflected light beam 50 is further split at the interface of the 
structured second surface 30 of the first substrate 24 into a second 
reflected light beam 52 and a second refracted light beam 54, whereby the 
reflected light beam 52 is returned by the retroreflecting polarizer 10 as 
the reflected light beam 22. The first refracted light beam 60 passes 
through a first air gap to the interface at the surface of the layer 28 of 
optical material and splits thereat into a third reflected light beam 70 
and a third refracted light beam 80. Because the angle of incidence at the 
interface is .theta..sub.B =tan.sup.-1 (n) which satisfies the Brewster 
condition for light passing from a low index of refraction medium to a 
high index of refraction medium, the third reflected light beam 70 is 
.sigma.-polarized and the third refracted light beam 80 is substantially 
.pi.-polarized. The processes of reflection and refraction under Brewster 
conditions continues within the retroreflecting polarizer 10, whereby the 
reflected light beam 22 retroreflected by the retroreflecting polarizer 10 
comprises .sigma.-polarized light and the transmitted light beam 20 
transmitted through the retroreflecting polarizer 10 comprises 
substantially .pi.-polarized light, whereby the percentage of 
.pi.-polarized light therein increases with an increasing number of layers 
28. 
It is known within the art that multiple index interfaces employed at a 
Brewster angle of incidence will provide substantially complete 
polarization of the incident light. Deviations from the Brewster angle 
therefore require significantly greater layers to achieve the same result. 
Since the angles for any given light beams upon the operative polarizing 
surfaces of instant invention is closer to the Brewster angle than for 
prior art prismatic polarizers, the polarization efficiency of each layer 
is much greater, reducing the number of required layers. Further, for an 
optical material having an index of refraction of about 1.732 the instant 
invention employs prismatic structures having apex angles of approximately 
120 degrees. For a given distance between adjacent elements of the 
structure, this results in over a 60% reduction in the material removal 
required to manufacture the structure compared to known devices, leading 
to greater control in the manufacturing process. Preferably, the depth of 
the prismatic elements comprising the structured surfaces should be 
relatively small so as to reduce the overall thickness of the 
retroreflecting polarizer. In one embodiment, the depth would be about 1 
mm. Furthermore, the number of layers is preferably between 10 and 30, and 
the angles of the faces of the structured first and second surfaces are 
within plus or minus 5 degrees of the nominal tan.sup.-1 (1/n) value. 
Referring FIG. 3, the instant invention is particularly attractive in that 
it may be readily manufactured using primarily only two distinct 
components 100 and 102. The top and bottom parts 100(a) and 100(b) are 
identical yet flipped, having a planar structure on one side an 
alternating surface on the other. Between these parts are placed ten to 
twenty units of a second part 102(a) to 102(x), each comprised of a single 
sheet of transparent material of uniform thickness, molded into an 
accordion shape with the alternating slanted structure. All parts will 
therefore easily mesh to form the retroreflecting polarizer 10 without 
special alignment arrangements, as indicated by the arrows shown in FIG. 
3. Further, since a small air space is typical upon placing optical 
elements lightly together, no special arrangements are necessary to 
separate the parts. However, the periphery of the retroreflecting 
polarizer 10 should be sealed to prevent moisture from entering the air 
spaces and nullifying its function. 
One of ordinary skill in the art will appreciate that while the instant 
invention is illustrated with air gaps between the layers of optical 
material, and between the first and second substrates and the layers of 
optical material proximate thereto, the air gaps could be replaced by a 
second optical material having an index of refraction lower than the index 
of refraction of the optical material from which the substrates and layers 
are constructed. 
While specific embodiments have been described in detail, those with 
ordinary skill in the art will appreciate that various modifications and 
alternatives to those details could be developed in light of the overall 
teachings of the disclosure. Accordingly, the particular arrangements 
disclosed are meant to be illustrative only and not limiting as to the 
scope of the invention, which is to be given the full breadth of the 
appended claims and any and all equivalents thereof.