U.S. patent application number 10/727103 was filed with the patent office on 2004-07-15 for polarization splitter, method of manufacturing same and ophthalmic lens incorporating projection inserts containing it.
Invention is credited to Cado, Herve, Moliton, Renaud.
Application Number | 20040136082 10/727103 |
Document ID | / |
Family ID | 32309951 |
Filed Date | 2004-07-15 |
United States Patent
Application |
20040136082 |
Kind Code |
A1 |
Cado, Herve ; et
al. |
July 15, 2004 |
Polarization splitter, method of manufacturing same and ophthalmic
lens incorporating projection inserts containing it
Abstract
Polarization splitter in which reflection for the 1 polarization
is locally centered on at least one emission peak of the
image-emitting source of a micro-display or in which reflection for
the 1 polarization is locally centered on at least one peak
corresponding to a wavelength selected from red, green and blue.
Application to an ophthalmic lens incorporating inserts for
projecting an image towards the user.
Inventors: |
Cado, Herve; (Champigny Sur
Marne, FR) ; Moliton, Renaud; (Paris, FR) |
Correspondence
Address: |
FISH & RICHARDSON PC
225 FRANKLIN ST
BOSTON
MA
02110
US
|
Family ID: |
32309951 |
Appl. No.: |
10/727103 |
Filed: |
December 2, 2003 |
Current U.S.
Class: |
351/159.56 ;
359/618 |
Current CPC
Class: |
G02B 5/30 20130101; G02B
27/283 20130101; G02B 27/0101 20130101; G02B 2027/0112 20130101;
G02B 2027/0118 20130101 |
Class at
Publication: |
359/634 ;
359/618; 351/159 |
International
Class: |
G02B 027/10; G02B
027/14; G02C 007/02 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 3, 2002 |
FR |
02 15 197 |
Claims
What is claimed is:
1. A polarization splitter in which reflection of the S
polarization is locally centered on at least one peak selected from
the group consisting of (i) at least one emission peak of an
image-emitting source of a micro-display, (ii) red, (iii) green and
(iv) blue.
2. The polarization splitter according to claim 1 in which
reflection is locally centered on at least two peaks.
3. The polarization splitter according to claim 1, in which
reflection is locally centered on at least one peak in the visible
spectrum.
4. The polarization splitter according to claim 2, in which
reflection is locally centered on at least two peaks in the visible
spectrum.
5. The polarization splitter according to claim 1, in which
reflection for the S polarization is centered on red, green and
blue.
6. The polarization splitter according to claim 1, in which peaks
of its spectral reflectance curve for the S polarization, locally
centered around the peaks, have their maximum level comprised
between 60 and 100%.
7. The polarization splitter according to claim 1, in which peaks
of its spectral reflectance curve for the S polarization, locally
centered around the peaks, have their maximum level comprised
between 80 and 100%.
8. The polarization splitter according to claim 1, in which its
spectral reflectance curve for the S polarization has a level
comprised between 0 and 35%, in all zones not locally centered
around the peaks.
9. The polarization splitter according to claim 1, in which its
spectral reflectance curve for the S polarization has a level
comprised between 0 and 20%, in all zones not locally centered
around the peaks.
10. The polarization splitter according to claim 1, in which each
peak of the spectral reflectance curve of the S polarization
centered around one of the peaks has a mid-height width of between
5 and 100 nm.
11. The polarization splitter according to claim 1, in which each
peak of the spectral reflectance curve of the S polarization
centered around one of the peaks has a mid-height width of between
20 and 80 nm.
12. The polarization splitter according to claim 1, in which each
peak of the curve resulting from the product of the spectral
transmittance for the P polarization and the spectral reflectance
for the S polarization, centered around one of the peaks, has a
mid-height width of between 5 and 100 nm.
13. The polarization splitter according to claim 1, in which each
peak of the curve resulting from the product of the spectral
transmittance for the P polarization and the spectral reflectance
for the S polarization, centered around one of the peaks, has a
mid-height width of between 20 and 80 nm.
14. The polarization splitter according to claim 1, in which
spectral transmittance for the P polarization is greater than 80%
on the emission spectrum of the source.
15. The polarization splitter according to claim 14, in which the
emission spectrum of the source is between 400 and 700 nm.
16. The polarization splitter according to claim 1, in which
spectral transmittance for the P polarization is greater than 90%,
on the emission spectrum of the source.
17. The polarization splitter according to claim 16, in which the
emission spectrum of the source is between 400 and 700 nm.
18. The polarization splitter according to claim 1, in which the
integrated average transmittance between 400 and 700 nm is greater
than 70%.
19. The polarization splitter according to claim 1, comprising a
substrate with a stack of thin layers.
20. The polarization splitter according to claim 1, comprising a
substrate with a holographic element.
21. The polarization splitter according to claim 19, in which one
of the materials is silicon dioxide.
22. The polarization splitter according to claim 19, in which one
of the materials is zirconium dioxide or praseodymium titanate.
23. The polarization splitter according to claim 1, in the form of
a cube made up of two prisms.
24. A method of manufacturing a polarization splitter in which
reflection of the S polarization is locally centered on at least
one peak selected from the group consisting of (i) at least one
emission peak of an image-emitting source of a micro-display, (ii)
red, (iii) green and (iv) blue, comprising the following steps: (i)
providing a substrate; and (ii) depositing thin layers.
25. An ophthalmic lens with inserts for projecting an image towards
the user, having a polarization splitter in which reflection of the
S polarization is locally centered on at least one peak selected
from the group consisting of (i) at least one emission peak of an
image-emitting source of a micro-display, (ii) red, (iii) green and
(iv) blue.
26. The ophthalmic lens according to claim 25, in which the
polarization splitter is provided in the form of a cube.
27. A device for projecting an image towards the user, comprising
an ophthalmic lens with inserts for projecting an image towards the
user, having a polarization splitter in which reflection of the S
polarization is locally centered on at least one peak selected from
the, group consisting of (i) at least one emission peak of an
image-emitting source of a micro-display, (ii) red, (iii) green and
(iv) blue.
28. The projection device according to claim 27, additionally
comprising a liquid crystal micro-screen.
29. The projection device according to claim 28, in which the
liquid crystal micro-display emits a P polarized light in red,
green and blue.
Description
[0001] The present invention relates to a polarization splitter.
The invention further relates to a method of manufacturing such a
polarization splitter. It additionally relates to ophthalmic lenses
having inserts for projecting an image towards the user, comprising
such a polarization splitter. The invention finally relates to
devices for projecting an image towards the user, comprising such
ophthalmic lenses.
[0002] Polarization splitters are optical elements which enable
light to be broken down into its different polarization components.
The direction of polarization of the light is defined with respect
to the oscillation plane of the electric field. More often than
not, non-polarized light is broken down into its two orthogonal
linear polarizations. This being the case, a distinction is made
between (perpendicular) S polarization and (parallel) P
polarization. In S polarized light, the oscillation plane is
perpendicular to the plane of incidence defined by the normal line
of the surface and the incidence vector. In P polarization light,
the oscillation plane is parallel to the plane of incidence. The
components can be split by absorption or by reflection.
[0003] Generally speaking, polarization splitters transmit P
polarization and reflect S polarization. It is generally accepted
that an ideal polarization splitter reflects all the polarized
light perpendicularly to the plane of incidence (S), whereas it
transmits all the polarized light parallel with the plane of
incidence (P) (for a given wavelength). The efficiency of the
polarization splitting function may be expressed as the product of
the spectral reflection of the S polarization (Rs) multiplied by
the spectral transmission of the P polarization (Tp), namely
(Rs).times.(Tp) (at a given wavelength). It is also generally
acknowledged that the aim in producing a polarization splitter is
to achieve an efficiency in excess of 80% and preferably in excess
of 90%.
[0004] Polarization splitters lend themselves to various
applications, which include ophthalmic lenses incorporating inserts
for projecting an image towards a user.
[0005] By this is meant ophthalmic lenses of image-combining
systems for spectacles or masks; an image is projected towards the
eye of the wearer via an optical path determined by the lens; the
term "lens" is then used to refer to the optical system containing
the inserts, which may be designed in particular to be mounted in a
spectacle frame or in a mask. The inserts may contain mirrors, beam
splitters, polarization splitter cubes, quarter-wave plates,
lenses, mirrors, concave reflective lenses (a Mangin mirror, for
example), diffraction lenses and/or holographic components. A
device for projecting images towards the user will then comprise
the lens mounted in spectacles or masks and an image source such as
a micro-screen, for example a liquid crystal micro-screen, more
specifically a Kobin CyberDisplay 320 micro-display.
[0006] In such applications, the polarization splitter elements are
used to process the polarized light emitted by the micro-screens
currently used, such as micro-displays.
[0007] One example of such an ophthalmic lens is illustrated in
FIG. 1. The image is emitted by a source 1. The source 1 may be a
miniaturized micro-screen such as a liquid crystal micro-display
emitting polarized light (P). The optical system of the projecting
ophthalmic lens 10 comprises a field lens 2. A mirror 3 and the
polarization splitter 4 are placed so as to intercept the optical
path travelled by the image inside the ophthalmic lens 10. Bonded
to the polarization splitter 4 is a quarter-wave plate 5 and a
Mangin mirror 6.
[0008] The ophthalmic lens 10 operates in the following manner.
Polarized light from the source 1 passes firstly through a field
lens 2. Having passed through it, it is reflected by a mirror 3,
which returns turns it through an angle of 90.degree.. The light
then passes through the polarization splitter 4, whereby one of the
polarization components (S) is reflected and the other (P) is
transmitted. The transmitted component passes through a
quarter-wave plate 5, the axes of which are arranged at 45.degree.
relative to the propagation direction P in the plane perpendicular
to the propagation direction, then strikes a Mangin mirror 6, which
reflects the light so that it is sent back through the quarter-wave
plate. The light, which is now S polarized, is reflected by the
polarization splitter towards the eye of the observer.
Consequently, this embodiment enables the polarized light
transmitted by the micro-screen to be sent back towards the eye
7.
[0009] However, a device of this type incorporating an "ideal"
polarization splitter has a drawback in that, in terms of
ophthalmic function, it directs only 50% of the light from an
object towards the eye because 50% of this light is S polarized and
is therefore reflected by the splitter.
[0010] The following definitions will be used throughout this
description.
[0011] See-through image: see-through image refers to the image of
a scene as viewed when the light rays pass directly through the
polarization splitter element.
[0012] Screen image: screen image refers to the image of a light
source (in our example a micro-screen) passing though the lens
inserted in the display glass, as illustrated in FIG. 1.
[0013] Efficiency of the polarization splitting function: see
above.
[0014] Efficiency of the vision see-through function: this refers
to the value of the mathematical integral of the product in terms
of non-polarized light of the polarization splitter [=1/2*(Tp+Ts)
multiplied by the emission spectrum of the source divided by the
integral of the emission spectrum of the source [in the spectral
domain in question].
[0015] Transmission of the imaging function of the display glass:
this refers to the value of the mathematical integral of the
product of the spectral transmission of the imaging function of the
display glass (defined by the optical path in FIG. 1) multiplied by
the emission spectrum of the source divided by the integral of the
emission spectrum of the source [in the spectral domain in
question].
[0016] It should be noted that transmission of the imaging function
of the display glass as well as transmission of the see-through
vision function may also be weighted by the spectral sensitivity,
of the eye. This is then referred to as "photopic transmission of
the imaging function of the display glass". The normalized curve Y
of the CIE (Commission International de l'Eclairage) 2.degree.
observer will be used as the curve representing the spectral
sensitivity of the eye.
[0017] Locally centered: a curve is said to be locally centered on
the emission spectrum of the light source when:
[0018] the spectrum of the emission source assumes the form of
peaks (see FIG. 2, for an example of this);
[0019] around an emission peak of the light source, within a
restricted spectral domain with an order of magnitude that is twice
the mid-height width of the emission peak of the source, the curve
follows a peak (or a valley), the local extremum of which is not
spectrally distant from the apex of the emission peak of the source
by greater than the value of the mid-height width of this peak;
[0020] alternatively, the Rayleigh criterion may also be applied,
on the assumption that two curves are locally centered.
[0021] The invention therefore provides a polarization splitter
which improves "see-through" vision whilst maintaining a high
screen transmission. The invention also enables a balance to be
maintained between the colors of the two images received by the
eye.
[0022] The invention is based, in particular, on the discovery that
it is sufficient for the polarization splitter to reflect the S
polarization around the frequencies at which the micro-display
emits, in other words, typically 630, 520 and 460 nm (corresponding
to the colors red, green and blue, respectively).
[0023] The invention therefore provides a polarization splitter, in
which reflection of the polarization is locally centered on at
least one emission peak of the emission source of a micro-screen
image.
[0024] Micro-screens or micro-displays are generally known and
nowadays are based on LCD technology; one example is the Kopin
micro-display. Moreover, it is an easy matter to measure the energy
emission spectrum of such a micro-display and determine at least
one emission wavelength from it.
[0025] According to one embodiment, the reflection of the splitter
is locally centered on at least two peaks.
[0026] According to one embodiment, the reflection of the splitter
is locally centered on at least one peak in the visible
spectrum.
[0027] The invention also proposes a polarization splitter in which
reflection of the S polarization is locally centered on at least
one peak corresponding to a wavelength selected from red, green and
blue.
[0028] Transmission of the S polarization is effected in a "comb"
pattern (if this transmission is centered on two wavelengths at
least).
[0029] According to one embodiment, the splitter reflects the
polarization S centered on the red, green and blue.
[0030] According to one embodiment, the peaks of the spectral
reflectance curve for the S polarization in the splitter, locally
centered around the peaks, have a maximum level of between 60 and
100%, preferably between 80 and 100%.
[0031] According to one embodiment, the spectral reflectance curve
for the S polarization in the splitter has a level of between 0 and
35%, preferably between 0 and 20%, in all zones that are not
locally centered around the peaks.
[0032] According to one embodiment, each peak of the spectral
reflectance curve of the S polarization in the splitter, centered
around one of the peaks, has a mid-height width of between 5 and
100 nm, preferably between 20 and 80 nm.
[0033] According to one embodiment, each peak of the curve in the
splitter resulting from the product of the spectral transmittance
for the P polarization and the spectral reflectance for the S
polarization, centered around one of the peaks, has a mid-height
width ranging between 5 and 100 nm, preferably between 20 and 80
nm.
[0034] According to one embodiment, the spectral transmittance for
the P polarization in the splitter is in excess of 80%, preferably
in excess of 90%, on the emission spectrum of the source, which is
preferably between 400 and 700 nm.
[0035] In one embodiment the integrated mean transmittance in the
splitter between 400 and 700 nm is greater than 70%.
[0036] According to one embodiment, the splitter has a substrate
comprising a stack of thin layers.
[0037] According to one embodiment, the splitter has a substrate
comprising a holographic element.
[0038] According to one embodiment, one of the materials in the
stack is silicon dioxide.
[0039] According to one embodiment, one of the materials in the
stack is zirconium dioxide or praseodymium titanate.
[0040] According to one embodiment, the splitter according to the
invention is in the form of a cube made up of two prisms.
[0041] The invention also relates to a method of manufacturing a
polarization splitter, comprising the following steps:
[0042] (i) providing a substrate; and
[0043] (ii) depositing thin layers.
[0044] The invention further relates to an ophthalmic lens having
inserts for projecting an image towards the user, comprising a
polarization splitter according to the invention.
[0045] According to one embodiment, the polarization splitter is in
the form of a cube.
[0046] The invention further relates to a device for projecting an
image towards the user, comprising a lens according to the
invention.
[0047] According to one embodiment, the projection device
additionally comprises a liquid crystal micro-screen.
[0048] According to one embodiment, the liquid crystal micro-screen
in the projection device emits a polarized light P in the red,
green and blue spectrum.
[0049] The invention will be described in more detail with
reference to the appended drawings, of which:
[0050] FIG. 1 is a schematic diagram of an ophthalmic lens with
inserts for projecting an image toward the user;
[0051] FIG. 2 represents the spectrum of the emission source (Kopin
Cyber display Color 320 micro-display);
[0052] FIG. 3 shows the spectral reflectance curve for the S
polarization for a polarization splitter according to the
invention;
[0053] FIG. 4 shows the spectral transmittance curve for the P
polarization for a polarization splitter according to the
invention.
[0054] The splitter according to the invention comprises a stack of
thin layers on a substrate of a given refractive index. It may also
comprise a substrate provided with a stack or holographic
element.
[0055] For example, it might be a stack containing only two
materials, one a "high refractive index" material and the other a
"low refractive index" material, in an alternating arrangement.
[0056] This embodiment has been chosen due to the fact that it is
easy to produce. However, it would also be possible to use 20
different materials, for example, arranged in a stack of 20 layers,
using materials which might be termed "medium refractive
index".
[0057] As a high refractive index material, ZrO.sub.2, a well known
material, might be used, or PrTiO.sub.3 (praseodymium titanate). In
the latter case, the material is deposited on the substrate
starting with a non-stoichiometric compound [available from the
Merck company under the name of Substance H2], which is deposited
by vacuum deposition in the presence of oxygen. The compound is
then in oxidized form and produces a transparent film corresponding
to the formula PrTiO.sub.3.
[0058] The refractive index of PrTiO.sub.3 is 2.0095 at 635 nm
(reference wavelength). The refractive index of ZrO.sub.2 is
1.9883.
[0059] The second material therefore has a lower refractive index
than the first. These materials include in particular SiO.sub.2 and
MgF.sub.2, the refractive index of SiO.sub.2, which is 1.4786 at
635 nm, having been found to be particularly suitable.
[0060] The substrate may be any transparent substrate that is
compatible with the materials constituting the stack and in
particular may be inorganic or organic substrates.
[0061] By "inorganic substrate" is meant a substrate of mineral
glass, as opposed to the concept of "organic substrate", which is
made from a polymer.
[0062] Appropriate materials for organic substrates are, for
example, polymers from the sort consisting of polythiourethanes,
obtained from a polythiol and a polyisocyanate. Such materials and
the process by which they are obtained are described in patents
U.S. Pat. No. 4,689,387 and U.S. Pat. No. 4,775,733, for
example.
[0063] Suitable polythiols might include, for example,
pentaerythrol tetrakis (thioglycolate), pentaerythrol
tetrakis(mercapto-propionate) or MDO
[4-mercapto-methyl-3,6-dithia-1,8-octane dithiol]. The
polyisocyanate may be xylylene diisocyanate in particular.
[0064] One particularly suitable organic substrate is obtained by
polymerising compositions based on xylylene diisocyanate,
pentaerythritol tetrakis(mercapto-propionate) and MDO. Such a
product is available from the Mitsui company under the name of
MR8.
[0065] In terms of the inorganic substrate, material 1.6 sold by
Corning, Code 60043, could be used, for example, the optical
constants of which are essentially identical to those of MR8.
[0066] BK7, sold by Schott Optical Glass, may also be used as an
inorganic substrate, for example.
[0067] The splitter according to the invention is obtained by
depositing successive thin layers. Generally speaking, the stack
will have 5 to 20, in particular 10 to 15, layers of material.
[0068] During the deposition phase, it may be preferable to
maintain the substrate at a temperature above ambient temperature,
for example between 80 and 120.degree. C. The substrate is
advantageously subjected to an ionic cleaning process prior to the
deposition stage, for example with argon.
[0069] During the deposition phase, the evaporation rate is
generally 1 to 10 nm/s, preferably 2 to 5 nm/s.
[0070] The layers and their respective thickness are determined in
a conventional manner by a person skilled in the art, as a function
of the wavelengths around which transmission is to be centered. The
conventional software program "Essential Macleod", version 8.5,
2002, sold by Thin Film Center Inc., 2745 E. Via Rotonda, Tucson,
Ariz. 85716, may be used for this purpose, for example. Details of
the substrate (indicating its refractive index), the optimisation
target adapted to the spectral emission curve of the source of
images and the list of materials are entered; the software then
simulates the stack.
[0071] The polarization splitter according to the invention is
particularly well suited to applications of the type involving
portable vision equipment, such as ophthalmic lenses with inserts
for projecting an image towards the user.
[0072] An example of such a lens is illustrated in FIG. 1, details
of which were described above. Compared with the embodiment known
from the prior art (in which the sought splitter is the "ideal"
splitter), the invention permits better "see-through" vision, i.e.
the ophthalmic vision of an object. In fact, to the extent that not
all the S polarized light is reflected by the splitter, the average
transmission is increased. As regards the micro-display, since only
some of the S polarized light returned by the mirror after passing
through the quarter-wave plate is reflected towards the eye of the
user, it produces a drop in the light transmission emitted by the
micro-display towards the eye of the user. However, this drop is
not significant because the splitter reflects the S polarization
around the emission wavelengths of the micro-display.
[0073] In the example of an display glass such as defined above,
efficiency of the see-through vision passes from a value of about
50% such as obtained with a conventional polarization splitter to a
value of about 75% using the polarization splitter according to the
invention. At the same time, transmission of the imaging function
drops to a value of approximately 40% only, which compares with 50%
obtained using a conventional polarization splitter. The above also
applies to the optical system, all things being equal, in
particular as regards sources of loss.
[0074] The ophthalmic lens is preferably made from the same
material as the substrate on which the stack of thin layers is
deposited, for example MR8 or BK7. This being the case, the
splitter is made in the form of a prism. In practice, the fact of
using a substrate of the same composition for the splitter and
hence with the same refractive index as the material used for the
ophthalmic lens, makes the polarization splitter less visible to
the wearer and thus reduces discomfort caused by the ophthalmic
function of the glass.
[0075] For this type of application, if the refractive index
n.sub.3 is substantially different from the refractive index of the
ophthalmic lens, the polarization splitter is advantageously
provided in the form of a splitter cube, made up of two prisms, one
of them having a stack of the type described above on one of its
faces. It would also be possible to design a polarization splitter
in the form of a plate embedded in the ophthalmic lens.
[0076] The examples described below are intended to illustrate the
invention without limiting it. The simulation software used is
"Code V", version 9.0, Sept. 2001, available from Optical Research
Associate, 3280 East Foothill Blvd, Pasadena, Calif. 91107. The
simulated transmission values are calculated on the basis of the
formula used for the thin layer stack. Furthermore, a Kopin
micro-display is used in all the examples (see FIG. 2).
EXAMPLE 1
[0077] A biplanar disc of BK7, refractive index 1.515, was cleaned
in an ultrasonic bath (Range M10, standard used for anti-reflective
treatments).
[0078] Having been thus prepared, the disc was then introduced into
a deposition unit under vacuum. It is then subjected to ionic
cleaning under argon at a 3.10-5 mbar pressure with a voltage of
120 V at the anode and 1 A of ionic current for 2 minutes.
[0079] A layer of PrTiO.sub.3 was then deposited at a thickness as
indicated in Table 1, at a pressure of 2.5.multidot.10.sup.-5 mbar
under the following conditions:
[0080] evaporation rate: 3 nm/s;
[0081] oxygen pressure: 5.multidot.10-5 mbar;
[0082] evaporation source: electron gun.
[0083] The thickness of the layer was monitored by means of a
quartz balance and evaporation halted when the thickness indicated
in Table 1 was reached.
[0084] A layer of SiO.sub.2 was then deposited in a thickness as
indicated in Table 1, under the same conditions.
[0085] A total of 14 alternating layers were then deposited. After
being processed in this manner, the disc was finally cut to obtain
polarization splitters of the desired shape.
1TABLE 1 Layer Material Refractive index Thickness [nm] 1 SiO.sub.2
1.4786 19.19 2 PrTiO.sub.3 2.0095 139.24 3 SiO.sub.2 1.4786 25.3 4
PrTiO.sub.3 2.0095 60.36 5 SiO.sub.2 1.4786 21.89 6 PrTiO.sub.3
2.0095 124.97 7 SiO.sub.2 1.4786 48.24 8 PrTiO.sub.3 2.0095 130.20
9 SiO.sub.2 1.4786 48.27 10 PrTiO.sub.3 2.0095 466.56 11 SiO.sub.2
1.4786 15.92 12 PrTiO.sub.3 2.0095 228.54 13 SiO.sub.2 1.4786 71.59
14 PrTiO.sub.3 2.0095 113.38
[0086] The optical characteristics of a polarization splitter thus
obtained are indicated in FIGS. 3 and 4 at an incidence of 45.
FIGS. 3 and 4 illustrate the spectral reflectance and transmittance
of the polarization splitter with regard to the S and P
polarization light respectively. It may be noted that the
transmittance is on average at least 95% for the P polarization
light whereas the S polarization light is reflected as a function
of the wavelength, this reflection (or transmission) being centered
around the red, green and blue wavelengths (at 630, 520 and 460 nm
respectively). Transmission is between approximately 25 and 30%,
the mid-height widths being between approximately 35 and 70 nm.
[0087] The integrated average transmission between 400 and 700
.mu.m is approximately 75%, which represents a gain of 25% in terms
of ophthalmic vision compared with a conventional splitter.
[0088] The visual transmission (weighting of the transmission by
the spectral response of the eye) is also approximately 75%.
[0089] The transmittance of the imaging function was also
determined for the center and edges of the field of vision in order
to determine the effect of angle on processing. The transmission is
calculated for a standardized CIE (Commission International de
l'Eclairage) user Y in the example of a display glass of the type
illustrated in FIG. 1. The values obtained are as follows (FOV:
Field of View, -4 to +4.degree. and -5 to +5, respectively).
2 Blue Y vs FOV -5 0 5 4 34.45 35.34 0 34.49 35.98 35.33 -4 34.45
35.34
[0090]
3 Green Y vs FOV -5 0 5 4 31.29 39.57 0 31.47 37.59 39.59 -4 31.27
39.57
[0091]
4 Red Y vs FOV -5 0 5 4 39.04 34.01 0 39.13 38.61 33.93 -4 39.02
34.01
[0092] It is therefore evident that the transmissions tended to be
well balanced between the three colors on the field of vision,
which means that perception of the colors is well preserved over
the whole image.
[0093] The visual transmission of white obtained at the center of
the field of the micro-display was 39% (compared with approximately
54% with an "ideal" splitter); the calculation of this transmission
takes account of other sources of loss. A slightly reduced value
such as this does not give rise to any significant perception by
the eye and does not cause discomfort.
[0094] The variation in transmittance as a function of the angle of
incidence was also determined. A slightly asymmetrical behaviour
was observed in the red and blue but overall stability was
preserved in the white spectrum cross the entire field.
[0095] The colorimetric co-ordinates of the blue displayed by the
micro-display were also analysed and the calorimetric shift of the
CIE co-ordinates x, y, z of the white displayed by the device
compared with the white displayed by the micro-display was
analysed. This shift is expressed here as being the distance of the
micro-display from white in the space x, y:
5 Distance relative to white micro-display (space x, y) D.sup.2 =
(x - x.sub.white).sup.2 + (y - y.sub.white).sup.2 0.025826253
0.0175353 0.025264825 0.012468111 0.019038742 0.025812775
0.0175353
[0096] It may be noted that the splitter according to the invention
has very little effect on the way colors are perceived in the
imaging process. It modifies the colors of the micro-display very
little.
[0097] The see-through transmission was also calculated for various
angles of incidence on the splitter. The values are 74.57%, 76.60%
and 76.28% for angle values of 40, 45.degree. and 50.degree.,
respectively. Transmission is therefore homogeneous.
[0098] A colorimetric study was also undertaken in order to
determine the effect of the splitter according to the invention in
"see-through" mode. The micro-display in "see-through" vision can
be used as the source of white light for this purpose and the
modification in the perception of this white light observed. The
results are expressed on the basis of the CIE co-ordinates x,y,z.
The angle of incidence is 40, 45.degree. and 50.degree.. The
results are set out below.
6 40.degree. x y z Sample 0.32528584 0.3162037 0.35851046
Micro-display 0.31273021 0.32903828 0.35823151 Delta 0.01255563
-0.01283458 0.00027896
[0099]
7 45.degree. x y z Sample 0.31966024 0.33805613 0.34228364
Micro-display 0.31273021 0.32903828 0.35823151 Delta 0.00693002
0.00901785 -0.01594787
[0100]
8 50.degree. x y z Sample 0.31092033 0.34921327 0.33986641
Micro-display 0.31273021 0.32903828 0.35823151 Delta -0.00180989
0.02017499 -0.0183651
[0101] This same study was undertaken for each color, red, green
and blue, and supports the results obtained above.
EXAMPLE 2
[0102] The procedure was the same as that described in Example 1,
but on a MR8 substrate (in fact an inorganic support corresponding
to MR8), with a refractive index 1.5931.
[0103] The values are set out in table 2.
9TABLE 2 Layer Material Thickness [nm] 1 PrTiO.sub.3 153.03 2
SiO.sub.2 240.94 3 PrTiO.sub.3 79.04 4 SiO.sub.2 50.45 5
PrTiO.sub.3 141.05 6 SiO.sub.2 38.16 7 PrTiO.sub.3 236.4 8
SiO.sub.2 21.19 9 PrTiO.sub.3 490.84 10 SiO.sub.2 51.32 11
PrTiO.sub.3 116.76
[0104] The optical characteristics of a polarization splitter thus
obtained are essentially identical to those given in FIG. 3
mentioned above, for an incidence of 45.degree..
[0105] The other conclusions drawn in respect of Example 1 apply to
this example mutatis mutandis.
EXAMPLE 3
[0106] The procedure was the same as that described in Example 1
but PrTiO.sub.3 was replaced by ZrO.sub.2 (the deposition
conditions were conventional).
[0107] The results are unchanged.
EXAMPLE 4
[0108] The procedure was the same as that described in respect of
Example 2, but PrTiO.sub.3 was replaced by ZrO.sub.2 (the
deposition conditions were conventional).
[0109] The results are unchanged.
[0110] Apart from the applications described above, the
polarization splitter may also be useful in applications involving
the supply and processing of polarized light. Furthermore, the
polarization splitter according to the invention may be used as a
means of splitting light into its circular or elliptical
polarization components.
* * * * *