U.S. patent application number 12/203167 was filed with the patent office on 2010-03-04 for flat panel lens.
This patent application is currently assigned to Microsoft Corporation. Invention is credited to Neil Emerton, Timothy A. Large, Adrian Travis.
Application Number | 20100053771 12/203167 |
Document ID | / |
Family ID | 41646460 |
Filed Date | 2010-03-04 |
United States Patent
Application |
20100053771 |
Kind Code |
A1 |
Travis; Adrian ; et
al. |
March 4, 2010 |
FLAT PANEL LENS
Abstract
A flat panel lens system as a tapered light guide that has
minimal or no margin for fan out. The tapered light guide includes
a thin end, and a thick end of which is a bevelled mirror or an
optical equivalent. Light is injected into the thin end and the
mirror is such that rays injected through a point at the thin end
emerge collimated from one of the light guide surfaces, and that
collimated rays injected at an appropriate angle through one of the
light guide surfaces emerge from a point at the thin end. Bragg
gratings can be utilized for color implementations as well. The
tapered light guide can be fabricated as a single piece, by
extrusion, injection molding, or the combination/variation of
extrusion and injection molding, as well as other commonly known
techniques.
Inventors: |
Travis; Adrian; (Seattle,
WA) ; Large; Timothy A.; (Bellevue, WA) ;
Emerton; Neil; (Redmond, WA) |
Correspondence
Address: |
MICROSOFT CORPORATION
ONE MICROSOFT WAY
REDMOND
WA
98052
US
|
Assignee: |
Microsoft Corporation
Redmond
WA
|
Family ID: |
41646460 |
Appl. No.: |
12/203167 |
Filed: |
September 3, 2008 |
Current U.S.
Class: |
359/726 |
Current CPC
Class: |
G02B 6/0046 20130101;
G06F 3/042 20130101 |
Class at
Publication: |
359/726 |
International
Class: |
G02B 17/00 20060101
G02B017/00 |
Claims
1. A lens system, comprising: a tapered light guide having first
and second surfaces that taper from a thick end to a thin end; a
receiving face at the thin end for receiving light injected into
the light guide; and a reflecting face at the thick end of the
light guide for deflecting incident light back to the first and
second surfaces, the deflected light exiting the first surface
before reaching the receiving face.
2. The system of claim 1, wherein the reflecting face is formed at
an angle such that the injected light deflected off the reflecting
face exits the first surface or the second surface as collimated
light.
3. The system of claim 1, wherein the reflecting face is formed at
an angle such that collimated light received at the first surface
or the second surface exits the receiving face at a point.
4. The system of claim 1, wherein the injected light fans out from
the receiving face and deflects from the reflecting face to
illuminate the first surface in its entirety.
5. The system of claim 1, wherein the reflecting face is
spherically curved according to a uniform radius of curvature and
an angle formed between the first surface and the curved reflecting
face is equivalent to an angle formed between the second surface
and the curved reflecting face.
6. The system of claim 1, wherein the first and second surfaces are
flat and the thick end is twice as thick as the thin end.
7. The system of claim 1, wherein the injected light entering the
receiving face includes rays that all undergo a same number of
reflections from exiting the receiving face to exiting the light
guide.
8. The system of claim 1, wherein the reflecting face is a
reflecting grating having layers with refractive indexes that
alternate between two index values and thickness that alternates
between two thickness values.
9. A lens system, comprising: a tapered light guide having a
display surface and a second guide surface that taper from a thick
end to a thin end, the thick end twice in thickness as the thin
end; a receiving face at the thin end for receiving light injected
into the light guide; and a reflecting face at the thick end of the
light guide for deflecting incident light back to the display
surface, the reflecting face tilted to reduce angles of specular
rays relative to a plane of the light guide so that the specular
rays exit the display surface before reaching the receiving
face.
10. The system of claim 9, wherein the tilted reflecting face is a
Fresnellated surface.
11. The system of claim 9, wherein the tilted reflecting face is
curved about a point distant from the thin end by one light guide
length in a direction parallel to an axis of the taper.
12. The system of claim 9, wherein the reflecting face includes one
or more reflecting gratings having layers with refractive indexes
that alternate between two index values and thickness that
alternates between two thickness values, the one or more reflective
grating eliminate aperture diffraction.
13. The system of claim 9, wherein the reflecting face is
spherically curved according to a uniform radius of curvature, and
an angle formed between the display surface and the curved
reflecting face is equivalent to an angle formed between the second
surface and the curved reflecting face.
14. The system of claim 13, wherein the display surface and the
second guide surface each form a right angle to the reflecting face
at the thick end.
15. The system of claim 9, wherein the reflecting face is comprised
of three sets of Bragg gratings for illuminating the display
surface using red, green and blue colors.
16. The system of claim 9, wherein the light is injected into a
center of the thin end to eliminate a blank margin for fan out.
17. The system of claim 9, wherein the reflecting face deflects the
incident light such that the specular light exits the display
surface at a point dependent on an angle of injection of the
light.
18. A lens system, comprising: a wedge-shaped light guide having a
display surface and a second guide surface that taper from a thick
end to a thin end; a receiving face at the thin end for receiving
light injected into the light guide; and a reflecting surface at
the thick end of the light guide for deflecting incident light back
to the display surface, the reflecting surface tilted and curved
about a point distant from the thin end of the light guide by one
light guide length in a direction parallel to an axis of the
taper.
19. The system of claim 18, wherein the display surface and the
second guide surface transition to a section at the thick end where
the display surface is parallel to the second guide surface, and
the thick end is twice as thick as the thin end.
20. The system of claim 19, wherein the reflecting surface is a
Fresnellated surface having alternating planes of reflective
surfaces at correspondingly alternating angles, the planes having
normals that share a same component resolved in a plane of the
light guide and equal but opposite components perpendicular to the
plane of the light guide.
Description
BACKGROUND
[0001] It is well known that rays emanating from a point in the
focal plane of a lens are collimated and that parallel rays
incident on a lens are concentrated to a point. A flat panel lens
collimates rays emanating from a point but needs no volume for rays
to fan out between the point and the plane of ray collimation. In a
flat panel lens, however, the point and plane periphery are
separate and there is an undesirable margin inbetween the point and
plane periphery.
[0002] There is space between an ordinary lens and each of its
focal planes and this can also make optical systems bulky. One
proposed solution discloses a flat panel lens that collimates rays
emanating from a point, or from a point to a flat panel lens,
within a slim tapered light guide. The focal point and exit surface
are approximately coplanar but displaced from one another because a
slab of light guide is disposed between the two in order that rays
can fan-out from the focal point to the exit surface.
[0003] Rear projection televisions are normally bulky because space
is required between the video projector and diffusive screen in
order that the picture fills the screen. The space may be replaced
by a flat panel lens but there is no need to collimate the light
because the screen necessarily destroys collimation. Thus, a simple
tapered optical waveguide with uniform cross-section can suffice.
However, a slab of light guide within which rays from the projector
fan-out forms a blank margin on the screen where users would rather
want to see the picture fill the screen.
[0004] Alternatively, a pair of prisms can be used to fold the slab
behind the screen, but the slab, tapered waveguide and prisms must
be carefully aligned and this has the potential to be costly. In
yet another attempted solution, the direction of rays may be
reversed in all linear optical systems and a tapered light guide
may be used to remove the space between a camera and the object
being photographed. However, a slab of light guide is again
required, this time so that rays can fan-in. Although the resulting
margin may be removed with folding prisms, these again, can be very
costly.
SUMMARY
[0005] The following presents a simplified summary in order to
provide a basic understanding of some novel embodiments described
herein. This summary is not an extensive overview, and it is not
intended to identify key/critical elements or to delineate the
scope thereof. Its sole purpose is to present some concepts in a
simplified form as a prelude to the more detailed description that
is presented later.
[0006] Disclosed is a flat panel lens as a tapered light guide that
has minimal or no margin. No folding prisms are utilized and the
light guide can be formed from a single piece of transparent
material. In one implementation, the tapered light guide includes a
thin end, and a thick end of which is a bevelled mirror or an
optical equivalent. The bevel is such that rays injected through a
point at the thin end emerge collimated from one of the light guide
surfaces, and that collimated rays injected at an appropriate angle
through one of the light guide surfaces emerge from a point at the
thin end.
[0007] The light guide finds particular application to flat panel
displays and displays that provide direct human tactile interaction
(independent of conventional input devices such as mice and
keyboards, for example), cameras, and projection televisions, to
name just a few. The tapered light guide can be fabricated as a
single piece, by extrusion, injection molding, or the
combination/variation of extrusion and injection molding, as well
as other commonly known techniques.
[0008] To the accomplishment of the foregoing and related ends,
certain illustrative aspects are described herein in connection
with the following description and the annexed drawings. These
aspects are indicative of the various ways in which the principles
disclosed herein can be practiced, all aspects and equivalents of
which are intended to be within the scope of the claimed subject
matter. Other advantages and novel features will become apparent
from the following detailed description when considered in
conjunction with the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 illustrates a lens system the characteristics and
dimensions of which reduce or eliminate the margin on a
display.
[0010] FIG. 2 illustrates examples of light injected into the
receiving face of the light guide at different angles from a light
source and the deflected light from the reflecting face at the
thick end.
[0011] FIG. 3 illustrates an alternative light guide having a light
guide with a receiving face oriented at a different angle suitable
to provide specular light that exits the first guide according to
the critical angle.
[0012] FIG. 4 illustrates that rays injected into the center of the
thin end of the light guide can fan out to the width of the light
guide.
[0013] FIG. 5 illustrates that the reflecting face can be
configured as a Fresnellated surface of alternate mirror
angles.
[0014] FIG. 6 illustrates an alternative embodiment of a light
guide having curved surfaces in which all rays undergo the same
number of reflections before exit.
[0015] FIG. 7 illustrates a light guide that provides a smooth
transition from the tapered section to a section with parallel
sides before light rays encounter the reflecting face.
[0016] FIG. 8 illustrates that the radius of curvature of the thick
end can be twice the length of the light guide.
[0017] FIG. 9 illustrates a diagram that employs a representation
of duplicate light guides in which the thick end of each is curved
according to the implementations described in FIG. 7 and FIG.
8.
[0018] FIG. 10 illustrates a technique for deriving a minimum
thickness of the light guide when the light guide surfaces are
flat.
[0019] FIG. 11 illustrates a technique for drawing FIG. 10 on a
spherical surface so that rays entering either end of the light
guide surface meet as when leaving the thin end.
[0020] FIG. 12 illustrates the optical properties that can be
employed for the reflecting face at the thick end.
[0021] FIG. 13 illustrates a lens system where the reflecting face
can be replaced by a Bragg reflector.
[0022] FIG. 14 illustrates that the reflecting face can be replaced
by a pair of Bragg reflectors.
[0023] FIG. 15 illustrates an alternative embodiment of a lens
system where a light guide uses a graded index between the thin end
and the thick end.
[0024] FIG. 16 illustrates a block diagram of a computing system
operable to employ the lens system in accordance with the disclosed
architecture.
DETAILED DESCRIPTION
[0025] Disclosed is a wedge (or tapered) light guide where the
point source of light is placed at the thin end and the thick end
is a reflector tilted so as to reduce the angle between rays and
the plane of the light guide. The reflector can include a zig-zag
(or Fresnellated) surface. The thick end can be curved about a
point distant from the thin end of the wedge by one wedge length in
a direction parallel to the axis of taper. Additionally, the thick
end can be twice as thick as the thin end. The thickness profile of
the wedge is proportional to sin [(z+L)/kL], where z is distance
from the thin end, L is the length of the wedge, and k is a
constant.
[0026] Reference is now made to the drawings, wherein like
reference numerals are used to refer to like elements throughout.
In the following description, for purposes of explanation, numerous
specific details are set forth in order to provide a thorough
understanding thereof. It may be evident, however, that the novel
embodiments can be practiced without these specific details. In
other instances, well known structures and devices are shown in
block diagram form in order to facilitate a description thereof.
The intention is to cover all modifications, equivalents, and
alternatives falling within the spirit and scope of the claimed
subject matter.
[0027] FIG. 1 illustrates a lens system 100 the characteristics and
dimensions of which reduce or eliminate the margin on a display.
The lens system 100 includes a wedge-shaped light guide 102 having
a first guide surface 104 and second guide surface 106 that tapers
from a thick end 108 to a thin end 110. The light guide 100 also
includes a receiving face 112 at the thin end 110 for receiving
light 114 injected into the light guide 102, and a reflecting face
116 (also referred to as a bevelled or tilted mirror, and
reflecting surface) at the thick end 108 of the light guide 102 for
deflecting incident light 118 back to the first guide surface 104
(and possibly the second guide surface 106), the deflected incident
light 120 (or specular light) exiting the first guide surface 104
(or display surface) before reaching the receiving face 112.
[0028] FIG. 2 illustrates examples 200 of light injected into the
receiving face 112 of the light guide 102 at different angles from
a light source 202 and the deflected light from the reflecting face
116 at the thick end 108. The reflecting face 116 (the bevelled
mirror) causes rays injected at the thin end 110 to be deflected in
such a way as to reach a critical angle (the angle at which the
light will depart the light guide 102 rather than reflect
internally) and depart the tapered waveguide (the light guide 102)
at a point dependent on the associated angle of injection by the
light source 202. The reflecting face 116 deflects incident rays so
that the deflected incident rays return toward the thin end 110 but
with a much reduced angle relative to an axis 204 perpendicular to
a plane 206 of the light guide 102. After deflection, the
diminishing thickness encountered in the direction of the thin end
110 by each ray causes the ray angle to reduce until the ray angle
is less than the critical angle (for the particular material of the
light guide 102), at which point the ray emerges from a guide
surface (first guide surface 104 and/or second guide surface 106)
of the light guide 102.
[0029] In the top example 200, the injected ray from the light
source 202 enters the guide 102 at a low angle, deflects off the
reflecting surface 116 and returns nearly to the receiving face 112
before exiting the first guide surface 102. In the middle example
200, the injected ray from the light source 202 enters the guide
102 at a higher angle than in the top example, deflects off the
reflecting surface 116 and returns to exit the light guide 102 near
the middle before reducing to the critical angle to exit the first
guide surface 102. In the bottom example 200, the injected ray from
the light source 202 enters the light guide 102 at a still higher
angle than in the middle example, deflects off the reflecting
surface 116 and returns to exit the light guide 102 at the first
surface 102 near the reflecting face 116. Thus, it is easy to see
that by adjusting the angle at which the light source injects the
light into the receiving face 112, the margin can be reduced or
eliminated entirely when viewed from the first surface 104.
[0030] FIG. 3 illustrates an alternative light guide system 300
having a light guide 302 with a receiving face 304 oriented at a
different angle suitable to provide specular light that exits the
first guide surface 104 according to the critical angle. The
reflecting face 116 (the bevelled mirror) causes rays injected into
the receiving face 304 at the thin end 110 to be deflected in such
a way as to reach the critical angle and depart the tapered
waveguide (the light guide 302) at a point dependent on the
associated angle of injection by the light source 202. The
reflecting face 116 deflects incident rays so that the deflected
incident rays (or specular rays) return toward the thin end 110 but
with a much reduced angle relative to an axis 306 perpendicular to
a plane 308 of the light guide 302. After reflection, the
diminishing thickness encountered in the direction of the thin end
110 by each ray causes the ray angle to reduce until the ray angle
is less than the critical angle, at which point the ray emerges
from a guide surface (the first guide surface 104 and/or second
guide surface 106) of the light guide 302.
[0031] FIG. 4 illustrates that rays 400 injected into the center of
the thin end 110 the light guide 102 can fan out to the width of
the light guide 102 so that after deflecting off the reflecting
face 116 (also called the bevelled mirror) at the thick end 108,
the rays 400 illuminate the whole area (the entirety) of a front
surface 402 (similar to the first guide surface 104) of the light
guide 102. Light exiting the whole area of the front surface 402 is
represented by ray arrows exiting near each of the four corners of
the front surface 402. There is therefore no need for a blank
margin within which rays can fan out, as in conventional systems in
the lower half of the light guide 102 near the light source
202.
[0032] It has been shown conventionally that for a smooth light
guide, the product of light guide thickness with the cosine of the
ray angle relative to the perpendicular of the light guide is a
constant number. It follows that thickness times the sine of the
ray angle relative to the plane of the light guide is also a
constant number. The sine of a small angle is approximately
proportional to the angle itself, so it can be said that the
product of the thickness with the ray angle itself is approximately
constant. Consider that light rays are injected into the thick end
108 into the light guide 102 by the light source 202 (e.g., laser,
LED) at a uniformly distributed range of angles between zero and
the critical angle, and that the light guide 102 gradually tapers
to half its input thickness (e.g., at the thin end 110). The angle
of rays reaching the thin end 110 will have doubled so half of the
injected rays have exceeded the critical angle and are no longer
guided.
[0033] Instead, as in the disclosed technique, rays are injected at
the thin end 110 and the angles of the rays 400 will all be less
than half the critical angle at the thick end 108. An appropriately
angled reflecting face 116 reflects the rays 400 so that all rays
400 are returned at greater than half the critical angle. Thus, all
rays 400 leave the light guide 102 before reaching the thin end
110. Therefore, in one implementation, the thick end 108 of the
light guide 102 is at or about twice the thickness of the thin end
110.
[0034] The three rays shown in the top, middle and bottom examples
200 of FIG. 2 are all incident on the reflecting face 116 at the
thick end 108 from the left-hand side in the light guide 102. Rays
will, however, also be incident from the right-hand side and these
rays will propagate back down the light guide 102 and be lost to
the system through the thin end 110, so there can be gaps in the
projected image.
[0035] FIG. 5 illustrates that the reflecting face 116 can be
configured as a Fresnellated surface 500 of alternate mirror
angles. The Fresnellated surface 500 can be embossed on the thick
end 108 to ensure that for any incident ray 502, a portion 504 of
the ray 502 is lost, but a specular portion 506 goes on to form an
image 508 so that the image is uniform and formed free of gaps
(margins). Accordingly, the Fresnellated surface 500 for the
reflecting face 116 on the thick end 108 comprises pairs of
reflecting planes (510 and 512) the normals of which share the same
component resolved in the plane of the light guide 102 but have
equal and opposite components perpendicular to the plane of the
light guide 102.
[0036] Discontinuities in an image projected through a tapered
light guide with straight sides will form at the point where a
small change in ray injection angle causes the ray to undergo an
extra reflection. The lower part of the projected image on the
front surface 402 in FIG. 4 is closer to the light source 202
(e.g., projector) than the upper part, so the projected image can
be subject to keystone distortion (a distortion in image dimensions
that makes the image look similar to a trapezoid) because the
reflecting face 116 at the thick end 108 is flat.
[0037] FIG. 6 illustrates an alternative embodiment of a light
guide 600 having curved surfaces in which all rays undergo the same
number of reflections before exit. The reflecting face 116 is
curved about an axis 602 perpendicular to the plane of the light
guide 600 and with a radius of curvature equal to twice the length
of the light guide 600. Rays no longer fan out after reflection so
keystone distortion is eliminated. However, it can be desirable
that the edges of a display be straight. Thus, the reflecting face
116 may instead be a Fresnel surface and the collimating action of
this may be integrated with the actions of the reflecting planes
510 and 512 (of FIG. 5) into a single prismatic surface.
[0038] The surface provided by curving the reflecting face 116 is
approximately cylindrical. A disadvantage of cylindrical mirrors
(or surfaces) is the introduction of an astigmatism that can
degrade resolution. This is resolved as follows.
[0039] FIG. 7 illustrates a light guide 700 that provides a smooth
transition from the tapered section 702 to a section 704 with
parallel sides before light rays encounter the reflecting face 116.
The reflecting face 116 at the end of the light guide 700 is curved
with a uniform radius of curvature such that a first corner 706
defined by the first guide surface 104 and the curved reflecting
face 116, and a second corner 708 defined by the second guide
surface 106 and the curved reflecting face 116, both are right
angles. When employing the Fresnel surface 500 of FIG. 5, the
reflecting planes (510 and 512) conform to this curvature so that
the angle made by each of the planes (510 and 512) relative to the
reflecting face 116 is constant.
[0040] FIG. 8 illustrates that the radius of curvature 800 of the
thick end 108 can be twice the length of the light guide 102. As
described herein above in one implementation, it can be optimal for
the thick end 108 of the light guide 102 to be double the thickness
of the thin end 110. Moreover, since the light guide 102 has flat
surfaces (the first guide surface 104 and second guide surface
106), the radius of curvature for the thick end 108 equals twice
the length of the light guide 102. This means that the reflecting
face 116 has the same radius of curvature about the axis 604
depicted in FIG. 6 as indicated about axis 602 in order to minimize
keystone distortion. The combined curvature of the thick end 108 is
therefore spherical, in which case the reflecting face 116
introduces no astigmatism and resolution is not degraded.
[0041] The path of a ray through a light guide can be found by
tracing the ray alternately to upper and lower surfaces and
calculating the angle of reflection off each surface; however, this
can be laborious and poorly informative. If the light guide has
flat surfaces, it is simpler and optically equivalent to draw a ray
straight through a stack of duplicates of the tapered light guide
until the ray crosses an intersection at less than the critical
angle, when the ray will in reality emerge into air. It is also
permissible to use a reverse approach by drawing the ray in
reverse; in other words, rather than injecting the ray at one end
of the light guide and tracing ray progression until it emerges
from a surface, the ray can be traced from the surface of a stack
of light guides back to a light guide end with the proviso that the
ray leaves the surface at the critical angle and never exceeds the
critical angle.
[0042] FIG. 9 illustrates such a diagram 900 that employs a
representation of duplicate light guides 902 in which the thick end
108 of each is curved according to the implementations described in
FIG. 7 and FIG. 8. Using the reverse approach described above,
incident rays 904 imposed on what can actually be an exit surface
906 (similar to first guide surface 104) reflect off the reflecting
face 116 of thick end 108. The duplicate light guides 902 at the
thick end 108 combine into a curve of constant radius, thereby
concentrating parallel rays approximately to a point 908 at a
distance equal to half the radius of curvature of the curve.
[0043] If the thick end 108 were perfectly smooth, this point 908
would be at the thin end 110 of the top light guide of the
duplicate light guides 902, but the action of the Fresnel planes
(510 and 512 of FIG. 5) is to deflect half of each ray such that
the point of focus of the deflected halves is at the thin end 110
of one of the duplicates 902 lower in the stack. In an optimum
implementation, all rays undergo the same number of reflections
between entrance and exit. FIG. 9 demonstrates that this is an
inherent property of the embodiment illustrated because each ray
crosses the same number of light guide interfaces as any other
ray.
[0044] Perpendicular to the plane of FIG. 9, rays traced from the
thin end 110 are approximately collimated by reflection off the
spherical surface of the reflecting face 116, so an image projected
through the light guide experiences little distortion. Collimated
rays have no skew, so the projected image exhibits uniformity.
[0045] Additionally, rays can be injected into the receiving face
at the thin end of the light guide so as to emerge from any point
on the display surface, thereby reducing, or eliminating margins
entirely. That the sides of the light guides have no curvature
helps preserve the resolution of an image projected through the
light guide. Moreover, this also means that the lens system is
simple to make.
[0046] To make the tapered light guide surfaces (e.g., first guide
surface 104 and second guide surface 106), the base of a polishing
machine can be tilted, a sheet of high transparency glass waxed on
top and polished flat. To make the spherically curved reflecting
face 116 (bevelled mirror), several such tapered light guides can
be waxed together in a stack and a sphere polished on the end as if
on a conventional lens surface. The Fresnellated planes (510 and
512) can then be cast on a film, stretched over the spherical
reflecting face 116, and then glued on.
[0047] FIG. 10 illustrates a technique 1000 for deriving a minimum
thickness of the light guide when the light guide surfaces are
flat. A spherical curvature on the reflecting face 116 reflects
parallel rays not to a point but to a spot of finite thickness, and
the light guide can be designed to be no thinner than the size of
the spot. FIG. 10 applies geometry to FIG. 9 to show how this
minimum thickness can be calculated. Two rays (1002 and 1004) are
shown, and the point of exit 1006 of ray 1002 is proximate the thin
end 110, while the point of exit 1006 of ray 1004 is proximate the
thick end 108. All rays (1002 and 1004) are at the critical angle
.phi. just before exiting. Tracing of both rays (1002 and 1004)
backwards at this angle is performed straight through the stack of
light guide duplicates 902 until the rays (1002 and 1004) meet the
thick end 108.
[0048] When employing the Fresnel planes (510 and 512), after
reflection off the Fresnel plane 510 at the thick end 108, tracing
of the ray 1002 back to the thin end 110 parallel to the plane of
the light guide duplicate can be performed. The reflected part of
ray 1002 is at some angle .theta. to the incident part of the ray,
which angle .theta. can be calculated, while Fresnel plane 510 is
at an angle .theta./2 to the spherical curvature of the reflecting
face 116.
[0049] Ray 1004 reflects off the same Fresnel plane 510. Thus, the
angle of the reflected part of ray 1004 can be calculated. By
symmetry, this angle equals .theta., if ray 1002 and ray 1004 are
to meet at the thin end 110. Geometry calculations show that if the
critical angle .phi. equals 42 degrees, .theta. equals 21 degrees,
while ray 1004 instead is reflected at 24 degrees to the wedge
surface (e.g., the first guide surface 104), a difference of about
three degrees.
[0050] Focus can be improved if very thin light guides are to be
made. However, the symmetry of FIG. 9 indicates that the thick end
108 be a simple curve. Rather than altering the geometry of the
thick end 108, the sides of the tapered light guide can be curved
and many such light guides joined in the manner of FIG. 9 will map
onto a spheroid, similar to the peel slices of a segmented orange.
The surface curvature desired of a very thin tapered light guide
can be found by drawing FIG. 10 on a spheroid whose radius of
curvature is such that all rays reflected off the thick end 108
meet at a point. This is now described.
[0051] FIG. 11 illustrates a technique 1100 for drawing FIG. 10 on
a spherical surface so that rays entering either end of the light
guide surface meet as when leaving the thin end 110. FIG. 11
depicts a section of a sphere 1102, having a North Pole N, where
the ray 1002 of FIG. 10 moves from position b at a latitude of
.psi. degrees to the North Pole N at a Southwesterly angle .phi.
according to a vector called track. The ray 1002 stops when at a
latitude of 2.psi., the final position denoted by vector a. The
initial direction of the ray 1002 was t, and the ray 1002 moved
along a great circle route, so a can be expressed as a linear sum
of b and t. If the arc ba of the great circle route has angle
.lamda. relative to the center C of the sphere, then,
a = cos .lamda.b + sin .lamda. t = cos .lamda. ( sin .psi. cos
.psi. 0 ) + sin .lamda. ( sin .phi. cos .psi. - sin .phi. sin .psi.
cos .phi. ) ##EQU00001##
[0052] Position a is at a latitude of 2.psi. so the j component of
a is equal to cos 2.psi.:
a.sub.j=cos 2.psi. {square root over (1-sin.sup.2 .lamda. cos
.psi.-sin .lamda. sin .phi. sin .psi.)}
So
(1-sin.sup.2 .lamda.)cos.sup.2 .psi.=cos.sup.2 2.psi.+sin.sup.2
.lamda. sin.sup.2 .phi. sin.sup.2 .psi.+2 cos 2.psi. sin .lamda.
sin .phi. sin .psi.
And,
0=(cos.sup.2 2.psi.-cos.sup.2 .psi.)+(2 cos 2.psi. sin .phi. sin
.psi.)sin .lamda.+(sin.sup.2 .phi. sin.sup.2 .psi.+cos.sup.2
.psi.)sin.sup.2 .lamda.
[0053] This is a quadratic equation solvable to obtain sin .lamda.;
then .lamda., and then a. The distance moved by the ray 1002 is
.lamda. times the radius of the sphere, but it is desirable to know
.theta., the direction of the ray 1002 as it reaches position a.
This can be determined by calculating the normal n, to the great
circle route, which the ray 1002 has just travelled; n is
perpendicular to both b and t, thus:
n = b .times. t = ( sin .psi. cos .psi. 0 ) .times. ( sin .phi. cos
.psi. - sin .phi. sin .psi. cos .phi. ) = ( cos .psi. cos .phi. -
sin .psi. cos .phi. - sin .phi. ) ##EQU00002##
[0054] A vector that is perpendicular to the circle of longitude
through the ray's final position can be obtained by calculating
j.times.a:
j .times. a = cos .lamda. ( 0 1 0 ) .times. ( sin .psi. cos .psi. 0
) + sin .lamda. ( 0 1 0 ) .times. ( sin .phi. cos .psi. - sin .phi.
sin .psi. cos .phi. ) = cos .lamda. ( 0 0 - sin .psi. ) + sin
.lamda. ( cos .phi. 0 - sin .phi. cos .psi. ) = cos .lamda. ( 0 0 -
sin .psi. ) + sin .lamda. ( cos .phi. 0 - sin .phi. cos .psi. )
##EQU00003##
[0055] Now, .theta. equals the angle between the line of travel and
the line of longitude through a, so angle .theta. is also equal to
the angle between the perpendiculars to both these lines.
Hence:
cos .theta. = n j .times. a j .times. a = ( cos .psi. cos .phi. -
sin .psi. cos .phi. - sin .phi. ) ( sin .lamda. cos .phi. 0 - cos
.lamda. sin .psi. - sin .lamda. sin .phi. cos .psi. ) j .times. a =
cos .psi. sin .lamda. cos 2 .phi. + sin .phi.cos.lamda.sin .psi. +
sin .lamda.sin 2 .phi.cos .psi. sin 2 .lamda. cos 2 .phi. + ( cos
.lamda. sin .psi. + sin .lamda.sin .phi. cos .psi. ) 2
##EQU00004##
[0056] By entering this into a spreadsheet, for example, and
changing .psi. until .theta.=24 degrees, it transpires that .psi.
must be 24.sup.0. Since the length L of the wedge (tapered light
guide 102) is equal to the spherical radius R times .psi. in
radians, it follows that R=L/.psi., which equals 2.39L.
[0057] It remains to calculate the variation of thickness with
length L of a wedge drawn on a spherical surface. At latitude
.beta. from the North Pole N, the radius of the line of latitude is
R sin .beta.. The wedge runs from latitude L/R to 2L/R, so wedge
thickness T is proportional to sin [(z+L)/2.39L], where z is the
distance along the wedge from the thin end to the thick end. This
can be put more generally as the light guide has a thickness
profile proportional to sin [(z+L)/kL], where z is distance from
the thin end, L is a length of the light guide, and k is a
constant.
[0058] The reflecting face 116 of FIG. 5 wastes half the light. The
following describes how to eliminate this loss. The loss can be
reduced or eliminated if Fresnel planes 510 and 512 are illuminated
only by those rays from the thin end which the Fresnel surface
reflects so as to emerge from the light guide surface, or
similarly, in reverse (from the thin end when the rays are
introduced to the light guide surface). For example, if a laser
beam scanned by a vibrating mirror is pointed into the thin end,
the beam should be switched on when the beam produces a reflection
such as specular portion 506 of FIG. 5, but switched off when the
beam produces a reflection such as portion 504. A photo-sensor can
be placed at the thin end and used to switch the laser off whenever
the unwanted reflection portion 504 is detected.
[0059] A laser beam may also be scanned by placing a liquid crystal
display (LCD) in front of the beam and expressing a diffraction
grating on the LCD, and an image can be produced by a superposition
of diffraction gratings in a technique known as holographic
projection. In this case, loss is minimized by ensuring that the
pattern on the LCD is comprised only of gratings which direct light
to Fresnel surfaces that do not produce wasted light, such as
portion 504.
[0060] It is well known that light reflected off small facets
(e.g., the Fresnel planes 510 and 512) is subject to aperture
diffraction which degrades resolution. Aperture diffraction can be
eliminated if a Bragg grating (a structure comprising layers whose
refractive index alternates between two values and whose thickness
alternates between two values) is used. By suitable choice of
thickness and index, it is possible to design the reflecting face
116 so that it reflects monochromatic light over a desired range of
angles but is otherwise transparent.
[0061] FIG. 12 illustrates the optical properties that can be
employed for the reflecting face 116 at the thick end. A graph 1200
shows that a single reflecting face 116 can be employed for
deflecting rays in a range from .theta..sub.0 to 2.theta..sub.0. A
second reflecting face can be employed in combination with the
reflecting face 116 to process rays in a range from zero to
-.theta..sub.0.
[0062] FIG. 13 illustrates a lens system 1300 where the reflecting
face can be replaced by a Bragg reflector 1302. The reflector 1302
can therefore be designed to reflect all rays (e.g., ray 1304) from
the lower surface (the second guide surface 106) of the tapered
light guide 102, but to transmit all those rays from the upper
surface (the first guide surface 104). A conventional planar or
Fresnellated mirror can then be embossed beyond the reflector 1302
to reflect rays from the upper surface (e.g., first guide surface
102).
[0063] FIG. 14 illustrates that the reflecting face can be replaced
by a pair of Bragg reflectors 1400. The two Bragg gratings 1400 may
be superposed so that a first grating 1402 reflects rays from the
lower surface (the second guide surface 106) of the light guide 102
and the other rays from the upper side (the first guide surface
104). Alternatively, the two gratings 1400 may be spatially
superposed into a single grating 1406 which performs both
functions.
[0064] Color displays are illuminated with red, green and blue
(RGB) light so at least three wavelengths can be reflected by the
Bragg gratings. These can be the wavelengths of lasers illuminating
the video projector in a rear projection television, for example.
Three pairs of separate gratings suffice to reflect rays of all
three wavelengths incident both from the left-hand and right-hand
sides but each pair reflects light only of its own wavelength. It
is fortunate that this will happen if the grating is designed for a
light guide with a refractive index typical of the most common
transparent material as the following paragraph will elaborate.
[0065] A common transparent material such as poly(methyl
methacrylate) or PMMA has a refractive index of 1.492 and a
critical angle in air of 42 degrees. Thus, a light guide made of
this material confines all rays at angles between 0 degrees and 48
degrees to the plane of the light guide. Rays are incident on the
thick end at angles less than 24 degrees because the thick end has
twice the thickness of the thin end, and all rays are reflected at
angles greater than 24 degrees to emerge from the display surface
when travelling back toward the thin end. In particular, a ray
incident at 24 degrees is reflected at minus 48 degrees as if off a
reflecting surface (mirror) angled at minus 12 degrees.
[0066] The Bragg grating which does this has a period equal to the
magnitude of the vector sum of the incident and reflected wave
vectors, which equals twice cos {[24.degree.-(-48.degree.)]/2}=0.81
per wavelength. A normally incident ray reflects off a grating with
two periods per wavelength, so a green ray will pass through a
grating designed for blue if the blue to green wavelength ratio is
0.81. The green-to-red wavelength ratio is the same and RGB
wavelengths of 430, 532 and 655 nanometers (nm), respectively, will
suffice.
[0067] When selected according to these wavelengths, no one
wavelength is affected by the gratings designed for the other two
wavelengths, so the three pairs of gratings can be superposed in a
single film. However, the eye's sensitivity at 655 nm may be poor.
In such cases, the critical angle of the light guide can be
increased so as to permit a smaller ratio between the
wavelengths.
[0068] It may be difficult to apply a Bragg mirror onto the end of
a light guide with the desired angular precision. An alternative
technique is to create the Bragg grating by directing an ion beam
through the side of the light guide. It is well known that the
passage of energetic ions through a polymer can cause bonds to
break and the refractive index to change. The ion beam can
therefore be used to create the alternating layers of high and low
refractive index that constitute a Bragg grating.
[0069] In one general, but non-exhaustive summary, a lens system
implementation is provided that comprised a tapered light guide
having a display surface and a second guide surface that taper from
a thick end to a thin end, the thick end twice in thickness as the
thin end; a receiving face at the thin end for receiving light
injected into the light guide; and, a reflecting face at the thick
end of the light guide for deflecting incident light back to the
display surface, the reflecting face tilted to reduce angles of
specular rays relative to a plane of the light guide so that the
specular rays exit the display surface before reaching the
receiving face.
[0070] The tilted reflecting face is a Fresnellated surface and can
be curved about a point distant from the thin end by one light
guide length in a direction parallel to an axis of taper. The light
guide has a thickness profile proportional to sin [(z+L)/kL], where
z is distance from the thin end, L is a length of the light guide,
and k is a constant. The reflecting face includes one or more
reflecting gratings having layers with refractive indexes that
alternate between two index values and thickness that alternates
between two thickness values, the one or more reflective grating
eliminate aperture diffraction.
[0071] The reflecting face is spherically curved according to a
uniform radius of curvature, and an angle formed between the
display surface and the curved reflecting face is equivalent to an
angle formed between the second surface and the curved reflecting
face, and the display surface and the second guide surface each
form a right angle to the reflecting face at the thick end.
[0072] The injected light entering the receiving face includes rays
all of which undergo a same number of reflections prior to exiting
the display surface. The reflecting face is comprised of three sets
of Bragg gratings for illuminating the display surface using red,
green, and blue colors. The light guide can be composed of poly
(methyl methacrylate).
[0073] FIG. 15 illustrates an alternative embodiment of a lens
system 1500 where a light guide 1502 uses a graded index between
the thin end 110 and the thick end 108. The lens system 1500
includes the light guide 1502 having the first guide surface 104
and second guide surface 106, which can be parallel, or that taper
from the thick end 108 to the thin end 110. The light guide 1502
also includes the receiving face 112 at the thin end 110 for
receiving light 114 injected into the light guide 1502, and the
reflecting face 116 (also referred to as a bevelled or tilted
mirror) at the thick end 108 of the light guide 1502 for deflecting
incident light 118 back to the first guide surface 104 (and
possibly the second guide surface 106), the deflected incident
light 120 (or specular light) exiting the first guide surface 104
before reaching the receiving face 112. The graded-index
characteristic of the light guide 1502 includes a refractive index
that decreases with increasing distance from the thin end 110, such
that the light rays follow sinusoidal paths from the thin end 110
to the thick end 108.
[0074] Referring now to FIG. 16, there is illustrated a block
diagram of a computing system 1600 operable to employ the lens
system in accordance with the disclosed architecture. In order to
provide additional context for various aspects thereof, FIG. 16 and
the following discussion are intended to provide a brief, general
description of the suitable computing system 1600 in which the
various aspects can be implemented. While the description above is
in the general context of computer-executable instructions that can
run on one or more computers, those skilled in the art will
recognize that a novel embodiment also can be implemented in
combination with other program modules and/or as a combination of
hardware and software.
[0075] The computing system 1600 for implementing various aspects
includes the computer 1602 having processing unit(s) 1604, a system
memory 1606, and a system bus 1608. The processing unit(s) 1604 can
be any of various commercially available processors such as
single-processor, multi-processor, single-core units and multi-core
units. Moreover, those skilled in the art will appreciate that the
novel methods can be practiced with other computer system
configurations, including minicomputers, mainframe computers, as
well as personal computers (e.g., desktop, laptop, etc.), hand-held
computing devices, microprocessor-based or programmable consumer
electronics, and the like, each of which can be operatively coupled
to one or more associated devices.
[0076] The system memory 1606 can include volatile (VOL) memory
1610 (e.g., random access memory (RAM)) and non-volatile memory
(NON-VOL) 1612 (e.g., ROM, EPROM, EEPROM, etc.). A basic
input/output system (BIOS) can be stored in the non-volatile memory
1612, and includes the basic routines that facilitate the
communication of data and signals between components within the
computer 1602, such as during startup. The volatile memory 1610 can
also include a high-speed RAM such as static RAM for caching
data.
[0077] The system bus 1608 provides an interface for system
components including, but not limited to, the memory subsystem 1606
to the processing unit(s) 1604. The system bus 1608 can be any of
several types of bus structure that can further interconnect to a
memory bus (with or without a memory controller), and a peripheral
bus (e.g., PCI, PCIe, AGP, LPC, etc.), using any of a variety of
commercially available bus architectures.
[0078] The computer 1602 further includes storage subsystem(s) 1614
and storage interface(s) 1616 for interfacing the storage
subsystem(s) 1614 to the system bus 1608 and other desired computer
components. The storage subsystem(s) 1614 can include one or more
of a hard disk drive (HDD), a magnetic floppy disk drive (FDD),
and/or optical disk storage drive (e.g., a CD-ROM drive DVD drive),
for example. The storage interface(s) 1616 can include interface
technologies such as EIDE, ATA, SATA, and IEEE 1394, for
example.
[0079] One or more programs and data can be stored in the memory
subsystem 1606, a removable memory subsystem 1618 (e.g., flash
drive form factor technology), and/or the storage subsystem(s)
1614, including an operating system 1620, one or more application
programs 1622, other program modules 1624, and program data 1626.
Generally, programs include routines, methods, data structures,
other software components, etc., that perform particular tasks or
implement particular abstract data types. All or portions of the
operating system 1620, applications 1622, modules 1624, and/or data
1626 can also be cached in memory such as the volatile memory 1610,
for example. It is to be appreciated that the disclosed
architecture can be implemented with various commercially available
operating systems or combinations of operating systems (e.g., as
virtual machines).
[0080] The storage subsystem(s) 1614 and memory subsystems (1606
and 1618) serve as computer readable media for volatile and
non-volatile storage of data, data structures, computer-executable
instructions, and so forth. Computer readable media can be any
available media that can be accessed by the computer 1602 and
includes volatile and non-volatile media, removable and
non-removable media. For the computer 1602, the media accommodate
the storage of data in any suitable digital format. It should be
appreciated by those skilled in the art that other types of
computer readable media can be employed such as zip drives,
magnetic tape, flash memory cards, cartridges, and the like, for
storing computer executable instructions for performing the novel
methods of the disclosed architecture.
[0081] A user can interact with the computer 1602, programs, and
data using external user input devices 1628 such as a keyboard and
a mouse. Other external user input devices 1628 can include a
microphone, an IR (infrared) remote control, a joystick, a game
pad, camera recognition systems, a stylus pen, touch screen,
gesture systems (e.g., eye movement, head movement, etc.), and/or
the like. The user can interact with the computer 1602, programs,
and data using onboard user input devices 1630 such a touchpad,
microphone, keyboard, etc., where the computer 1602 is a portable
computer, for example. These and other input devices are connected
to the processing unit(s) 1604 through input/output (I/O) device
interface(s) 1632 via the system bus 1608, but can be connected by
other interfaces such as a parallel port, IEEE 1394 serial port, a
game port, a USB port, an IR interface, etc. The I/O device
interface(s) 1632 also facilitate the use of output peripherals
1634 such as printers, audio devices, camera devices, and so on,
such as a sound card and/or onboard audio processing
capability.
[0082] One or more graphics interface(s) 1636 (also commonly
referred to as a graphics processing unit (GPU)) provide graphics
and video signals between the computer 1602 and external display(s)
1638 (e.g., LCD, plasma) and/or onboard displays 1640 (e.g., for
portable computer). The graphics interface(s) 1636 can also be
manufactured as part of the computer system board. The external
display(s) 1638 and/or onboard display 1640 can include the lens
system disclosed herein.
[0083] The computer 1602 can operate in a networked environment
(e.g., IP) using logical connections via a wire/wireless
communications subsystem 1642 to one or more networks and/or other
computers. The other computers can include workstations, servers,
routers, personal computers, microprocessor-based entertainment
appliance, a peer device or other common network node, and
typically include many or all of the elements described relative to
the computer 1602. The logical connections can include
wire/wireless connectivity to a local area network (LAN), a wide
area network (WAN), hotspot, and so on. LAN and WAN networking
environments are commonplace in offices and companies and
facilitate enterprise-wide computer networks, such as intranets,
all of which may connect to a global communications network such as
the Internet.
[0084] When used in a networking environment the computer 1602
connects to the network via a wire/wireless communication subsystem
1642 (e.g., a network interface adapter, onboard transceiver
subsystem, etc.) to communicate with wire/wireless networks,
wire/wireless printers, wire/wireless input devices 1644, and so
on. The computer 1602 can include a modem or has other means for
establishing communications over the network. In a networked
environment, programs and data relative to the computer 1602 can be
stored in the remote memory/storage device, as is associated with a
distributed system. It will be appreciated that the network
connections shown are exemplary and other means of establishing a
communications link between the computers can be used.
[0085] The computer 1602 is operable to communicate with
wire/wireless devices or entities using the radio technologies such
as the IEEE 802.xx family of standards, such as wireless devices
operatively disposed in wireless communication (e.g., IEEE 802.11
over-the-air modulation techniques) with, for example, a printer,
scanner, desktop and/or portable computer, personal digital
assistant (PDA), communications satellite, any piece of equipment
or location associated with a wirelessly detectable tag (e.g., a
kiosk, news stand, restroom), and telephone. This includes at least
Wi-Fi (or Wireless Fidelity) for hotspots, WiMax, and Bluetooth.TM.
wireless technologies. Thus, the communications can be a predefined
structure as with a conventional network or simply an ad hoc
communication between at least two devices. Wi-Fi networks use
radio technologies called IEEE 802.11x (a, b, g, etc.) to provide
secure, reliable, fast wireless connectivity. A Wi-Fi network can
be used to connect computers to each other, to the Internet, and to
wire networks (which use IEEE 802.3-related media and
functions).
[0086] The illustrated aspects can also be practiced in distributed
computing environments where certain tasks are performed by remote
processing devices that are linked through a communications
network. In a distributed computing environment, program modules
can be located in local and/or remote storage and/or memory
system.
[0087] What has been described above includes examples of the
disclosed architecture. It is, of course, not possible to describe
every conceivable combination of components and/or methodologies,
but one of ordinary skill in the art may recognize that many
further combinations and permutations are possible. Accordingly,
the novel architecture is intended to embrace all such alterations,
modifications and variations that fall within the spirit and scope
of the appended claims. Furthermore, to the extent that the term
"includes" is used in either the detailed description or the
claims, such term is intended to be inclusive in a manner similar
to the term "comprising" as "comprising" is interpreted when
employed as a transitional word in a claim.
* * * * *