U.S. patent application number 15/368978 was filed with the patent office on 2017-06-22 for curved back-projection screen.
This patent application is currently assigned to BARCO N.V.. The applicant listed for this patent is BARCO N.V.. Invention is credited to Patrick CANDRY, Stefaan DUCASTEELE, Geert MATTHYS, Koenraad VERMEIRSCH.
Application Number | 20170176847 15/368978 |
Document ID | / |
Family ID | 43243378 |
Filed Date | 2017-06-22 |
United States Patent
Application |
20170176847 |
Kind Code |
A1 |
VERMEIRSCH; Koenraad ; et
al. |
June 22, 2017 |
CURVED BACK-PROJECTION SCREEN
Abstract
A curved back-projection screen having an angle of curvature
greater than 180.degree., such as a wrap-around cylindrical or dome
screen. The screen includes a first layer and a second synthetic
resin diffusing layer on the first layer, the second synthetic
resin diffusing layer containing a light absorbing material and
light diffusing particles embedded in a resin material, with the
second synthetic resin diffusing layer having a value of the
product of the absorption coefficient and thickness of between 0.1
and 2. The second synthetic resin diffusing layer can be applied by
spraying.
Inventors: |
VERMEIRSCH; Koenraad;
(Bellegem, BE) ; DUCASTEELE; Stefaan; (Zonnebeke,
BE) ; MATTHYS; Geert; (Haaltert, BE) ; CANDRY;
Patrick; (Harelbeke, BE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
BARCO N.V. |
Bellegem |
|
BE |
|
|
Assignee: |
BARCO N.V.
Bellegem
BE
|
Family ID: |
43243378 |
Appl. No.: |
15/368978 |
Filed: |
December 5, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13876696 |
Mar 28, 2013 |
9513541 |
|
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PCT/BE2010/000074 |
Oct 28, 2010 |
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15368978 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G03B 37/04 20130101;
G02B 1/04 20130101; G02B 5/0278 20130101; B05D 1/02 20130101; G03B
21/60 20130101; G03B 21/62 20130101 |
International
Class: |
G03B 21/62 20060101
G03B021/62; B05D 1/02 20060101 B05D001/02; G02B 1/04 20060101
G02B001/04; G03B 37/04 20060101 G03B037/04; G03B 21/60 20060101
G03B021/60; G02B 5/02 20060101 G02B005/02 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 1, 2010 |
GB |
1016566.0 |
Claims
1-18. (canceled)
19. A curved back-projection screen having an angle of curvature
greater than 180.degree., the screen comprising a first layer and a
second diffusing layer on the first layer, the second synthetic
diffusing layer containing a light absorbing material and light
diffusing particles, the second synthetic resin diffusing layer
being a sprayed-on layer on an inside surface of the first
layer.
20. The curved back-projection screen according to claim 19, the
second synthetic resin diffusing layer having a uniform
thickness.
21. The curved back-projection screen according to claim 19,
wherein the second diffusing layer comprises multiple layers.
22. The curved back-projection screen according to claim 19,
wherein the second diffusing layer comprises a mix of diffusing
layers and light absorbing layers.
23. The curved back-projection screen according to claim 22,
wherein the second diffusing layer comprises alternating diffusing
layers and light absorbing layers.
24. The curved back-projection screen according to claim 19,
wherein the first layer is a synthetic resin layer or is made of
glass.
25. The curved back-projection screen according to claim 19,
wherein the light diffusing particles are embedded in a resin
material.
26. The curved back-projection screen according to claim 19,
wherein the gain for backscattering (Gr) has a value Gr<0.1 for
an angle >40.degree..
27. The curved back-projection screen according to claim 26,
wherein the value of Gr<0.07.
28. A method of making a curved back-projection screen, the curved
back-projection screen having an angle of curvature greater than
180.degree. and comprising a first layer and a second synthetic
resin diffusing layer on the first layer, the second synthetic
resin diffusing layer containing a light absorbing material and
light diffusing particles, the method comprising spraying the
second synthetic resin diffusing layer onto in an inside surface of
the first layer.
29. The method according to claim 28 comprising spraying the second
synthetic resin diffusing layer having a uniform thickness onto in
an inside surface of the first layer.
30. The method according to claim 28, wherein the second synthetic
resin diffusing layer is applied multiple layers.
31. The method according to claim 28, wherein the second diffusing
layer is applied with a mix of diffusing layers and light absorbing
layers.
32. The method according to claim 31, wherein the second diffusing
layer is applied with alternating diffusing layers and light
absorbing layers.
33. The method according to claim 28, wherein the first layer is
made of a synthetic resin or is made of glass.
34. The method according to claim 28, further comprising embedding
the light diffusing particles in a resin material.
35. The method according to claim 28, wherein the gain for
backscattering (Gr) has a value Gr<0.1 for an angle
>40.degree..
36. The method according to claim 35, wherein the value of
Gr<0.07.
37. A display or a simulator having a curved back-projection
screen, the curved back-projection screen having an angle of
curvature greater than 180.degree., the curved back-projection
screen comprising a first layer and a second diffusing layer on the
first layer, the second diffusing layer containing a light
absorbing material and light diffusing particles; and at least one
projector configured for back-projecting an image onto the first
layer wherein light of the image exits the second diffusing layer
towards a viewer.
38. A method of displaying an image onto a curved back-projection
screen having an angle of curvature greater than 180.degree. by
back projection, the screen comprising a first layer and a second
diffusing layer on the first layer, the second diffusing layer
containing a light absorbing material and light diffusing
particles; the method comprising: projecting the image with at
least one projector, and arranging the projector so that the image
is projected onto the first layer and light of the image exits the
second diffusing layer towards a viewer.
Description
[0001] The present invention relates to a curved back-projection
screen such as a wrap-around cylindrical or dome screen, e.g. a
curved screen especially a polygonal, cylindrical or spheroidal
back-projection screen and also to an immersive display or a
simulator using that screen.
TECHNICAL BACKGROUND
[0002] Ed Lantz provided a survey of large-scale immersive displays
as published by ACM SIGGRAPH in the Emerging Display Technology
Conference Proceedings, August, 2007. Wrap-around cylindrical or
dome screens are said to be preferred over rectilinear immersive
screens in cinematic applications as they provide a more seamless
appearance over a greater range of viewing angles and
conditions.
[0003] There are specific problems in providing a back-projection
screen e.g. for a simulator which is basically spherical in shape
(spheroidal) as shown schematically in FIG. 1 or has a significant
angular extent e.g. more than 180.degree., e.g. a half sphere or
half cylinder screen. Outside a translucent spherical (truncated
spherical) screen 2, projectors 4 are arranged to project the
images required for the simulator onto the screen 2. The images
from the projectors overlap so that care needs to be taken at the
overlap positions so that the images remain realistic and are not
subject to distortions.
[0004] The screen operates in back projection for which a diffusing
screen is required. Although diffusing flat back projection screens
are known, these are not so easily adapted for spherical use. One
problem with viewing inside a spherical dome is that light from one
side of the screen will impinge on the other. This differs from the
flat screen for which the only light projected onto the screen is
that from ambient light sources and that can be reduced by suitable
shading within the enclosure where the simulator is located. But
for the spherical screen it is the image itself on one side of the
screen which becomes the ambient light for the image on the other
side. This effects the contrast that can be obtained and renders
materials suitable for flat screens to be not suitable for
spherical screens if the same levels of contrast are to be
achieved.
[0005] At present there are no commercial diffusers that combine
all the conflicting requirements for spherical screens. Some
diffusers have a broad HGA at the expense of sharpness. Other
diffusers have excellent sharpness at the expense of speckle. Even
other diffusers have a very good balance of transmissive/reflective
gain, which ensures a high image brightness and high contrast but
at the expense of transmissive half gain angle.
SUMMARY OF THE INVENTION
[0006] It is an object of the present invention to provide a curved
back-projection screen such as a wrap-around cylindrical or dome
screen, e.g. a curved screen especially a polygonal, cylindrical or
spheroidal back-projection screen and also an immersive display or
a simulator using that screen.
[0007] The present invention provides a curved back-projection
screen having an angle of curvature greater than 180.degree. such
as a wrap-around cylindrical or dome screen, the screen comprising
a first layer and a second diffusing layer on the first layer, the
second diffusing layer containing a light absorbing material and
light diffusing particles. Such a wrap-around cylindrical or dome
screen may have a radius from 1 meter up to in excess of 15
metres.
[0008] The second diffusing layer may be made of a synthetic resin.
The light diffusing particles embedded in the resin material. The
light absorbing material may be embedded in the resin material. The
second diffusing layer can be adapted to have a value of the
product of the absorption coefficient and thickness of between 0.1
and 5, e.g. 0.5-5. The second diffusing layer can be applied by
spraying.
[0009] Parameters of the second layer:
TABLE-US-00001 A. Diffusing particles Size particles <d> =
2.0-40.0 [um] Concentration particles .sup. c = 1%-25% (by weight)
B. Medium containing particles, formulation Thickness layer
<t> = 200 .sup. [um] RI difference between medium and
.DELTA.n = 0.01 particles C. Absorber dyes or pigments Absorption
factor .sup. .alpha. = 200-6000 [1/m]
[0010] The present invention provides an article comprising a
bilayer sheet for use as a rear projection screen which comprises a
first-layer and a second synthetic resin diffusing layer, the first
layer being glass or a synthetic resin layer, the second synthetic
resin diffusing layer containing light diffusing particles and a
light absorbing material, the light diffusing particles being in a
weight concentration in the second synthetic resin diffusing layer
of 1-25%, the magnitude of the difference in refractive index
between the particles and the resin material being 0.01-0.15, the
thickness variation of the diffusing layer being less than 5%
within a spherical cap with base radius 30 cm, and the article
having a transmissive half gain viewing angle of >45.degree. or
>55.degree. and a transmissive peak gain >0.25.
[0011] The reflective gain can be <0.1 for a scattering angle
>30.degree. and <0.2 for a scattering angle >15.degree.,
scattering angle being the angle between scattering direction and
specular reflection direction.
[0012] The absorption coefficient of the second synthetic resin
diffusing layer can be in the range 200-6000 per meter.
[0013] The particles can have an equivalent spherical diameter of
2.0-40 micron.
[0014] The present invention also provides an article comprising a
multiple layer sheet for use as a rear projection screen which
comprises at least a synthetic resin diffusing layer containing
light diffusing particles and a light absorbing material, the light
diffusing particles being in a weight concentration in the
synthetic resin diffusing layer of 1-25%, the magnitude of the
difference in refractive index between the particles and the resin
material being 0.01-0.15, the global thickness variation of all
absorptive layers being less than 5% of their summarized thickness
within a spherical cap with base radius 30 cm and the article
having a transmissive half gain viewing angle >55.degree. and a
transmissive peak gain >0.25.
[0015] The reflective gain can be <0.1 for a scattering angle
>30.degree. and <0.2 for a scattering angle >15.degree.,
scattering angle being the angle between scattering direction and
specular reflection direction.
[0016] The absorption coefficient of at least one synthetic resin
diffusing layer can be 200-6000 per meter.
[0017] The particles can have an equivalent spherical diameter of
2.0-40 micron.
[0018] The present invention also provides an article comprising a
multiple layer sheet for use as a rear projection screen which
comprises at least a synthetic resin diffusing layer containing
light diffusing particles and at least one synthetic resin
absorbing layer, the light diffusing particles being in a weight
concentration of at least one synthetic resin diffusing layer of
1-25%, the magnitude of the difference in refractive index between
the particles and the resin material being 0.01-0.15, the global
thickness variations of all absorptive layers being less than 5% of
their summarized thickness within a spherical cap with base radius
30 cm and the article having a transmissive half gain viewing angle
>55.degree. and a transmissive peak gain >0.25.
[0019] The reflective gain can be <0.1 for a scattering angle
>30.degree. and <0.2 for a scattering angle >15.degree.,
scattering angle being the angle between scattering direction and
specular reflection direction.
[0020] The absorption coefficient can be 200-6000 per meter.
[0021] The particles can have an equivalent spherical diameter of
2.0-40 micron.
BRIEF DESCRIPTION OF THE FIGURES
[0022] FIG. 1 illustrates a dome screen as used in the present
invention.
[0023] FIG. 2 illustrates a dome screen as used in the present
invention with an entrance hole.
[0024] FIG. 3 illustrates forward scattering of light with a
bilayer screen according to the present invention.
[0025] FIG. 4 shows the luminance as a unction of viewing angle
obtained in accordance with an embodiment of the present
invention.
[0026] FIG. 5 shows how incident light from other parts of a screen
affect contrast.
[0027] FIG. 6 illustrates backscattered light and surface
reflection for a screen as used in the present invention.
[0028] FIG. 7 illustrates backward scattering for normal incidence
on an embodiment of the present invention.
[0029] FIGS. 8 and 9 show incident light flux of a collimated beam
onto a screen as used in the present invention.
[0030] FIG. 10 shows values for transmission gain on a coating
according to an embodiment of the present invention.
[0031] FIG. 11 shows values for reflection gain on a coating
according to an embodiment of the present invention.
[0032] FIGS. 12(a) and 12(b) and 13 show spraying patterns for
applying coating in accordance with embodiments of the present
invention.
[0033] FIG. 14 shows the relationship between % of tinting additive
to peak gain as obtained with coatings in accordance with
embodiments of the present invention.
[0034] FIG. 15 illustrates forward and backward scattering.
[0035] FIG. 16 illustrates bulk scattering.
[0036] FIG. 17 illustrates bulk diffusion transmission (left) and
reflection (right).
[0037] FIG. 18 illustrates transmissive (left) and reflective
(right) screen gain.
[0038] FIG. 19 illustrates sharpness loss due to bulk
diffusion.
[0039] FIG. 20 illustrates contrast decrease due to ambient
light.
[0040] FIG. 21 illustrates contrast decrease due to back reflection
of front scattered light.
[0041] FIG. 22 illustrates back projection onto a substantially
curved screen as used in embodiments of the present invention.
[0042] FIG. 23 illustrates brightness uniformity for an observer,
left in front of screen.
DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS
[0043] The present invention will be described with respect to
particular embodiments and with reference to certain drawings but
the invention is not limited thereto but only by the claims. The
drawings described are only schematic and are non-limiting. In the
drawings, the size of some of the elements may be exaggerated and
not drawn on scale for illustrative purposes. Where the term
"comprising" is used in the present description and claims, it does
not exclude other elements or steps. Furthermore, the terms first,
second, third and the like in the description and in the claims,
are used for distinguishing between similar elements and not
necessarily for describing a sequential or chronological order. It
is to be understood that the terms so used are interchangeable
under appropriate circumstances and that the embodiments of the
invention described herein are capable of operation in other
sequences than described or illustrated herein.
[0044] The present invention relates to a rear projection or back
projection screen. A rear projection (RP) screen scatters incoming
light from the projectors in different directions (FIG. 15). The
light scattering can be split up into forward scattering and
backward scattering. Backward scattered rays are lying on the same
side of the screen as the incoming light, forward scattered rays
are lying on the opposite side. For rear projection screens the
light that is scattered in the forward direction is useful light as
it will be viewed. Backscattered light is loss and should be
minimized. The backscatter pattern of the light rays can be
characterized by the bidirectional reflectance distribution
function (BRDF), the forward scattered pattern by the bidirectional
transmittance function (BTDF). These functions contain as
parameters the direction of the incoming and outgoing light ray.
Rear projection screens are often characterized by a more simple
function, the screen gain. The screen gain is defined by the ratio
of the luminance of the screen to the luminance of an ideal, zero
absorption Lambertian screen. The luminance of an ideal, zero
absorption Lambertian screen is defined by
L = E .pi. ##EQU00001##
[0045] In which E is the illuminance of the backside of the screen
and L is the luminance seen by an observer in front of the screen.
In general the luminance of an ideal Lambertian screen only depends
on the illumination of the screen surface.
[0046] Bulk diffusion rear projection screens make use of bulk
scattering (FIG. 16) to scatter incoming light in different
directions. These screens are often circular symmetric, i.e. the
brightness depends only on the angle between observation direction
and specular direction (FIG. 17):
gain ( .theta. ) = .pi. . L ( .theta. ) E ##EQU00002##
[0047] In that case the gain characteristics of the rear projection
screen can be represented by two 2-dimensional graphs (FIG. 18).
The gain at zero angle is called peak gain, the angle for which the
gain is half of the peak gain is called the half gain angle
(=HGA).
[0048] The optical properties of a bulk diffusion screen are
important. One straightforward way to realize a RP screen is by
adding barium sulfate, aluminum oxide, zinc oxide, magnesium
carbonate, calcium carbonate, calcium sulfate, sodium silicate,
clay, chalk, etc. . . . to a clear bulk material, for instance
polyester, polycarbonate or PMMA. The weight percentage of this
addition is in the range of 20%. By adding these minerals, cavities
are introduced causing the material to look opaque. The quality of
such a RP screen will be very poor. A lot of light is backscattered
and/or ends up in neighbouring pixels, thus destroying sharpness
and contrast (FIG. 19). This will be referred to as pixel
cross-talk. Besides this the transmissive gain will be poor and the
screen will appear milky.
[0049] Even with a well defined bulk diffusion screen that has low
pixel cross-talk one may end up with a low contrast. If the screen
is put in an environment with a lot of ambient light, this ambient
light may destroy the contrast. Each pixel, including the dark
ones, is illuminated by ambient light, and backscatters light
partly in direction of the observer (FIG. 20). Therefore the dark
pixel is perceived brighter than without ambient light, i.e. the
contrast ratio of the image is decreased.
[0050] If we assume a Lambertian screen, the contrast ratio can be
written as
CR = ( E bright . g trans + E amb . g refl ) / .pi. ( E dark . g
trans + E amb . g refl ) / .pi. ##EQU00003##
[0051] If we now assume that the dark illumination E_dark is very
low, we end up with
CR = E bright . g trans + E amb . g refl E amb . g refl
##EQU00004##
[0052] From this equation it is clear that there are two ways to
increase the contrast, first of all by decreasing the ambient
illumination and secondly by introducing an unbalance in the
transmissive/reflective gain characteristics (g_trans
>>g_refl).
[0053] In case of a curved screen part of the forward scattered
light ends up in dark pixels where it is back scattered in all
directions, including the observer direction (FIG. 21). So also in
this case contrast ratio is decreased. From the equation above it
is clear that for a given ambient illumination condition,
introducing an unbalance in the transmissive/reflective gain
characteristics (g_trans >>g_refl) will lead to a higher
contrast ratio.
[0054] The above reasoning also holds for the case where a non
Lambertian screen is considered.
[0055] In accordance with embodiments of the present invention the
optical properties of a diffusive coating have been optimised. A
screen including the coating is suited for rear projection screens
that are substantially curved, e.g. have an angle of curvature of
180.degree. or more. Curved also includes a polygonal curvature,
e.g. similar to the simulator screen of the windows of a ship's
bridge. The viewing side of the screen is the hollow side, the
projection side is the convex side in a back-projection arrangement
(FIG. 22). By substantially curved we refer to screens that contain
at the viewing side at least two screen normals that are
substantially opposite to each other. In other words there exist at
least 2 screen normals that make an angle close to 180.degree.. For
such screens there is at least one region that receives forward
scattered light coming from the screen in the opposite direction
(see FIG. 21). From the above it can be derived there is a need to
create an unbalance between transmissive and reflective gain
characteristics in order to end up with a good contrast ratio.
[0056] A second requirement of a bulk diffusive screen coating is
the transmissive half gain angle, which should be as large as
possible (see FIG. 18). Both for multi-viewer applications and
applications where the observer head moves this is necessary. If
the HGA would be very small, the screen would be very directive.
This implies that without electronic or optical compensation (i.e.
illuminance on the backside of the screen does not depend on the
screen position) the brightness uniformity of the screen would be
very poor (see FIG. 23). Electronic or optical compensation may
uniformize the brightness but this only goes for one observer.
[0057] Besides these requirements, there are other requirements as
for instance image sharpness and speckle.
[0058] In accordance with embodiments of the present invention the
parameters that determine the forward and backward scattering
properties of the coating are: the light diffusing particles
included within the diffusing layer, the medium that contains these
particles and the absorber dyes, pigments or other materials that
are used for light absorption. The light diffusing particles are
characterized by their shape, size, refractive index and
concentration. The medium that contains the particles is
characterized by its refractive index and thickness. This medium
should be very stable in time, no discoloration over time may
occur. The absorber materials such as pigments are characterized by
their stability, concentration and should be color neutral.
[0059] By adjusting all parameters that influence the optical
parameters of the diffusive coating a parameter combination can be
found that results in a high resolution, high contrast ratio, high
brightness, high half-gain-angle, speckle free diffusive coating.
The asymmetry achieved can be characterized by a forward scattering
peak gain that is larger than 0.25, a forward scattering
half-gain-angle that is larger than 45.degree. and more preferably
greater than 55.degree. and less than 80.degree. or 70.degree. and
a backward scattering gain that is smaller than 0.20 for angles
larger than 15.degree. and smaller than 0.10 for angles larger than
30.degree..
[0060] In one embodiment the present invention relates to a curved
screen especially a domed screen. A domed screen can be a
polygonal, cylindrical or spheroidal or spherical rear projection
screen 2 (See FIGS. 1 and 2). For example the viewing space inside
a dome such as a sphere can have a field of view is theoretically
0<.theta.<2.pi. and 0<.phi.<2.pi. (usual spherical
co-ordinates (r, .theta., .phi.) with origin in the center of the
sphere). The dome, e.g. sphere can have one section cut away
(truncated) to provide an opening 4 at the bottom. This can be used
for access or a separate opening 6 can be provided for access. In
this case the field of view can be 0<.theta.<2.pi. and
0>.phi.<2.pi. (usual spherical coordinates (r, .theta.,
.phi.) with origin in the center of the sphere). However
embodiments of the present invention also find advantageous use
with a curved screen with a field of view can be
0<.theta..ltoreq..pi. and 0>.phi.<.pi. Such a screen can
be used in a simulator of a ship's bridge for example, as the view
out of the windows of the bridge can be limited, e.g. the roof
prevents a view out in the vertical direction. Another example is a
half-dome screen where the viewer only faces forwards. In this case
the field of view can be 0<.theta.<.pi. and
0>.phi.<.pi. (usual spherical co-ordinates (r, .theta.,
.phi.) with origin in the center of the sphere).
[0061] The curved screen or dome especially the polygonal,
cylindrical or spheroidal screen, e.g. sphere comprises an
optically transparent structure with mechanical characteristics and
dimensions that guarantee a stable mechanical structure. One
surface, typically the inside surface of the curved screen, e.g.
the dome or sphere has an inner-layer with the required diffusing
optical characteristics. A good balance of the characteristics:
high contrast ratio, high resolution (MTF-value), freedom from
artifacts and large viewing angle are usually required. Several
projectors 8 are located outside the curved screen, e.g. sphere and
the image from these projectors is formed on the inner-layer. MTF
stands for Modulation Transfer Function and is used to characterize
how well an optical system can resolve black and white images. This
is linked to the eye limiting properties of the dome. For example
in a dome used for flight simulation it is important to have so
much resolution that a pilot can seeing a simulated plane from
several kilometers--i.e. the simulator has a resolution that
matches the actual limit for 20/20 eyesight. See
http://www.bobatkins.com/photography/technical/mtf/mtfl.html,
http://www.videovantage.com/?p=805.
[0062] Without being limited by theory the following optical
characteristics are preferred for a good image formation on the
inner-layer:
[0063] For an ideal projection screen the luminance is independent
of the viewing angle (Lambertian emitter: L(.theta.')=constant=L0).
The gain g of the screen is defined as the luminance of the screen
in the forward direction with respect to the luminance of an ideal
Lambertian reflectance standard with 100% reflectance. Although in
some applications the observation volume (eye-box of the viewer) is
limited it is preferred to approximate a Lambertian emitter because
also a good blending is required in the overlap region of the
images from different projectors.
[0064] The level of the luminance is preferably higher than a
certain minimum level for good visibility. The luminance level is
function of the light flux produced by the projector, the area of
the image and the optical characteristics of the inner-layer.
[0065] For good image reproduction the contrast ratio of the image
viewed inside the curved screen, e.g. sphere is preferably higher
than a certain minimum level. The contrast ratio is usually
measured using a checkerboard pattern. Light scattered from
illuminated parts of the screen will illuminate the dark parts of
the image (cross-talk and integration effect) and consequently
reduce the contrast ratio. The inner-layer of the curved screen,
e.g. sphere should minimize the influence of the scattered light
that illuminates the dark parts of the image. The backscattering
characteristics of the inner-layer reduce the influence of the
cross-talk on the contrast ratio.
[0066] The inner-layer must conserve the resolution of the image.
Due to scattering of light in the inner-layer a blurring effect on
the image of the pixels is possible. This can be expressed as the
impulse response or an MTF-value can quantify this effect. The
MTF-reduction due to the screen should be minimal.
[0067] The inner-layer preferably does not produce speckle noise.
Speckle noise is a granular pattern superimposed on the displayed
image and is a consequence of the spatial and temporal coherence of
light. Wavelets scattered by surface roughness or scattering
particles can interfere at the observation point and generate a
granular pattern.
Forward Scattering
[0068] Forward scattering of light (see FIG. 3) can be caused by
surface diffusion, holographic diffusion or bulk diffusion. Bulk
diffusion is realized by scattering particles (also called
light-diffusing particles) in the bulk of the inner-layer, in this
case the scattering particles, with refractive index n1, are
distributed in a material such as a resin or glass with refractive
index n2. The particles are preferably spherical in shape but they
could have other shapes such as spheroidal, potato-shaped,
cylindrical, ellipsoidal, oval, etc. In the case of bulk scattering
the scattering cone angle .theta..sub.s depends on .DELTA.n=n1-n2,
the average diameter d of the scattering spherical particles, the
weight concentration c of the scattering particles and the
thickness t of the inner-layer:
.theta. s .varies. .DELTA. n t c ln ( d ) ##EQU00005##
[0069] A diffuser is characterized by an amplitude transmittance
|d(x, y)|, this is a random variable. The autocorrelation function
R(x, y) of the diffuser's amplitude transmission is for many types
of diffusers given
by: R(x, y)=exp.left brkt-bot.-.pi.(x.sup.2+y.sup.2)/L.sup.2.right
brkt-bot.; L is the correlation length of the diffuser. The
scattering angle .theta..sub.s of the diffuser is related to the
correlation length:
sin ( .theta. s 2 ) = .lamda. 2 L ##EQU00006##
[0070] The bulk diffuser is designed to provide the desired
luminance distribution L(.theta..sub.v). In a practical realization
FIG. 4 shows the luminance as a function of the viewing angle
.theta..sub.v obtained in accordance with an embodiment of the
present invention.
[0071] An ideal Lambertian emitter would have a luminance that is
constant for -.pi./2<.theta..sub.v<.pi./2.
[0072] An important characteristic is the angle for which the
luminance is at 50% of the maximum luminance level, called the
"half-gain angle". In this embodiment the half-gain angle is
approximately 70.degree..apprxeq.1.22 rad. The measured 0.degree.
luminance value was approximately 74 cd/m.sup.2. In this case the
0.degree. luminance of an ideal Lambertian emitter is 191
cd/m.sup.2. The screen gain g at 0.degree. (called "screen peak
gain") is in this case 0.38.
[0073] An important drawback of the diffusion by the scattering
particles is usually the increased blurriness of the image by
increasing the scattering cone angle.
Backward Scattering
[0074] Incident light from other parts of the image inside the
sphere (FIG. 5) reduce the contrast ratio. In order to minimize
this contrast reduction the reflection and the backward scattering
should be reduced as much as possible. This is realized by adding a
light absorbing material in the inner layer, such as a dark or
black material of which a dye or pigment or tint are examples, with
a weight concentration C.sub.dye to the inner-layer. The
inner-layer also contains a light scattering particles with a
refractive index n1. The refractive index of the light scattering
particles is smaller than the refractive index n2 of the resin,
n1<n2. The black dye will result in a reflection coefficient
.rho.<1 and a light absorption coefficient .alpha. (units
m.sup.-1).
[0075] The reflected light has two components: backscattered light
from the diffusers in the resin and surface reflection that depends
on the surface characteristics of the coating (see FIG. 6).
[0076] Measured backward scattering for normal incidence on an
embodiment of the present invention is shown in FIG. 7.
[0077] Compared with the forward scattering, the backward
scattering is more specular. The stronger specular reflection
results in a substantially higher contrast ratio.
[0078] The backward scattering has a relative small scattering
angle. This is explained as follows: [0079] a) because of the
absorption a of the light in the light absorber such as dye or
particles of the inner layer [0080] b) and the average traveling
distance of the light for back reflection is 2 times the average
traveling distance of the light for the forward direction a thinner
layer (average thickness .delta.<t) of the inner-layer will
contribute to the backscattering and will consequently have a
smaller scattering angle.
Resolution
[0081] In accordance with embodiments of the present invention
light absorbing material is present in, e.g. is added to the inner
layer of the screen, e.g. a dark material such as a black dye or
pigment is added to the inner-layer to reduce the blurring effect
of the diffuser and realize an image with a high MTF-value.
Pigments, tints or inherent light absorbing properties of materials
can be used to achieve this in accordance with the present
invention. One example is carbon particles.
[0082] Absorption coefficient .alpha. of the inner-layer.
[0083] Concentration of dark material such as a black dye or
pigment=C.sub.dye
[0084] (Consider for simplicity of the notation 1 dimensional
case).
[0085] We assume an impulse response function I(x) centered around
x=0 and symmetrical around x=0 for an inner layer without absorbing
dye. If we now assume to add a dark material such as a dark dye or
pigment with absorption coefficient .alpha.. For larger |x|-values
the light has to travel (on the average) a longer distance through
the inner-layer and consequently undergoes a stronger absorption.
The point spread function for an inner-layer with a dark material
such as a dye or pigment with absorption coefficient .alpha. is
therefore: I(x)exp(-.alpha.f (x)).
f(x)=f(-x),f(x)>0 and
df ( x ) dx > 0. ##EQU00007##
[0086] This explains the strong reduction on MTF degradation when a
dark material such as a dye or pigment is added.
Speckle Noise
[0087] Although the light of a projection system is not
monochromatic and is produced by an extended light source the
images formed on screens can show speckle noise. By the van
Cittert-Zernike theorem the coherence width can be calculated (M.
Born and E. Wolf, Principles of Optics). For a projection lens with
opening 2 .theta.' the coherence area diameter is given by:
.DELTA. D = .lamda. sin ( .theta. ' ) ##EQU00008##
[0088] For a projection system with magnification m and f-number
F/# this gives:
.DELTA.D=2.lamda.mF/#
[0089] Projectors with small light modulators (diagonal .about.1
inch) require large magnification and because the f-number in such
projectors is around 2.5 the area of high degree of coherence is
relative large. This means that light from such an area, when
scattered, can interfere and produce speckle noise at the receptor
side.
Example
[0090] For .lamda.=550 nm, m=80 and F/2.5 we have a coherence width
of 220 .mu.m;
[0091] (.theta.'.apprxeq.2.510.sup.-3 rad)
[0092] A diffuser layer with scattering cone angle 140.degree. has
a coherence area with diameter.apprxeq.0.59 .mu.m.
[0093] The eye has a resolving power of approximately 1
arc-minute=0.2910.sup.-3 rad and has a resolution cell of
diameter.apprxeq.436 .mu.m at a viewing distance of 1.5 m. The
number of statistically independent coherence areas in an eye
resolution spot is in this case approximately 50010.sup.3. The
speckle noise will be effectively reduced by this diffuser.
("Speckle-free rear-projection screen using two close screens in
slow relative motion", E. Rowson, A. Nafarrate, R. Norton, J.
Goodman, J. Opt. Soc. Am. Vol. 66, No. 11, November 1976).
[0094] In accordance with an aspect of the present invention a
tinted inner layer (a, t) with scattering particles (d, c,
.DELTA.n, n1<n2) conserves a sharp image with a low MTF
degradation for a suitable choice of the parameters a, t, d, c and
.DELTA.n. This is applicable on flat screens and on curved screens.
However different parameters are required for curved screens than
flat screens.
[0095] In accordance with an aspect of the present invention a
tinted inner layer (having an absorption a) with scattering
particles (d, c, .DELTA.n, n1<n2) can realize a forward
scattering approximating a Lambertian emitter (half-gain angle
.gtoreq.50.degree.) and a backward scattering approximating a
specular reflector for a suitable choice of .alpha., d, c,
.DELTA.n.
[0096] In accordance with an aspect of the present invention a
contrast ratio >10:1 can be realized inside a sphere or for a
screen with a certain curvature when the forward scattering
approximates a Lambertian emitter and the backward scattering
approximates a specular reflector.
[0097] In accordance with an aspect of the present invention a low
speckle noise level can be realized with a suitable choice of the
parameters d, c, t, .DELTA.n. This is applicable on flat screens
and on curved screens.
[0098] In accordance with an aspect of the present invention a low
speckle noise level can be realized in combination with a good
conservation of the screen MTF. This is applicable on flat screens
and on curved screens.
[0099] In accordance with an aspect of the present invention the
optimal .alpha. (absorption coefficient) for a curved screen or
dome need not be the same as for a planar structure.
[0100] In accordance with an embodiment of the present invention
the value of a suitable for a dome is around 1650 per meter for a
coating thickness of 560 .mu.m which leads to a product of the two,
.alpha.*d=0.924. .alpha.*d is a dimensionless constant. As the
absorption is dependent on the product .alpha.*thickness d, the
product of the two is a useful parameter for assessing the quality
of such a coating.
[0101] For another embodiment value of a suitable for a dome is
around 3000 per meter for a coating thickness of 560 .mu.m which
leads to a product of the two, .alpha.*d=1.64.
[0102] For yet another embodiment value of a suitable for a dome is
around 4200 per meter for a coating thickness of 560 .mu.m which
leads to a product of the two, .alpha.*d=2.3.
[0103] The value of .alpha.*d is useful for the coating composition
ranges described below in the table with reference to the present
invention and is believed to be more or less independent of the
exact nature of the binder/polymer+additives used. For example, a
suitable range would be 0.8 to 1.2 for .alpha.*d or 0.5 to 1.5 for
.alpha.*d or under some conditions 0.1 to 2 for .dbd.*d. More
heavily tinted layers provide better results for contrast and/or
resolution so that other suitable ranges are 0.1 to 5 for .alpha.*d
or for example are 0.5 to 5 for .alpha.*d.
[0104] It is not expected that the materials used to obtain an
absorption a have a significant effect on the present invention
which is not limited to a specific tint or additive e.g. carbon nor
to a specific concentration for the tint (e.g. in ppm).
[0105] In accordance with an aspect of the present invention the
g.sub.r (the gain for backscattering) is at large angles a function
of mainly .alpha.. In accordance with an aspect of the present
invention an optimal result is obtained for g.sub.r<0.1 for an
angle >40.degree.. For example a preferred value would be
g.sub.r<0.07.
[0106] Embodiments of the present invention make use of a range
parameters for the inner layer coating of the screen as given
below:
TABLE-US-00002 Range parameters coating dome: min max Diameter
spherical d [.mu.m] 2.0 40.0 particles Weight concentration c [%] 1
25 spherical particles Thickness coating (inner- t [.mu.m] 200 2000
layer) Absolute value difference |.DELTA.n| [--] 0.01 0.15
spherical diffusive particles and resin Absorption coefficient
.alpha. [m-1] 200 6000
[0107] The particles may be made of polyorgano-silsesquioxane for
example and the resin material in which they are embedded can be an
acrylic polymer such as PMMA. The light absorbing pigment can be
carbon particles.
[0108] Difference forward scattering (transmissive gain) and
backward scattering (reflective gain).
[0109] The difference between the forward scattering and backward
scattering characteristics is an important aspect of this
invention.
[0110] A meaningful and practical measure for difference between
the forward scattering and backward scattering is the difference of
the transmissive gain and the reflective gain.
[0111] The ratio of
.DELTA. g t ( .PHI. ) .DELTA..PHI. and .DELTA. g r ( .PHI. )
.DELTA..PHI. ##EQU00009##
a good measure for the difference between the transmissive gain and
the reflective gain. [0112] g.sub.r(.phi.): transmissive gain
[0113] g.sub.r(.phi.): reflective gain [0114] .phi.: viewing
angle
[0114] g r ( .PHI. ) = .pi. L ( .PHI. ) S .PHI. ##EQU00010##
[0115] .PHI. is the incident light flux of a collimated light beam
on an area S of the screen. L(.phi.) is the measured luminance (see
FIGS. 8 and 9).
g r ( .PHI. ) = .pi. L ( .PHI. ) S .PHI. ##EQU00011##
[0116] Values for measured on materials according to embodiments of
the present invention for reflective and transmissive gain are
shown in FIGS. 10 and 11.
[0117] From the measurement g.sub.r (.phi.) and g.sub.r (.phi.) we
can calculate:
.DELTA. g t .DELTA..PHI. for 10 deg < .PHI. < 40 deg
.DELTA..PHI. = 30 deg ##EQU00012## and ##EQU00012.2## .DELTA. g r
.DELTA..PHI. for 6 deg < .PHI. < 15 deg .DELTA..PHI. = 10 deg
##EQU00012.3## .DELTA. g t .DELTA..PHI. .apprxeq. - 0.165 rad - 1
##EQU00012.4## .DELTA. g r .DELTA..PHI. .apprxeq. - 2.28 rad - 1
##EQU00012.5## .DELTA. g t .DELTA..PHI. .DELTA. g r .DELTA..PHI.
> .gamma. ##EQU00012.6##
In this case is .gamma..apprxeq.14 .gamma. must be sufficiently
large to obtain the required balance between the characteristics:
luminance, contrast ratio and image sharpness.
[0118] In accordance with embodiments of the present invention a
sufficiently large value is .gamma.>7
[0119] Another important condition to achieve a high CR is the
value of the reflective gain g.sub.r(.theta.) for large
.theta.:
g.sub.r(.theta.)<0.1 for .theta.>40 deg
[0120] An important aspect of the present invention is a coating
for a rear projection curved, e.g. spherical screen. The present
invention provides a high quality rear projection screen coating
for the inside of a transparent dome. Based on the formula for the
square root integral (=SQRI) one can calculate that the
requirements for a dome screen coating are different from the
requirements of a general flat screen coating. The square root
integral is the number for the image quality of a display. It's
expressed in units of just noticeable differences. The bigger this
value, the higher the screen quality. Very important is the
influence of unwanted light on the contrast ratio. In case of a
flat screen this unwanted light is ambient light at a certain
constant level. In case of a spherical screen this unwanted light
is light reflected by the screen coating due to the integrating
character of a spherical screen. All projected light that does not
end up in the observer's eye immediately after scattering through
the coating may end up in the observer's eye after multiple
reflections on the spherical screen. Therefore the dark zones on
the screen are illuminated by the bright zones, i.e. the contrast
ratio of the spherical screen is diminished.
Influence of Ambient Light on the Modulation Depth
[0121] For a certain spatial frequency the SQRI is proportional to
the square root of the modulation depth divided by the threshold
modulation. Contrast sensitivity of the human eye and its effects
on image quality, P. G. J. Barten, p 157. If we assume a constant
luminance level, the threshold modulation is also constant and so
we can focus on the ambient light influence on the modulation
depth. The modulation depth or contrast modulation M is defined as
the ratio of the amplitude of the luminance variation to the
average value of the luminance.
M = L max - L min L max + L min = .DELTA. L 2 L ( 1 )
##EQU00013##
in which .DELTA.L is the difference between maximum and minimum
luminance and <L> is the average luminance.
[0122] Ambient light will have an equal impact on the maximum and
the minimum luminance. The expression for the modulation depth
changes to
M ' = ( L max + A ) - ( L min + A ) ( L max + A ) + ( L min + A ) =
.DELTA. L 2 L + A = M 1 1 + A L ( 2 ) ##EQU00014##
where A is the ambient light luminance as seen by the observer
after reflection on the screen.
[0123] In case of a flat screen this ambient light luminance A is
constant. The larger the ambient light level the bigger the
decrease in modulation depth and the smaller the SQRI value. To
diminish the ambient light influence it makes sense to raise the
light level of the display.
[0124] In case of a spherical screen (as an example of a curved
screen) this ambient light luminance A is proportional to the
average luminance in the sphere. The expression for the modulation
depth is in this case:
M ' = M 1 1 + .beta. L L = M 1 1 + .beta. ( 3 ) ##EQU00015##
[0125] Raising the light level, of the display has no influence on
the modulation depth, it is mandatory to decrease .beta..
[0126] Influence of absorptive materials such as pigments or dyes
on the modulation depth
[0127] Suppose the screen coating contains at least one layer such
that the coating absorption can be characterized by an absorption
coefficient .alpha. and a thickness d. The light that passes once
through the layer is attenuated by exp(-.alpha.d)
L'.sub.max=L.sub.maxe.sup.-.alpha.d
L'.sub.min=L.sub.mine.sup.-.alpha.d (4)
[0128] The ambient light passes twice through the layer and is
attenuated by exp(-2.alpha.d)
A'=Ae.sup.-2.alpha.d
.beta.'L'=.beta.e.sup.-.alpha.dLe.sup.-.alpha.d (5)
[0129] For a flat screen this results in following expression for
the modulation depth
M ' = M 1 1 + A e - .alpha. d L ( 6 ) ##EQU00016##
[0130] For a spherical screen (as an example of a curved screen)
this results in following expression for the modulation depth:
M ' = M 1 1 + .beta. e - .alpha. d ( 7 ) ##EQU00017##
[0131] In realistic rear projection situations the ambient light A
is only a fraction of the average display light <L>, let say
0.1. Therefore no absorption or only a limited absorption is
necessary to preserve a good modulation depth (equation 6). For a
spherical screen there is no way to preserve the modulation depth
except by increasing the absorption (equation 7).
[0132] Now let us aim for the same modulation depth in both cases,
flat screen and spherical screen. This imposes that
A e - .alpha. 2 d L = .beta. e - .alpha. 2 d ( 8 ) ##EQU00018##
where we assumed a similar coating thickness. Since A is the
ambient light luminance as seen by the observer after reflection on
the screen, therefore it can be written as
A=.beta.'A' (9)
[0133] In which A' is the luminance of the incident ambient light
and .beta.' is a reflectance value, which is slightly bigger than
the value of .beta.. Theoretically in the limit these can be equal.
For realistic projection situations the ambient light A is a
fraction of the average luminance of the screen, let us say 20% of
the average luminance. This gives the following equation for the
reflectance values
.beta. ' e - .alpha. 2 d 5 = .beta. e - .alpha. 2 d ( 10 )
##EQU00019##
[0134] If we neglect the small difference between the reflectance
values, i.e. if we assume
.beta.'.apprxeq..beta. (11)
[0135] We get the following equation for the difference in
absorption factors
.alpha. 2 - .alpha. 1 = ln ( 5 ) d ( 12 ) ##EQU00020##
[0136] I.e. for a similar image quality the absorption factor of
the spherical screen coating should be ln(5)/d higher than the
absorption factor of the flat screen coating. I.e. the spherical
screen should absorb a lot more than a flat screen to result in a
similar image quality. If we assume a coating thickness of 350
micron, we end up with a difference of
.alpha. 2 - .alpha. 1 = 4600 [ 1 m ] ( 13 ) ##EQU00021##
[0137] This means that the difference between .alpha.*d for a flat
screen and a curved screen with the same thickness of coating is
4600.times.d. Thus the curved screen coatings in accordance with
embodiments of the present invention differ significantly in the
value of .alpha.*d compared with conventional coatings for
back-projection flat screens.
Adding Absorbing Materials
[0138] Adding absorbing materials to at least one layer has serious
consequences. The coating can be extremely sensitive to thickness
variations, which is obvious when looking at the exponential factor
in the equation for the luminance (equation 4). In order to achieve
an acceptable luminance uniformity, the thickness variation has to
be controlled meticulously. One possible approach to apply this
coating is by spray painting. If the local brightness variation is
restricted to 3%, the local thickness variation may have to be less
than 2.7% for a certain coating configuration. If the local
brightness variation is restricted to 5%, the local thickness
variation may have to be less than 4.5% for a certain coating
configuration.
[0139] In accordance with a preferred embodiment the coating is
applied to the inside of the sphere by means of spraying. The
coating is preferably applied as an aqueous suspension. Of the
kinds of spraying that can be used airless spraying is less
preferred because the thickness uniformity is not good. The coating
must be applied with a good surface quality since this is important
for the thickness tolerance. Thickness variations when viewed in
transmission from the inside of the spherical screen are easily
seen by the eye and are disturbing. Further a rather thick layer
has to be applied e.g. >70 .mu.m From experiment it has been
found that conventional air spraying methods are also not very
suitable. In accordance with an embodiment of the present invention
either an air assisted method is preferred or a rotational bell cup
method is preferred. The air assisted method is a method that lies
between airless and conventional spraying techniques. Air assisted
spraying typically uses air pressure and fluid pressure of
2,100-21,000 kPa to achieve atomization of the coating. This
equipment provides high transfer and increased application.
[0140] The fluid pressure is provided by an airless pump, which
allows much heavier materials to be sprayed than is possible with
an airspray gun. Compressed air is introduced into the spray from
an airless tip (nozzle) to improve the fineness of atomisation.
[0141] A rotational bell cup method uses a rotary atomizer as a
paint applicator. The typical bell applicator consists of four or
five major elements: the valve module, the bell cup, the turbine,
the shaping air shroud, and optionally an electrostatic system.
[0142] The valve module is a manifold consisting of passages for
paint, solvent, and compressed air, and valves to control the flow
of materials for paint delivery, cleaning and purging with solvent,
and management of compressed air to the valves, turbine, and
shaping air shroud. The bell cup is a conical or curved disc fixed
to the shaft of the turbine. Paint is injected into the center of
the rear of the disc, and is atomized by being forced out to the
edge of the cup by centrifugal forces. The flow of the paint over
the cup and off the edge breaks up the paint into atomized
droplets. The turbine is a high speed, high precision air motor
that rotates the bell cup at speeds ranging from 10,000 rpm to
70,000 rpm, depending on the cup diameter, atomization desired, and
physical properties of the paint. Typical turbines for this
application use an air bearing, where the spinning shaft is
suspended in a cushion of flowing compressed air, with virtually no
frictional resistance. The shaping air shroud, or shaping air ring,
is simply a ring with passages for air to flow out the front of the
atomizer, outside of the cup diameter, to manage the size of the
spray pattern produced. As more air is forced through the shroud,
the atomized paint is forced into a smaller pattern.
[0143] The electrostatic system is optional and can be internal or
external (or direct or indirect charge), and supplies high voltage
(30,000 to 100,000 volts DC) charge to the applicator, or the air
surrounding it. This has the effect of negatively charging the
paint, while causing a region of positive charge to form on the
workpiece, resulting in electrostatic attraction between the paint
and the workpiece. The electrostatic system is visible only on an
external (or indirect) charge applicator, where it appears as a
series of 4-8 forward-facing electrodes in a circular array around
the bell.
[0144] Since thickness tolerance is so important and the geometry
on the inside of a sphere is complicated, it is very important to
calculate a customized spray path. It is preferred if the spray
path does have any overlaps. This means that each pass of the
spraying head abuts the previous path. In case of a rectangular
substrate the path this is straightforward (see FIG. 12(a)). In
case of a spherical substrate the path is less straightforward. In
accordance with embodiments of the present invention the substrate,
i.e. the sphere can be kept static and the spray head moves
preferably under the control of a robot, or the sphere is made to
move and the spray head is kept sensibly still or the sphere is
made to move and the spray head is also allowed some movement. In
the case of a static sphere it is preferred to use a circular spray
pattern, as for instance the spray pattern from a round beam or
rotational bell. The path may be a spiral (FIG. 13). In the case of
a dynamic movement of the spherical substrate, this may rotate
around its rotational axis, that case it may also be possible to
use a flat beam spray technique.
[0145] For all the spray techniques multiple layers (e.g. 6-9) are
applied. In one embodiment a mix diffusive and absorbing layers are
used (e.g. 10-13 layers) (schematic representation in FIG. 12(b) of
side view of FIG. 12(a)) rather using a single paint composition so
for example alternating diffusive and absorbing layers can be
applied.
[0146] FIG. 14 shows the relationship between % of tinting additive
to peak gain as obtained with coatings in accordance with
embodiments of the present invention. The percentage of tinting
material is based on the liquid paint for spraying which has 52%
solid matter. Hence to obtain the percentages of absorbtive
material in the final coating the values on the X axis of this
graph must be divided by 0.52.
* * * * *
References