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Description  |
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TECHNICAL FIELD OF THE INVENTION
The present invention is directed, in general, to display screens and, more
specifically, to a polymer-dispersed liquid crystal ("PDLC") shutter for a
teleconferencing system capable of transitioning rapid between alternative
transparent and scattering states and a low parallax display screen
employing the shutter.
BACKGROUND OF THE INVENTION
Teleconferencing was introduced decades ago in a simplified form with
picture telephones wherein bidirectional video and audio links were
established between calling and called parties. With the advent of
personal, desktop computers, teleconferencing has assumed a more complex
form. Digital images, such as text and graphics, are displayed on each
conferee's terminal display screen, while video images of the conferees
are also displayed in a portion of the display screen. The latter are made
possible by positioning a camera to one side (top, bottom, left or right)
of the display screen for recording images of the particular conferee
viewing the display screen. Since the conferee naturally focuses attention
on the display screen and because the camera is positioned off to one side
of the display screen, eye contact is lacking between the conferee and the
camera. This is known as "parallax"--a problem that arises when axes are
misaligned, such as those of the camera and the display screen. Parallax
is also a problem in the broadcasting industry where text prompting
devices are employed.
Eye contact with the camera establishes eye-to-eye contact with each of the
conferees shown on the display screen, thereby creating a feeling of
interest among the conferees. Similarly, a lack of direct eye contact with
the camera causes a loss of eye-to-eye contact with each of the conferees
shown on the display screen that, in turn, creates a perception of
disinterest or preoccupation.
From the perspective of the camera suffering a parallax problem, the
conferee appears to be dozing when the camera is above the display screen,
gazing to the left or right when the camera is right or left of the
display screen or looking at the ceiling when the camera is below the
display screen. As the conferee's scrutiny of the display screen becomes
closer, the problem of parallax intensifies.
While some teleconferencing display terminals continue to be produced with
an inherent parallax problem, conventional solutions have been proposed.
One such solution involves a combination of a cathode ray tube ("CRT")
display screen with a side-mounted camera focused on the conferee through
a properly-angled beam splitter. The conferee can concentrate on the
display screen while maintaining simultaneous eye contact with the camera.
Although such a display terminal conquers the parallax problem, it cannot
be overlooked that the cost of success is quite high. The CRT-beam
splitter display terminal is extremely bulky, covering an area of several
feet square. This is a significant portion of a standard desk surface.
Bulkiness is an inherent problem caused by the introduction of a beam
splitter.
U.S. Pat. No. 5,159,445, issued on Oct. 27, 1992, entitled
"Teleconferencing Video Display System for Improving Eye Contact,"
commonly assigned with the present invention and incorporated herein by
reference is directed to a video display system including a camera
positioned behind, or opposite the viewing angle of, a display screen
having first and second states of operation. As a result, the camera and
the conferee are on opposite sides of the display screen. The display
screen is controlled to switch from the first, or image display, state to
the second, or substantially transparent, state. When the display screen
is in the substantially transparent state, the camera is controlled to
record images appearing on the viewing side of the display screen. Thus,
this video display system achieves both direct eye-to-eye contact and
compactness.
It has been found, however, that as display and camera frame rate increase,
the light transmitted from the conferee to the camera through the display
screen diminishes unacceptably, resulting in a reduction in image quality.
The effect is particularly prominent in color display screens.
Accordingly, U.S. Pat. No. 5,243,413, issued on Sep. 7, 1993, entitled
"Color Parallax-Free Camera and Display," commonly assigned with the
present invention and incorporated herein by reference is directed to a
teleconferencing display terminal wherein the amount of light transmitted
to the camera (the so-called "light budget") is increased. In the
illustrated embodiment, light-attenuating devices present in a liquid
crystal display ("LCD"), such as color filters, are repositioned out of
the path of light entering the camera, allowing light transmission to
increase without compromising the ability of the display screen to display
color. The system introduces, in one embodiment, a shutter, mounted
between the conferee and the LCD, that is capable of switching between
transparent and scattering states in synchronicity with the LCD and the
camera. In the transparent state, light is allowed to pass from the
conferee through the shutter and the LCD to the camera. In the scattering
state, the shutter behaves as a translucent, rear-projection screen to
receive light from a projection lamp through the LCD. Conventionally, the
shutter comprises a film of polymer-dispersed liquid crystal (PDLC)
material under control of driver circuitry.
When the shutter is in its scattering state, it should have the properties
of a good rear-projection screen. The PDLC material should act as a
Lambertian scatterer, resulting in a display having brightness
substantially independent of viewing angles. If the scattering is less
than Lambertian, a bright spot will be apparent on the display
corresponding to an image of the projection lamp. As scattering decreases,
the projection lamp itself will become visible.
A further consideration in the design of the PDLC shutter is the residual
scattering of the shutter in the transparent state. To the extent that
light continues to be scattered in this state, image quality suffers.
Therefore, it is desirable that there be great contrast between the
shutter's transparent and scattering states and that the shutter's
transitions between transparent and scattering states are as swift as
possible. At 60 Hertz, approximately 8 milliseconds (ms) is available for
camera image acquisition and 8 ms for image display.
The most critical transition occurs when the shutter switches from
transparent to scattering states. If the projection lamp is activated
before the PDLC material has fully transitioned to the scattering state,
the conferee will see the bright image of the projection lamp through the
shutter. Of course, activation of the projection lamp may be delayed to
accommodate the transition interval, but at the cost of a significant
reduction in display screen brightness.
The physical properties inherent in the PDLC material govern the speed of
this transition; variations in drive voltage and waveform are of no
effect. Therefore, what is needed in the art is an improvement in the
physical properties of a PDLC shutter for a teleconferencing system that
will allow the shutter to transition between the transparent and
scattering states faster, thereby improving display quality.
SUMMARY OF THE INVENTION
To address the above-discussed deficiencies of the prior art, the present
invention provides, for use in the display screen of a teleconferencing
system, a shutter capable of assuming alternative transparent and
scattering states and a method of manufacture therefor. In a preferred
embodiment, the shutter includes a film of a polymer dispersed liquid
crystal (PDLC) composition comprising a polymer and a liquid crystal
material wherein the liquid crystal material comprises from about 60% to
about 90% by weight of the film. The liquid crystal preferably has a
viscosity less than 40 cst (centistokes). Optimum liquid crystal
morphology is achieved when the film is cured at a temperature ranging
from about 30.degree. C. to about 40.degree. C. The present invention also
includes first and second layers of a transparent conductive material for
containing the film there between and driver circuitry, connected to the
film, for causing the film to have a response time that is equal to or
less than 8 ms.
The present invention therefore in a preferred embodiment introduces a PDLC
shutter of a liquid crystal of a weight fraction or composition that
minimizes droplet size to thereby decrease switching times, most notably,
from the transparent state to the scattering state. In a preferred method
of manufacture of the present invention, the polymer may be selected from
those known in the art to be used in the manufacture of PDLC screen
displays. However, in those embodiments where the polymer is produced by
photopolymerization, as acrylate, vinyl ether or epoxy monomer is
preferred, and in a more preferred embodiment, the monomer is an acrylate
monomer. The polymerization is initiated, preferably by irradiating the
homogenous mixture of monomer and liquid crystal with ultraviolet light
having an intensity of at least one milliwatt per cm.sup.2. Of course,
however, it will be appreciated that the polymerization can be achieved by
other methods as well, such as thermal initiation. The polymerized
monomer-fraction of the PDLC is cured at an elevated temperature
preferably above 30.degree. C. and that more preferably ranges from about
35.degree. C. to about 38.degree. C. It has been unexpectedly found that
the elevated temperatures within the stated ranges during cure lead to a
film having a response time that is equal to or less than 8 ms.
In a preferred embodiment of the present invention, the liquid crystal
comprises from about 75% to about 80% by weight of the film. In a more
preferred embodiment, however, the liquid crystal comprises about 78% by
weight of the film. In a manner to be described more fully, a film of
about 78% liquid crystal material has been empirically determined to have
optimal transition speed characteristics when cured at about 38.degree. C.
In a preferred embodiment of the present invention, the film has a response
time ranging from about 1 millisecond to about 8 milliseconds. More
preferably, however, the film switches from the transparent state to the
scattering state in about 1.3 ms. In some applications, such response
times may not be necessary, however, the broad scope of the present
invention contemplates faster response times of less than or equal to 1
ms.
In a preferred embodiment of the present invention, the film is contained
in at least 5 .mu.m thick cells. More preferably, however, the film is
contained in about 20 .mu.m thick cells, and exhibit faster switching
times.
In a preferred embodiment of the present invention, the cells have a
substantially uniform cell gap of at least 15 .mu.m. In a more preferred
embodiment, the cells have a substantially uniform cell gap of about 20
.mu.m.
The foregoing has outlined, rather broadly, preferred and alternative
features of the present invention so that those skilled in the art may
better understand the detailed description of the invention that follows.
Additional features of the invention that form the subject of the claims
of the invention are described below. Those skilled in the art should
appreciate that they can readily use the disclosed conception and specific
embodiment as a basis for designing or modifying other structures for
carrying out the same purposes of the present invention. Those skilled in
the art should also realize that such equivalent constructions do not
depart from the spirit and scope of the invention in its broadest form.
BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding of the present invention, reference is
now made to the following descriptions taken in conjunction with the
accompanying drawings, in which:
FIG. 1 illustrates a simplified block diagram of a conventional LCD screen,
together with a light source and video camera;
FIG. 2 illustrates a simplified block diagram of a video display system
employing a display screen incorporating an embodiment of a PDLC shutter
constructed according to the principles of the present invention;
FIG. 3 illustrates a graph of response time measurements as a function of
composition for different temperatures as measured for a series of 8 .mu.m
thick test cells;
FIG. 4 illustrates a graph of switching voltage data as a function of the
liquid crystals fractional weight for different temperatures; and
FIG. 5 illustrates an oscilloscope trace of transition times for a PDLC
shutter constructed according to the principles of the present invention.
DETAILED DESCRIPTION
Referring initially to FIG. 1, illustrated is a simplified block diagram of
a conventional LCD screen 100, together with an area or distributed light
source 110 and video camera 120. The LCD screen 100 is a single integrated
unit that includes a front polarizer 130, front glass layer 140, liquid
crystal 150, color filters 160, rear glass layer 170 and rear polarizer
180. The liquid crystal array 150 contains additional elements (not shown)
including, without limitation, transparent conducting layer, circuit layer
and alignment layers on both sides of the liquid crystal 150. The light
passing through the layers of the LCD screen 100 to the video camera 120
is attenuated introducing losses therein. Images are displayed on the LCD
screen 100 by illuminating white light through the rear polarizer 180 via
the light source 110.
The rear polarizer 180 polarizes the light emanating from the light source
110. The cells of the liquid crystal screen 100 are independently
controllable. Each liquid crystal cell operates by rotating the
polarization direction of the light passing through it. The angle through
which the polarization is rotated in each liquid crystal cell depends upon
the voltage applied to it. After passing through the liquid crystal array
150, the light passes through the front polarizer 130 to a user viewing
the LCD screen 100.
Three cells in combination form the pixels that constitute the LCD screen
100. The cells, synonymously designated subpixels, are associated with a
respective one of red, green or blue color filters. To produce a color on
the LCD screen 100, the light passes through the color filters 160 and is
therein proportioned with the three subpixels to achieve a desired color.
When the LCD screen 100 passes light to the video camera 120, the presence
of the color filters 160 and other elements of the LCD screen 100 reduce
the amount of light transmitted to the video camera 120 An optimal color
filter 160 allows about one-third of the white light to pass therethrough.
Typically, color filters 160 pass less than one-hundred percent of the
light in their passband thereby blocking at least two-thirds of the white
light passing through their cells. For instance, the red light is blocked
by the green and blue filters; the green light is blocked by the red and
blue filters; the blue light is blocked by the red and green filters.
Turning now to FIG. 2, illustrated is a simplified block diagram of a video
display system 200 employing a display screen 210 incorporating an
embodiment of a PDLC shutter 220 constructed according to the principles
of the present invention. Along with the display screen 210, the video
display system 200 includes a video camera 260, light source or projection
lamp 270 and driver circuitry 280. The display screen 210 includes the
PDLC shutter 220 and a front polarizer 230, front glass layer 235, liquid
crystal array 240, rear glass layer 245, color filters 250 and rear
polarizer 255. The color filters 250 and rear polarizer 255 are located in
the path of the light source 270, but not in the path of the video camera
260 to increase the light from the front polarizer 230 to the video camera
260 while maintaining the color capability of the display screen 210.
In the illustrated embodiment, the light source 270 is a dot or stripe
projector that projects a two dimensional array of red, green and blue
dots or stripes of polarized light through the back surface of the liquid
crystal array 240 to the back surface of the PDLC shutter 220. The red,
green and blue dots are permanently focused on one of three subpixels that
form each pixel of the display screen 210 during the manufacturing
process. The front polarizer 230 in conjunction with the liquid crystal
array 240 attenuates and modulates the intensity of the dots of polarized,
colored light. The PDLC shutter 220 diffuses the dots of polarized,
colored light to form an image on the display screen 210.
Although the principles of the present invention are described in the
context of a twisted nematic liquid crystal display, it should be apparent
to one of ordinary skill in the related art that the principles of the
present invention are equally applicable to other video display systems
incorporating different display screens. For instance, the principles of
the present invention can be implemented in a system employing a flat
screen display such as a ferro-electric liquid crystal display.
The video camera 260 is mounted in a centralized position behind the
display screen 210 (i.e., such that the PDLC shutter 220 is centered about
an optical axis of the video camera 260) in a line of sight with a user
looking at the display screen 210. The positioning of the video camera 260
eliminates parallax by maintaining direct eye contact between the video
camera 260 and the user. The video camera 260 employs an electrical,
optical or mechanical shutter (not shown) to prevent light from the
projector 270 reaching the video camera 260 when the video camera is on.
The video camera 260 should have sufficient light sensitivity to maintain
an acceptable video output to the user positioned at a normal viewing
distance from the front polarizer 230 when recording images. The
transparency of the display screen 210 can be optimized to minimize any
losses of the available light. Any standard video camera 260 may be
employed to advantage; any charge coupled device ("CCD") sensored array or
camera having imaging tubes may also be employed together with the
appropriate lens and focusing and shuttering apparatus.
The PDLC shutter 220, video camera 260 and light source 270 are multiplexed
to provide two modes of operation, namely, the scattering mode image
display and transparent mode image acquisition. In the scattering mode,
information is displayed on the PDLC shutter 220 via the light source 270
while the video camera 260 is inactive. In the transparent mode, the video
camera 260 integrates light to produce a video representation of a
captured image of the user. In this mode of operation, the light source
270 is inactive and the PDLC shutter 220 is substantially transparent. The
shutter 220 has a response time that can range from about 1 millisecond
(ms) to about 8 ms; that is, the shutter, in a preferred embodiment,
switches from the transparent state to the scattering state within that
range. However, in a more preferred embodiment, the shutter 220 has a
response time that is less than or equal to 1 ms. In some applications,
such switching speeds may not be necessary. However, the broad scope of
the present invention contemplates switching speeds faster than 1.3 ms.
The video display system 200 also incorporates a 50% duty cycle between
the image display mode and image display mode, although other ratios of
duty cycles are well within the scope of the present invention.
Regardless, the rate and ratio that the video display 210 and video camera
260 operate are selected to minimize flicker therein.
The PDLC shutter 220 includes droplets or bubbles of liquid crystal 222
dispersed in a polymer matrix film. The PDLC shutter also includes first
and second layers 226, 228 of transparent conductive material for
containing the film 222 therebetween. Preferably, the first and second
layers 226, 228 are glass plates that are coated with a conventional
transparent conductive material that allows the plates to act as
electrodes. The first and second layers 226, 228 are coupled to the driver
circuitry 280 to thereby cause the film 222 to switch between the
transparent and scattering states. In a preferred embodiment, the
combination of larger cell gaps that preferably range from about 5 .mu.m
to about 20 .mu.m, lower viscosity liquid crystal below 40 cst, a liquid
crystal fraction that ranges from about 60% to about 90%, and elevated
photopolymerization temperatures that range from about 30.degree. C. to
about 40.degree. C. during the curing process provide a film that produces
a PDLC film with a morphology that switches more rapidly from the
transparent state to the scattering state.
In a preferred embodiment, the film 222 is formed from a polymer dispersed,
liquid crystal material composition wherein the liquid crystal comprises
from about 60% to about 90% by weight of the film 222. More preferably,
however, the film 222 is comprised from about 75% to about 80% by weight
of liquid crystal, and in an even more preferred embodiment, the film 222
comprises about 78% by weight of liquid crystal. The film 222 is cured at
temperatures ranging from about 30.degree. C. to about 40.degree. C. and
more preferably, is cured at a temperature of 38.degree. C. It has been
unexpectedly found that curing the film with the stated liquid crystal
fractional weight within the above-stated temperature range results in a
film that switches from the transparent state to the scattering state in
no more than 1.3 ms and at a voltage of about 140 volts RMS for a 20 .mu.m
thick film.
The polymer may be one of several selected from a group of polymers that
are known in the PDLC art. In these embodiments, the phase separation that
forms the dispersion occurs upon polymerization of the monomer mixture and
liquid crystal. A preferable commercially available example of the liquid
crystal mixture used in the PDLC shutter of the present invention is
LICRILITE.RTM. TL216, which is available from Merck House, Poole, Dorset,
BH15 1TD, England. This particular liquid crystal has the following
physical properties: 1)S to N transition of <-20.degree. C., 2)a clearing
point of 80.3.degree. C., 3)viscosity at +20.degree. C. of 36, cSt,
4)dielectric anistropy .DELTA..di-elect cons.1 kHz at 20.degree. C. is
5.5, .di-elect cons..sub..parallel. 1 kHz at 20.degree. C. is 9.7,
.di-elect cons..perp.1 kHz at 20.degree. C. is 4.2, 5)optical anistropy
(20.degree. C., 589 nm) .DELTA.n of 0.2106, n.sub.o of 1.5234, n.sub.e of
1.7340, 6) multiplex properties measured at 90.degree. twist V(90,0,20)
(saturation) 3.53V, V(10,0,20) (threshold) 2.63V 7)elastic constants
K.sub.1 at +20.degree. C. 14.40 10.sup.-12 N, K.sub.3 +20.degree. C. 19.60
10.sup.-12 N and K.sub.3 /K.sub.1 of 1.36.
In such embodiments, the monomer is mixed with the liquid crystal material
in the above-stated weight fraction ranges given for the liquid crystal
material to form a homogeneous mixture. The homogeneous mixture is
positioned between the two glass plates 226, 228, and is then subjected to
conditions that cause the monomer to polymerize while mixed with the
liquid crystal material to form a polymer matrix about the liquid crystal
material. In a preferred embodiment, the polymerization is initiated by
exposing the homogeneous mixture to ultra violet light at an intensity of
at least one milliwatt per cm.sup.2 for a total dose of at least about 6.4
cm.sup.2. Of course, it will be appreciated by those of ordinary skill in
the art that other processes may be used to initiate the polymerization as
well. During polymerization, the mixture is preferably cured at a
temperature above 30.degree. C. In a more preferred embodiment, the
mixture is cured at temperature ranging from about 35.degree. C. to about
40.degree. C., and more preferably, at about 38.degree. C. The monomer is
preferably selected from the group consisting of acrylate, vinyl ethers or
epoxies, and the liquid crystal is selected to have a birefringence
greater than 0.2.
As the monomer undergoes polymerization, the resulting polymer separates
from the liquid crystal material, and thus, forms a polymer matrix about
the liquid crystal material that results in a liquid crystal drop-size and
morphology that provides an optimal response time.
In the illustrated embodiment, the film contains about 78% liquid crystal
material, although other percentages of liquid crystal material are well
within the scope of the present invention. Additionally, the film 222 is
contained in about 20 .mu.m cells that comprise the PDLC shutter 220.
However, the broad scope of the present invention encompasses cells of any
size, spacing or degree of uniformity.
Synchronization between the PDLC shutter 220, video camera 260 and light
source 270 is maintained by the drive circuitry 280 including a display
processor 285 and control circuitry 290. Again, the driver circuitry 280
provides rapid time switching between the transparent and scattering modes
to allow the user to view images on the display screen 210 at apparently
the same time the video camera 260 is recording images of the user. The
driver circuitry 280 receives images destined for the display screen 210
via a video in lead 287. The control circuit 285 develops synchronization
from an externally supplied signal on a sync lead 292. The control circuit
285 employs industry standard internal circuitry (not shown) to manage the
operation of the PDLC shutter 220, video camera 260 and light source 270.
The display processor 290 converts the input video images to a format
compatible with the display screen 210 and, also, controls the operation
of the light source 270.
The driver circuitry 280 generally operates as follows. The synchronization
signal is supplied externally via the sync lead 292. The synchronization
signal typically is supplied by a device external to the video display
system 200, but may alternatively be supplied from the video camera 260 or
display processor 285. The synchronization signal provides a reference for
establishing the time intervals based upon, for instance, a recording
period of the video camera 260 or image display period of the display
screen 210. Coordination between the display processor 285 and control
circuitry 290 in the driver circuitry 280 is established by signals on a
control lead 295.
An additional advantage of positioning the rear polarizer 255 out of the
path of the video camera 260 is that there is no longer a requirement to
multiplex the liquid crystal array 240 between displaying information and
a clear state for image acquisition. Although the video camera 260
receives polarized light whose orientation depends on display information
(due to the operation of the front polarizer 230), the liquid crystal
array 240 is effectively clear to the video camera 260 because the video
camera 260 is insensitive to polarization. Thus, the alignment of the rear
polarizer 255 eliminates the effect of the state of the liquid crystal
array 240 on the video camera 260. While application of the present
invention has been discussed with respect to a display screen, it should
be understood that the display screen is an example of but one | | |
