LC-TEC DISPLAYS has a long experience working with liquid crystal technologies. Below are some technical
papers & patents covering some of the development
work carried out at LC-TEC DISPLAYS.
The Optical Response of Liquid Crystal Cells to a
Low Frequency Driving Voltage
This report investigates the
optical response of liquid crystal cells to a low frequency square wave
voltage of 0.1Hz. It is found that there are predominately three physical
phenomena that dominate the overall properties of the device. The first is
the discharging effect whereby the effective voltage over the liquid
crystal layer decreases as a function of time and occurs due to mobile
ions being present within the liquid crystal material. That of the second
is the charging-up of the cell where the effective voltage increases with
time. This is attributed to charge separation taking place within the
polyimide layer upon application of the DC voltage component. Finally, the
third effect is cell asymmetry whereby the effective voltage depends upon
the polarity of the externally applied field. This transpires due to a
locked-in DC holding voltage being present within the cell layers. These
three effects are analyzed in some detail with the view of developing a
liquid crystal cell capable of being driven with a low frequency square
wave voltage. A model of a liquid crystal cell in which the liquid crystal
material can dissolve impurity ions from the alignment layers and in which
the ions can then become re-adsorbed into the polyimide layer is
deduced.
(194Kb)
Fast, automatically darkening welding filter
offering an improved level of safety
A mode of operation is introduced
for the standard 90°twisted nematic (TN) liquid-crystal cell when
placed together with an interference fitter and positioned between crossed
polarizers such that a small stimulating voltage of between ± 2.0 and ±
3.0 V is required in order to attain the light state. Further
incrementation of the driving electronics reverts the system back to a
darker phase. Such cells offer advantages over those of the standard
90°TN device operating in the normally white mode, in that the
unit maintains the fast response time from the light to the dark state
associated with the employment of TN cells placed between crossed
polarizers. In addition, a low transmittance state is achieved when the
unit is in the inactivated phase; this is an effect usually correlated
with the normally black mode of operation. These cells are therefore ideal
candidates for incorporation into fast, automatically darkening, welding
filters that are designed to change rapidly from the light to the dark
protective state, while offering an improved level of safety by not
holding in a potentially hazardous light state should the controlling
electronics malfunction. The requirement for this phenomenon to be
observed is that the cell displays a low optical transmittance over the
green wavelengths of the visible spectrum when in the inactivated phase
and placed between crossed polarizers. The presence of an interference
filter that possesses a peak transmittance over the central part of the
visible spectrum is also necessary. It is shown that there are only two
possible cell types that satisfy this criteria, and the optical properties
of such cells are analyzed in some detail.
(79Kb)
Optical angular properties of twisted-nematic
liquid-crystal cells with twist angles of less than 90 degres
I analyze the optical angular
properties of twisted-nematic liquid-crystal cells that operate in the
normally white mode with twist angles of less than 90°. It is demonstrated
that, although a reduction of the twist angle produces an increase in the
asymmetry of the birefringence generated by a single cell when in the
active phase, the positioning of two cells such that the face-to-face rub
directions are crossed gives rise to a large amount of cell compensation,
giving an enhanced field of view. The effect of the polarizer arrangement
is investigated, and it is shown that the configuration required in order
to maximize the overall optical transmittance when in the inactive phase
also maintains the asymmetry displayed by the single cell when stimulated
by voltages lying below that of the saturation voltage. Although
employment of this polarizer design together with a lowering of the twist
angle degrades cell contrast, a fast optical shutter from the light to the
dark state based on a double-cell construction possessing improved optical
angular properties is introduced. Such systems find applications in
automatically darkening welding filter visors for which it is critical to
combine both a fast response time to the protective state together with a
large viewing cone.
(190Kb)
Liquid-crystal cell with a wide viewing angle and
a high cell contrast
A liquid-crystal cell operating in
the normally white mode is introduced that displays a wide viewing angle
together with a high cell contrast. A wide viewing angle is achieved by
reducing the twist angle in the cell to below 90° and by reducing the Dnd
parameter, while a high cell contrast is obtained by incorporating an
in-plane, low-valued compensating retardation film with values lying in
the range of 10-50 nm. The optimization of the cell parameters together
with the polarizer arrangement permit cell transmittances approaching 100%
to be obtained when the cell is in the inactivated phase and with Dnd
values down toward the theoretical limit of 0.27 µm. Further improvements
in the optical angular properties can be obtained by the placing together
of two such cells so that the angular bisector of the surface alignment
directors of the two cells lies antiparallel. Such optical shutters find
applications in fast, automatically darkening welding filters where a fast
response time from the light to the dark protective state is required,
together with both a large viewing cone in the low transmittance state and
a clear field of view in the light phase.
(126Kb)
Electronic Removal
of Mechanical Pressure Marks in Cholesteric Liquid Crystal Displays
The bistable, cholesteric liquid crystal
(Ch-LC) technology is expected to gain significant market penetration over
the next few years with the possibility of manufacturing low power,
reflective LCD's that employ the passive matrix driving scheme, providing
for low manufacturing costs. A large fill factor combined with the fact
that no polariser is required offers the possibility of producing LCD's
possessing higher reflectances than that of competing LCD technologies.
However, cholesteric LCD's are pressure sensitive whereby applied
mechanical pressure to the surface of the LCD generates lateral flow of
the liquid crystal (LC) material. This disrupts the bistable LC textures,
resulting in an optical defect mark being formed around the pressure
point. Frame refreshing rectifies the textures of the Ch-LC material at
each pixel point. However, the textures of the Ch-LC material in the
inter-pixel regions remains unchanged and hence the pressure mark remains
visible. This paper discloses a method by which mechanical pressure marks
in Ch-LCD's using a passive matrix driving scheme are removed via
application of an electric voltage.
(92Kb)
Automatically darkening welding filters
based on liquid crystal
technology
This technical-paper
introduces the concept of welding and briefly discusses the different
techniques that currently exist. The necessity of using some type of eye
protector whilst welding is presented and the advantages of utilising an
automatically darkening welding filter over that of a conventional
welding-filter are shown. The general construction of automatically
darkening welding filters based on liquid crystal technology is analysed
together with the fundamental properties displayed by the individual
liquid crystal optical-shutters used in the
device.
Furthermore, this paper
summarises the optical properties displayed by automatically darkening
welding filters that are based on liquid crystal technology and
specifically analyses the characteristics of liquid crystal cells both
when operated with low voltages and also when in the inactivated (OFF)
state. The poor optical angular properties of such devices is indicated
and the current techniques for alleviating these problems are introduced.
The final section deals with the inactivated (OFF) state and demonstraits
the necessity of optimising the intrinsic parameters of liquid crystal
cells in order to maximise the overall optical transmittance when
functioning in the normally white mode (liquid crystal cell placed between
crossed polarisers).
The present invention
relates to a Liquid Crystal (LC) colour-filter consisting of two (2) or
more independent liquid crystal optical-shutters stacked together and
placed between two (2) linear-polarizers that are either mutually parallel
or alternatively mutually perpendicular.
By applying independent
voltages to the individual liquid crystal optical-shutters, the overall
transmittance of the LC-colour-filter can be electronically switched so as
to possess a high transmittance over selected wavelengths of the visible
spectrum. In such way, the colour of the LC-colour-filter can be rapidly
switched or modulated between different colours.
Such fast switching
LC-colour-filters find applications in products such as, but not limited
to, back-lit, direct-driven (TN) segment liquid crystal displays (LCDs).
Here, the LC-colour-filter can be placed between the back-lighting and
LCD. With the back-lighting emitting a constant and stable white-light,
the LC-colour-filter can rapidly modulate the colour from said
back-lighting. Furthermore, by synchronising the activation of specific
display-segments in the LCD with the colour-modulation from the
back-lighting, full-colour displays can be obtained via sequential
colouration.
(45Kb)
Fast Optical Shutter
(Patent 02238SE)
The present invention
relates generally to liquid crystal optical shutters that are to be
operated with high electrical voltages and more specifically to a large
sized, high voltage optical shutter using cholesteric liquid crystal
materials.
A liquid crystal optical
shutter is disclosed that possesses an aperture window (124) comprising of
first and second electrode patterns (112; 212) arranged on respective
planar substrates (110; 210). The first and second substrates are provided
at a predetermined mutual distance. By providing the electrode patterns so
that there is a high internal electrical resistance in series with any
point in the liquid crystal optical shutter, the occurrence of electrical
sparking is significantly reduced. In a preferred embodiment, the
electrode patterns (112, 122) each comprises a series of essentially
linear row electrodes in the aperture window.
(229Kb)
Method of driving
liquid crystals (Patent
01038SE)
The present invention relates generally to a
method of driving liquid crystal displays and more specifically to a
method of changing the liquid crystal phase of bistable, cholesteric
liquid crystals in a display.
A method of changing the liquid
crystal phase of bistable, cholesteric liquid crystals in a display is
provided. In the display there is provided at least two active display
areas with inactive display areas lying between said active display areas,
and a D.C voltage is applied for a predetermined period of time to
neighbouring active display areas, thereby causing the liquid crystal
phase of the inactive display areas between the active areas to changed to
a predetermined state. In such way, mechanical pressure marks in
cholesteric liquid crystal displays can be removed.
(105Kb)
Homogeneous Liquid Crystal
Cell (Patent 02444SE)
The present invention relates generally to liquid crystal displays
and liquid crystal optical shutters that possess homogeneous
electro-optical properties and more specifically to a method of designing
large sized, liquid crystal displays and liquid crystal optical shutters
that are to be homogeneously filled with liquid crystal
materials.
Liquid crystal displays and liquid
crystal optical shutters typically consist of at least two substrates
placed a small distance apart and which are bonded together around the
edges with a gasket adhesive material. Liquid crystal material is
typically injected into the volume bounded by said substrates and said
gasket through one or more holes or filling-ports present in the gasket. The
filling-port is thereafter sealed with a curing
adhesive.
During the
injection process, a wave-front of liquid crystal material enters the
empty liquid crystal cell and traverses through the volume of said cell
thereafter. However, the variations in the flow-velocity of the wave-front
during the filling process together with the sometimes high surface-energy
associated with the wave-front geometry may induce and generate both phase
separation of the liquid crystal components as well as affect and perturb
the anchoring-energy between the cell alignment layers and said liquid
crystal materials. This results in the liquid crystal display or liquid
crystal optical shutter possessing inhomogeneous electro-optical
properties.
The present invention
relates to a design for a liquid crystal cell that can be homogeneously
filled with liquid crystal materials whereby both the large and sometimes
rapid variations in the flow-velocity of the liquid crystal material as
well as the surface-energy associated with the wave-front geometry during
the filling process are minimised. This enables large sized, liquid
crystal displays and liquid crystal optical shutters to be manufactured
that possess homogeneous optical and electro-optical
characteristics.
