Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD FOR MAINTAINING THE WHITE COLOUR POINT IN A
FIELD-SEQUENTIAL LCD OVER TIME
100011 The present invention relates to the field of liquid crystal display
and, particularly,
to the field of white colour point of a liquid crystal display screen.
BACKGROUND OF THE INVENTION
[00021 Field sequential liquid crystal displays (LCD) use three colour light
emitting
diodes (LED) to provide full colour displays. If the current supplied to the
LEDs were
finely regulated, the white colour point formed by the three colours would
remain the
same. Because the LEDs are voltage controlled, over time, the forward voltage
(Vf) of
each LED varies (increases) so that the calibrated white colour point formed
by operation
of three colours drifts. Thus, there is a need for a method for maintaining
the white
colour point for a field sequential LCD.
SUMMARY OF THE INVENTION
100031 In addressing the problem of maintaining the proper white colour point
during the
life of the LCD, the forward voltages (Vf) of the light emitting diodes for
illuminating the
LCD are adjusted to calibrate the white colour point established as a
combination of the
light emitting diode colours. This adjustment may occur through monitoring the
ON time
and, optionally, brightness of each light emitting diode and comparing a
resulting value
with thresholds stored in software code, look up tables, arrays, hardwired
values, etc.
100041 Other aspects and features of the present invention will become
apparent to those
of ordinary skill in the art upon review of the following description of
specific
embodiments of the invention in conjunction with the accompanying figures.
BRIEF DESCRIPTION OF THE DRAWINGS
100051 Embodiments of present invention will now be described by way of
example with
reference to attached figures, wherein:
FIG. 1 is a block diagram that illustrates pertinent components of a wireless
communications device that communicates within a wireless communication
network
according to the present invention;
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FIG. 2 is a more detailed diagram of a preferred wireless communications
device of FIG.
1;
FIG. 3 illustrates an embodiment of a backlit liquid crystal display;
FIG. 4 illustrates an embodiment of the liquid crystal display and liquid
crystal display
controller;
FIG. 5 illustrates a timing scheme for the light source and the display scans;
FIG. 6 illustrates an embodiment of a section of the gate line driver;
FIG. 7 illustrates a general overview of the method of illuminating an LCD;
FIG. 8 illustrates further detail of an embodiment of the scanning for one
colour within
one frame;
FIG. 9 illustrates an embodiment of a general method;
FIG. 10 illustrates a block diagram of an embodiment of an implementation of
compensation circuitry for one light emitting diode; and
FIG. 11 illustrates an embodiment of a process for compensating the white
colour point of
a display.
DETAILED DESCRIPTION
100061 A method and device, especially a mobile station such as a handheld
communications device, acts to stabilize a white colour point in a display by
compensating for behavioural changes in the light source illuminating the
display over
time. Preferably, the display is a liquid crystal display and the light source
includes light
emitting diodes (LEDs) of different colours. The liquid crystal display may be
operated
at a rate of 30 or more frames per second. The LEDs of the light source
preferably will
include red, green, and blue colours. Other colour schemes, such as cyan,
magenta, and
yellow, are contemplated. Although directed to a liquid crystal display per
se, the
preferred use of the LCD is in a mobile station, such as a wireless portable
handheld
communications device. Cell phones and pagers are amongst the many handheld
devices
contemplated.
100071 FIG. 1 is a block diagram of a communication system 100 that includes a
mobile
station 102 that communicates through a wireless communication network. Mobile
station 102 preferably includes a visual display 112, a keyboard 114, and
perhaps one or
more auxiliary user interfaces (UI) 116, each of which is coupled to a
controller 106.
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Controller 106 is also coupled to radio frequency (RF) transceiver circuitry
108 and an
antenna 110.
[0008] Typically, controller 106 is embodied as a central processing unit
(CPU) which
runs operating system software in a memory component (not shown). Controller
106 will
normally control overall operation of mobile station 102, whereas signal
processing
operations associated with communication functions are typically performed in
RF
transceiver circuitry 108. Controller 106 interfaces with device display 112
to display
received information, stored information, user inputs, and the like. Keyboard
114, which
may be a telephone type keypad or full alphanumeric keyboard (e.g., QWERTY or
DVORAK), is normally provided for entering data for storage in mobile station
102,
information for transmission to network, a telephone number to place a
telephone call,
commands to be executed on mobile station 102, and possibly other or different
user
inputs.
[0009] Mobile station 102 sends communication signals to and receives
communication
signals from the wireless network over a wireless link via antenna 110. RF
transceiver
circuitry 108 performs functions similar to those of a base station and a base
station
controller (BSC) (not shown), including for example modulation/demodulation
and
possibly encoding/decoding and encryption/decryption. It is also contemplated
that RF
transceiver circuitry 108 may perform certain functions in addition to those
performed by
a BSC. It will be apparent to those skilled in art that RF transceiver
circuitry 108 will be
adapted to particular wireless network or networks in which mobile station 102
is
intended to operate.
[0010] Mobile station 102 includes a battery interface (IF) 134 for receiving
one or more
rechargeable batteries 132. Battery 132 provides electrical power to
electrical circuitry in
mobile station 102, and battery IF 132 provides for a mechanical and
electrical
connection for battery 132. Battery IF 132 is coupled to a regulator 136 which
regulates
power to the device. When mobile station 102 is fully operational, an RF
transmitter of
RF transceiver circuitry 108 is typically keyed or turned on only when it is
sending to
network, and is otherwise turned off to conserve resources. Similarly, an RF
receiver of
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RF transceiver circuitry 108 is typically periodically turned off to conserve
power until it
is needed to receive signals or information (if at all) during designated time
periods.
[0011] Mobile station 102 operates using a Subscriber Identity Module (SIM)
140 which
is connected to or inserted in mobile station 102 at a SIM interface (IF) 142.
SIM 140 is
one type of a conventional "smart card" used to identify an end user (or
subscriber) of
mobile station 102 and to personalize the device, among other things. Without
SIM 140,
the mobile station terminal is not fully operational for communication through
the
wireless network. By inserting SIM 140 into mobile station 102, an end user
can have
access to any and all of his/her subscribed services. SIM 140 generally
includes a
processor and memory for storing information. Since SIM 140 is coupled to SIM
IF 142,
it is coupled to controller 106 through communication lines 144. In order to
identify the
subscriber, SIM 140 contains some user parameters such as an International
Mobile
Subscriber Identity (IMSI). An advantage of using SIM 140 is that end users
are not
necessarily bound by any single physical mobile station. SIM 140 may store
additional
user information for the mobile station as well, including datebook (or
calendar)
information and recent call information.
[00121 Mobile station 102 may consist of a single unit, such as a data
communication
device, a multiple-function communication device with data and voice
communication
capabilities, a personal digital assistant (PDA) enabled for wireless
communication, or a
computer incorporating an internal modem. Alternatively, mobile station 102
may be a
multiple-module unit comprising a plurality of separate components, including
but in no
way limited to a computer or other device connected to a wireless modem. In
particular,
for example, in the mobile station block diagram of FIG. 1, RF transceiver
circuitry 108
and antenna 110 may be implemented as a radio modem unit that may be inserted
into a
port on a laptop computer. In this case, the laptop computer would include
display 112,
keyboard 114, one or more auxiliary Uls 116, and controller 106 embodied as
the
computer's CPU. It is also contemplated that a computer or other equipment not
normally capable of wireless communication may be adapted to connect to and
effectively assume control of RF transceiver circuitry 108 and antenna 110 of
a single-
unit device such as one of those described above. Such a mobile station 102
may have a
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more particular implementation as described later in relation to mobile
station 202 of FIG.
2.
100131 FIG. 2 is a detailed block diagram of a preferred mobile station 202.
Mobile
station 202 is preferably a two-way communication device having at least voice
and
advanced data communication capabilities, including the capability to
communicate with
other computer systems. Depending on the functionality provided by mobile
station 202,
it may be referred to as a data messaging device, a two-way pager, a cellular
telephone
with data messaging capabilities, a wireless Internet appliance, or a data
communication
device (with or without telephony capabilities). Mobile station 202 may
communicate
with any one of a plurality of fixed transceiver stations 200 within its
geographic
coverage area.
100141 Mobile station 202 will normally incorporate a communication subsystem
211,
which includes a receiver, a transmitter, and associated components, such as
one or more
(preferably embedded or internal) antenna elements and, local oscillators
(LOs), and a
processing module such as a digital signal processor (DSP) (all not shown).
Communication subsystem 211 is analogous to RF transceiver circuitry 108 and
antenna
110 shown in FIG. 1. As will be apparent to those skilled in field of
communications,
particular design of communication subsystem 211 depends on the communication
network in which mobile station 202 is intended to operate.
[00151 Network access is associated with a subscriber or user of mobile
station 202 and
therefore mobile station 202 requires a Subscriber Identity Module or "SIM"
card 262 to
be inserted in a SIM IF 264 in order to operate in the network. SIM 262
includes those
features described in relation to FIG. 1. Mobile station 202 is a battery-
powered device
so it also includes a battery IF 254 for receiving one or more rechargeable
batteries 256.
Such a battery 256 provides electrical power to most if not all electrical
circuitry in
mobile station 202, and battery IF 254 provides for a mechanical and
electrical
connection for it. The battery IF 254 is coupled to a regulator (not shown)
which
provides power V+ to all of the circuitry.
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100161 Mobile station 202 includes a processor 238 (which is one
implementation of
controller 106 of FIG. 1) which controls overall operation of mobile station
202.
Communication functions, including at least data and voice communications, are
performed through communication subsystem 211. Processor 238 (e.g., a
microprocessor
or processing circuit or core) also interacts with additional device
subsystems such as a
display 222, a flash memory 224, a random access memory (RAM) 226, auxiliary
input/output (I/O) subsystems 228, a serial port 230, a keyboard 232, a
speaker 234, a
microphone 236, a short-range communications subsystem 240, and any other
device
subsystems generally designated at 242. Some of the subsystems shown in FIG. 2
perform communication-related functions, whereas other subsystems may provide
"resident" or on-device functions. Notably, some subsystems, such as keyboard
232 and
display 222, for example, may be used for both communication-related
functions, such as
entering a text message for transmission over a communication network, and
device-
resident functions such as a calculator or task list. Operating system
software used by
processor 238 is preferably stored in a persistent store such as flash memory
224, which
may alternatively be a read-only memory (ROM) or similar storage element (not
shown).
Those skilled in the art will appreciate that the operating system, specific
device
applications, or parts thereof, may be temporarily loaded into a volatile
store such as
RAM 226.
[00171 Processor 238, in addition to its operating system functions,
preferably enables
execution of software applications on mobile station 202. A predetermined set
of
applications which control basic device operations, including at least data
and voice
communication applications, will normally be installed on mobile station 202
during its
manufacture. A preferred application that may be loaded onto mobile station
202 may be
a personal information manager (PIM) application having the ability to
organize and
manage data items relating to the user such as, but not limited to, instant
messaging (IM),
e-mail, calendar events, voice mails, appointments, and task items. Naturally,
one or
more memory stores are available on mobile station 202 and SIM 262 to
facilitate storage
of PIM data items and other information.
[00181 The PIM application preferably has the ability to send and receive data
items via
the wireless network. In a preferred embodiment, PIM data items are seamlessly
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integrated, synchronized, and updated via the wireless network, with the
mobile station
user's corresponding data items stored and/or associated with a host computer
system
thereby creating a mirrored host computer on mobile station 202 with respect
to such
items. This is especially advantageous where the host computer system is the
mobile
station user's office computer system. Additional applications may also be
loaded onto
mobile station 202 through network 200, an auxiliary I/O subsystem 228, serial
port 230,
short-range communications subsystem 240, or any other suitable subsystem 242,
and
installed by a user in RAM 226 or preferably a non-volatile store (not shown)
for
execution by processor 238. Such flexibility in application installation
increases the
functionality of mobile station 202 and may provide enhanced on-device
functions,
communication-related functions, or both. For example, secure communication
applications may enable electronic commerce functions and other such financial
transactions to be performed using mobile station 202.
100191 In a data communication mode, a received signal such as a text message,
an e-mail
message, or web page download will be processed by communication subsystem 211
and
input to processor 238. Processor 238 will preferably further process the
signal for output
to display 222, to auxiliary I/O device 228 or both as described further
herein below with
reference to Figures 3 and 4. A user of mobile station 202 may also compose
data items,
such as e-mail messages, for example, using keyboard 232 in conjunction with
display
222 and possibly auxiliary I/O device 228. Keyboard 232 is preferably a
complete
alphanumeric keyboard and/or telephone-type keypad. These composed items may
be
transmitted over a communication network through communication subsystem 211.
[00201 For voice communications, the overall operation of mobile station 202
is
substantially similar, except that the received signals would be output to
speaker 234 and
signals for transmission would be generated by microphone 236. Alternative
voice or
audio I/O subsystems, such as a voice message recording subsystem, may also be
implemented on mobile station 202. Although voice or audio signal output is
preferably
accomplished primarily through speaker 234, display 222 may also be used to
provide an
indication of the identity of a calling party, duration of a voice call, or
other voice call
related information, as some examples.
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]00211 Serial port 230 in FIG. 2 is normally implemented in a personal digital
assistant
(PDA)-type communication device for which synchronization with a user's
desktop
computer is a desirable, albeit optional, component. Serial port 230 enables a
user to set
preferences through an external device or software application and extends the
capabilities of mobile station 202 by providing for information or software
downloads to
mobile station 202 other than through a wireless communication network. The
alternate
download path may, for example, be used to load an encryption key onto mobile
station
202 through a direct and thus reliable and trusted connection to thereby
provide secure
device communication.
10022] Short-range communications subsystem 240 of FIG. 2 is an additional
optional
component which provides for communication between mobile station 202 and
different
systems or devices, which need not necessarily be similar devices. For
example,
subsystem 240 may include an infrared device and associated circuits and
components, or
a BluetoothTM communication module to provide for communication with similarly-
enabled systems and devices. BluetoothTM is a registered trademark of
Bluetooth SIG,
Inc.
]00231 In accordance with an embodiment, mobile station 202 is a multi-tasking
handheld
wireless communications device configured for sending and receiving data items
and for
making and receiving voice calls. To provide a user-friendly environment to
control the
operation of mobile station 202, an operating system resident on station 202
(not shown)
provides a GUI having a main screen and a plurality of sub-screens navigable
from the
main screen.
]0024] The liquid crystal display cell 222 is shown in greater detail in
Figure 3 in which
a light source formed from multiple LEDs 322, 324, 326 is used as a backlight.
Preferably, the LCD is a field sequential liquid crystal display (FS LCD). LCD
controller
316 provides a voltage to the common electrode(s) 308 and the active elements
310 of the
active matrix. The active elements are preferably thin film transistors. The
common
electrode(s) 308 and active elements are supported on substrates 306 and 312,
respectively. Alternatively, the LCD may be a passive matrix. The LCD
preferably
contains a brightness enhancing film or layer 304 to optimize the distribution
of light for a
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viewer and a diffusing layer. As the preferred liquid crystal material is
super twisted
nematic, polarizers 302 and 314 are used. The LCD controller 316 sets the
pixel grey
scale of the LCD. An optional processor 318 may coordinate synchronization of
the LCD
controller 316 with the light source controller 320. Preferably, the LCD
controller 316
and the processor 318 are integrated into a single device 317, which may
simply be
referred to as an LCD controller having the capability of controlling a light
source
controller 320. The light source may be implemented by using red, green, and
blue LEDs
322, 324, 326. In a specific embodiment, four green, four red, and two blue
LEDs are
used to provide full colour and/or black and white display. The LED controller
320 may
sequence the three colours or may simultaneously energize LEDs of all of the
colours and
terminate power to the LEDs simultaneously. Other combinations of LEDs are
contemplated. The light guide 328 may have a tapered block construction and
may have
approximately a trapezoidal, cross sectional form to more evenly distribute
the light into
the LCD. The light guide may also have uneven areas 330, 332 that scatter the
light so as
to avoid shadowing effects in the LCD image. Although uneven area 330 is shown
to
project out from the surface of the light guide 328 and uneven area 332 is
shown to
project inward to the surface of the light guide 328, the uneven areas may be
arranged
differently so long as the arrangement effectively scatters the light from the
LEDs 322,
324, 326. The uneven areas may be abraded, molded, corrugated, chemically
etched, or
the like. Preferably, to maximize the utilization of light, the LEDs 322, 324,
326 and the
light guide 328 are partially enclosed by a reflector such that the only
opening is fully
bounded by the light transmissive area of the LCD.
[00251 Figure 4 illustrates an embodiment of the LCD controller 402 and LCD
430 for
the method. The LED controller may be internally adapted to provide a sequence
of
lights each centered on a specific wavelength according to the LEDs energized,
followed
by light generated simultaneously from all LEDs or at least two LEDs
generating light
centered on two different wavelengths. In Figure 4, in synchronization with
the LED
controller, the LCD controller 402 creates a grey scale pattern for each light
centred on a
specific wavelength according to column driver 440 (source driver) according
to data and
control signals 410 and row selectors 450 (gate driver) from a data bit line
and a LOAD
LINE clock in a X-Y matrix arrangement. For a red light pattern, only pixels
selectable
by the column driver 440 may be set to a variable transmissive state to
provide a desired
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grey scale pattern. Pixels that do not have a red component of light are
turned off. For
green and blue light patterns, similar procedures are followed. When all red,
green, and
blue colours are transmitted through a given pixel, that pixel may have a
white or whitish
appearance because of the blending of the three primary colours perceived by a
viewer.
Advantages in using the light source to determine colours include elimination
of a colour
filter layer, thus enhancing brightness of the display by reducing a light
absorbing layer,
and increasing the resolution as only one pixel is needed to provide full
colour instead of
separate red, green, and blue pixels. The size of a pixel is allowed to
increase while
resolution is improved; in other words, using the light source and not the LCD
to
determine colour optimizes substrate real estate usage.
[00261 Figure 5 illustrates a colour only mode in which either the entire
display screen is
in colour or the non-colour portion of the display screen is in the off state.
In operation,
pixel grey scale is achieved through pulses written to a pixel during
scanning. Each
colour frame 502 is divided into three parts (or fields) 504, 506, 508 for the
three colours
in full colour mode. Each pixel to be illuminated by a specific colour of
light achieves a
grey scale value from a pulse pattern into the source of the thin film
transistor providing
charge to the pixel. The pulse pattern (i.e., colour scans) includes multiple
high and/or
low pulses for each pixel. One pulse is applied to each colour pixel during a
scan of the
colour region that includes the colour pixel. During the colour region scan
(or sweep)
532, the actual scanning occupies most of the time allotted 530 for a given
colour. It is
the successive scans of the colour pixels during a frame that establishes a
grey scale
value. A smaller portion of the time allotted in a scan period is idle time
534. During
most of the scan period, the light source is turned off 514. In alternative
embodiments,
the light source may remain on for most or all of the scan period and/or the
actual
scanning may occupy a different portion of the time allotted for a given
colour. Once the
final grey scale value for a row or line of pixels is fairly well established,
the light source
(e.g., light emitting diode) is turned on 512. In some embodiments, during the
light
source turn on time, the common electrode of the display is inverted from a
first voltage
bias level 522 to a second voltage bias level 524 to prevent charge buildup in
the liquid
crystal that would degrade performance and damage the display. The inversion
of the
common electrode voltage occurs for each colour for each frame. Thus, for a
red, green,
and blue pixel LCD, the common electrode voltage is inverted three times.
Other
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inversion modes are contemplated such as line inversion and pixel inversion.
In line
inversion, a given line may be alternately supplied through the source driver
with voltages
from a first set of a polarity and then supplied with voltages from a second
set of a
polarity opposite to that of the first set; that is, a non-inverting pair of
voltages may be
applied and an inverting pair of voltages may later be applied. In pixel
inversion,
alternate columns may be supplied for each row with voltage sets of opposing
polarities.
100271 Figure 6 represents a more specific embodiment. An output shift
register (e.g.,
serial in/ parallel out shift register) may be used for scanning the display
screen. The shift
register contains initialization values for the gate shift register. It
preferably contains a
one-hot encoding of the starting line number of display screen. (As used in an
embodiment, one-hot encoding refers to a single active bit that is shifted
through the shift
register such that only one line at a time of pixels is written to from the
source driver.)
The shift register is loaded and then used to sweep the display. A LINE CLOCK
rate is
relatively high; for example, a 10 MHz clock rate may be used. The storage
elements
may be latches 618, 626 that latch data on the rising or falling edges of a
clock, D type
flip flops, or the like. A counter 602 may be used to hold the number of lines
in the
display screen.
[00281 Figure 7 illustrates an overview of the embodiment of a method
corresponding to
the display scanning system. In the general method, initialization occurs 704
(e.g.,
registers are initialized) and the three colour fields are cycled through 706-
710 through
successive scans during a frame.
[00291 Figure 8 illustrates a more detailed embodiment of a scan for a field.
The gate line
driver is shifted once 804. The load pattern is deasserted 806. A new source
pattern is
loaded 807. The source lines on the display matrix are driven 808. The line
count is
reduced by one 810. As long as the counter does not expire (e.g., the line
count remains
greater than zero in a count down mode) 812, scanning resumes at step 804.
[00301 A field sequential liquid crystal display maintains its white colour
point through
compensation values to at least one colour light emitting diode that
illuminates the
display. A degradation curve may be used to calculate extrapolate the
theoretical forward
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voltage of the light emitting diode. Additional complexity arises from the
need for
calculating uptime for multiple light emitting diodes of different colours.
Brightness
levels may also be factored in.
[00311 Figure 9 illustrates an embodiment of a general method for determining
the
application of compensation to a light emitting diode of a single colour A
according to the
time of use or a more complicated function of time of use and brightness per
use. It is to
be understood that in a colour display, there will be two or more light
emitting diodes of
different colours - for example, red, green, and blue - or one or more light
emitting diode
that produces two or more colours. Colour A, as used here, may be any colour -
including red, green, or blue. LED compensation is preferably performed
through pulse
width modulation (PWM) techniques or through current control. A determination
is
periodically made as to whether a light emitting diode is turned on 904. If
so, then the
time of use value is adjusted to correspond to the time the light emitting
diode has been
turned on 906. For example, the time of use value may be expressed as E;=1 k{
unit time
At} where the unit time At may be uniform or non-uniform in duration. A
degradation
curve may be used to calculate or extrapolate the theoretical forward voltage
of an LED
based on usage time. An algorithm may be used to keep track of display
"uptime" and to
insert Vf compensation values as required to pull a white point back to a
specified value.
In another embodiment, a more complicated function value is adjusted and
stored in
which the function correlates time of use and intensity of the light emitting
diode being
monitored to determine a cumulative intensity-time value. In this embodiment,
the
display brightness level must be tracked. For example, the cumulative
intensity-time
value may be expressed as 11k {intensity during unit time I * unit time At}
where the unit
time At may be uniform or non-uniform in duration. Because LEDs of different
colours
(e.g., red, green, blue) are likely to be used, there is additional complexity
for calculating
uptime in a field sequential LCD since the amount of ON versus OFF time for
red, green,
and blue is different. Through multiple LEDs having two or more different
colours, a
synergy may arise that further complicates the adjustment values to maintain
the white
colour point. Thresholds are stored for determining the amount of compensation
to be
applied to the LED. The thresholds may be stored in a data structure, an
array, a look up
table (e.g., an aging table), or the like. If the time of use value or the
cumulative
intensity-time value for the light emitting diode exceeds a first threshold
908 and is less
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than or equal to a second threshold level 910, then a first compensation
element or
arrangement is turned on 912. A compensation element/ arrangement may be
resistive or
capacitive in effect and includes one or more passive and/or active
components, such as a
resistor, a capacitor, or a transistor. In the case of PWM techniques, the
compensation
arrangement may entail the processor altering a set of pulses applied to the
LED being
controlled. For example, the number of pulses may be varied in a unit interval
of time. If
the time of use value or the cumulative intensity-time value for the light
emitting diode
exceeds a second threshold level 910, but not a third threshold level 914, a
second
compensation element or arrangement is switched on 916. In this case, the
first
compensation element or arrangement may be switched off or may remain switched
on.
If the time of use value or the cumulative intensity-time value for the light
emitting diode
exceeds a third threshold, then the third compensation element or arrangement
is switched
on 918. Either or both of the first and second compensation elements or
arrangements
may be switched off in this case.
100321 Figure 11 illustrates an embodiment of a general method for a process
for
determining the white point compensation of a field sequential liquid crystal
display. In
step 1102, a white point is calibrated at the factory. For a red, green, blue
colour scheme
in which red, green, blue light emitting diodes are used, the calibrated may
be set by the
following equations:
RT = Xc seconds
GT = Yc seconds
BT = Zc seconds
At some point, later or earlier than step 1102, an ageing table is created,
step 1104, for the
particular model, sampled batches, or individual field sequential liquid
crystal displays.
An exemplary ageing table is presented below:
RT GT BT
1 hour Al S21 01
10 hours A2 922 J2
1000 hours A3 K23 I3
10,000 hours A4 924 04
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After steps 1102 and 1104, through actual usage of the FS LCD, the white
colour point is
compensated automatically. For example, when usage time is greater than or
equal to one hour
but less than 10 hours, the R, G, B values may be set as RT = XC + A2 ; GT =
YC + 02 ; and BT =
ZC+02.
[00331 Figure 10 illustrates a block diagram of an arrangement of a current
compensation
scheme for a light emitting diode of one of the three colours. It is to be
understood that light
emitting diodes of one or both of the other colours will similarly be
compensated for behavioural
changes over the lifespan of the LED. In Figure 10, light emitting diode LEDI
may have series
compensation A or parallel compensation B or both. The switches SWA and SWB
may be
implemented as complementary metal oxide semiconductor field effect
transistors (CMOS FET)
or as another active circuit element. A processor 1002 controls a switch
internally or externally,
such as one of switches SWA1, SWA2, and SWAN. In an embodiment, only one
switch of the
A switches may be activated (i.e., turned) or two or more switches may be
activated through
processor 1002 or other control circuitry. Because it is not desirable to keep
an LED on
continuously, it is necessary that the current path from power +V through a
current limiting
resistor RES be interruptible, so a switch is always required at the power
receiving end of the
LED. The activated switch permits compensation element(s) A to modify the
current and
voltage applied to LED 1. In an embodiment, it may be desirable to have one of
the
compensation elements A to have negligible resistance and capacitance such as
through the
absence of any impedance element CEAN. Additionally or largely alternatively
to series
compensation elements A, compensation elements B may be placed in parallel
with LED 1.
Processor 1002 or other control circuitry may also be used to control
switching of switches
SWB1, SWB2, through SWBN to activate compensation elements CEB1, CEB2, and
CEBN. It
is to be understood that Figure 10 may be varied so as there may be a single
switch A or multiple
switches A in conjunction with zero or more switches B. Other compensation
arrangements are
contemplated. Preferably, processor 1002 and the compensation circuitry for
the light emitting
diode or diodes are incorporated within the same integrated circuit.
Alternatively, processor
1002 and the compensation circuitry may be formed separately in which case the
processor may
control the switches through various interface circuitry through addressing
information or may
directly control the switches. In the case of pulse width modulation (PWM),
the processor may
directly control an LED without an
CA 02519967 2005-09-16
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impedance element by controlling the number of uniform pulses per unit time or
by
altering the pulse width of one or more pulses in a pulse train.
[00341 The above-described embodiments of the present application are intended
to be
examples only. Those of skill in the art may effect alterations, modifications
and
variations to the particular embodiments without departing from the scope of
the
application. The invention described herein in the recited claims intends to
cover and
embrace all suitable changes in technology.
