U.S. patent application number 13/595842 was filed with the patent office on 2012-12-20 for electro-optical device, method of driving the same, and electronic apparatus.
This patent application is currently assigned to SEIKO EPSON CORPORATION. Invention is credited to Tetsuro MATSUMOTO, Tomoyuki OKUYAMA.
Application Number | 20120319934 13/595842 |
Document ID | / |
Family ID | 41681044 |
Filed Date | 2012-12-20 |
United States Patent
Application |
20120319934 |
Kind Code |
A1 |
OKUYAMA; Tomoyuki ; et
al. |
December 20, 2012 |
ELECTRO-OPTICAL DEVICE, METHOD OF DRIVING THE SAME, AND ELECTRONIC
APPARATUS
Abstract
An electro-optical device includes a pixel that has an
electro-optical element with a light transmittance that changes to
a predetermined value in response to an applied voltage, which
changes in response to the temperature. A light source irradiates
light to the pixel. A driving unit includes a predetermined number
of sub-fields having the same length in one frame. The driving unit
provides either an on-voltage indicating light-on or an off-voltage
indicating light-off to the pixel in each of the predetermined
number of the sub-fields in response to the grayscale to be
displayed. A temperature measuring unit measures the temperature
and outputs a temperature signal. A time controlling unit controls
the length of one sub-field on the basis of the temperature signal.
A light intensity controlling unit controls the intensity of the
light incident on the pixel on the basis of the temperature
signal.
Inventors: |
OKUYAMA; Tomoyuki;
(Chino-shi, JP) ; MATSUMOTO; Tetsuro; (Chino-shi,
JP) |
Assignee: |
SEIKO EPSON CORPORATION
Tokyo
JP
|
Family ID: |
41681044 |
Appl. No.: |
13/595842 |
Filed: |
August 27, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12542033 |
Aug 17, 2009 |
8274462 |
|
|
13595842 |
|
|
|
|
Current U.S.
Class: |
345/89 |
Current CPC
Class: |
G09G 3/2025 20130101;
G09G 3/2014 20130101; G02F 1/1362 20130101; G09G 2320/041 20130101;
G09G 3/3648 20130101; G02F 2203/30 20130101; G02F 2203/60
20130101 |
Class at
Publication: |
345/89 |
International
Class: |
G09G 3/36 20060101
G09G003/36 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 18, 2008 |
JP |
2008-209510 |
Claims
1. An electro-optical device, comprising: a pixel that has an
electro-optical element of which brightness is controlled by at
least one datum; a driving unit that supplies a voltage
corresponding to the at least one datum to the pixel in each of a
plurality of sub-fields; a temperature measuring unit that measures
the temperature and outputs a temperature signal; and a time
controlling unit that controls a length of at least one sub-field
of the plurality of sub-fields on the basis of the temperature
signal.
2. The electro-optical device according to claim 1, wherein the
time controlling unit controls a length of each of the plurality of
sub-fields.
3. The electro-optical device according to claim 1, wherein: the
time controlling unit controls a length of one sub-fields of the
plurality of sub-fields; and a ratio of a response time during
which a light transmittance of the electro-optical element changes
to a predetermined value after the voltage is supplied to the pixel
to the length of the one sub-field is constant even if the
temperature changes.
Description
[0001] This application is a Continuation of U.S. application Ser.
No. 12/542,033, filed Aug. 17, 2009, which claims priority to
Japanese Patent Application No: 2008-209510, filed Aug. 18, 2008.
The foregoing patent applications are incorporated herein by
reference.
BACKGROUND
[0002] 1. Technical Field
[0003] The present invention relates to an electro-optical device
using an electro-optical material such as a liquid crystal, a
method of driving the same, and an electronic apparatus.
[0004] 2. Related Art
[0005] A liquid crystal is known as an electro-optical material
that has optical characteristics that are changed by electrical
energy. The liquid crystal has transmittance that changes in
response to an applied voltage. Liquid crystal devices using the
change in transmittance are known. Liquid crystal devices include a
plurality of scan lines, a plurality of data lines, and a plurality
of pixels provided to correspond to intersections of the scan lines
and the data lines. The plurality of pixels has a pixel electrode,
an opposite electrode, and a liquid crystal element composed of the
liquid crystal which is interposed therebetween.
[0006] As a method of digitally driving liquid crystal devices,
there is known a sub-field driving method that divides one field
into a plurality of sub-fields along a time axis, and in each
sub-field, applies an on-voltage or an off-voltage to each pixel in
accordance to a gray scale. This method is of changing the
effective voltage which offers the voltage to be applied to the
liquid crystal is performed not by the voltage level but by the
application time of a voltage pulse, to control transmittance of a
liquid crystal panel. In the method, the voltage level necessary to
drive the liquid crystal is only two values, an on-level and an
off-level. In a simple sub-field driving method, the grayscales
that are able to be displayed are limited by the number of divided
sub-fields.
[0007] However, at a constant temperature, the response
characteristics of the liquid crystal are such that they have a
response speed which quickens in response to the magnitude of
electrical field applied to the liquid crystal layer in the
transition from a steady state (oriented state). The transition
from a state where the electrical field is applied to the liquid
crystal layer to the oriented state requires a certain amount of
response time. This response time is typically several times longer
than the time for which the electrical field is applied to the
liquid crystal layer.
[0008] Japanese Patent No. 4,023,517 discloses technology that can
display a number of grayscales which is the same as or greater than
the number of sub-fields by using the rise time and fall time of
the liquid crystal.
[0009] However, the response time of the liquid crystal changes to
a great extent due to the temperature. For this reason, even though
a grayscale corresponding to a value of digital data can be
accurately minced at a certain temperature, the grayscale that has
to be displayed becomes inaccurate if the temperature changes. In
some instances, even when the value of digital data becomes large,
the reversion of a grayscale may arise where the displayed
grayscale becomes dark.
SUMMARY
[0010] An advantage of some aspects of the invention is to provide
an electro-optical device and an electronic apparatus capable of
preventing the reversion of the grayscales while displaying a
number of grayscales equal to or greater than the number of
sub-fields.
[0011] According to a first aspect of the invention, there is
provided an electro-optical device, including: a pixel that has an
electro-optical element of which light transmittance changes in
response to an applied voltage, and response time, for which the
light transmittance changes to a predetermined value after the
voltage is applied, changes in response to temperature; a light
source that irradiates light to the pixel; a driving unit that
includes a predetermined number of sub-fields having the same
length in one frame, and that provides either an on-voltage
indicating light-on or an off-voltage indicating light-off to the
pixel in each of the predetermined number of sub-fields in response
to the grayscale to be displayed; a temperature measuring unit that
measures the temperature and outputs a temperature signal; a time
controlling unit that controls the length of one sub-field on the
basis of the temperature signal; and a light intensity controlling
unit that controls the intensity of the light incident on the pixel
on the basis of the temperature signal.
[0012] According to this invention, the length of one sub-field is
adjusted and the intensity of the light incident on the pixel is
controlled even if the response time of the electro-optical element
changes in response to the temperature. This thereby makes it
possible to keep the display luminance constant even if the
temperature changes. Specifically, if the response time becomes
longer in response to the temperature, it is possible to make the
temporal alteration in the transmittance (for example, the response
waveform of the case where the transmittance of the liquid crystal
is taken on the longitudinal axis, and the time is taken on the
transverse axis) constant by making shorter the length of one
sub-field even if the temperature changes. In other words, it is
preferable to control the length of one sub-field so that the
temporal alteration in the transmittance is constant even if there
is a change in the temperature. The sub-field is one unit that can
control the lighting and/or the turning off of a pixel. However, if
the length of the sub-field changes, the length of the residual
period changes during which it is not possible to control the
lighting and/or the turning off, which can be compensated by
controlling the intensity of the light incident on the pixel. As a
result, it is possible to prevent a reversion of the grayscale
while displaying a number of grayscales equal to or greater than
the number of sub-fields.
[0013] In addition, a typical example of the electro-optical
element is a liquid crystal element. Further, the response time for
which the light transmittance changes to a predetermined value
after the voltage is applied is, for example, the time for which it
takes the transmittance to change from 10% to 90%.
[0014] It is preferable that the time controlling unit controls the
length of the one sub-field on the basis of the temperature signal
so that the ratio of the response time to the length of the one
sub-field is constant even if the temperature changes. In this
case, it is possible to make the temporal alteration in the
transmittance constant even if the temperature changes.
[0015] Moreover, it is preferable that when a residual period is
defined as the period in which the predetermined number of the
sub-fields are removed from one frame, the light intensity
controlling unit controls the intensity of the light incident on
the pixel on the basis of the temperature signal in compensation
for the length of the residual period that has changed due to the
temperature so that the grayscale to be displayed is constant. In
this case, the length of the residual period can be compensated for
in the case where the change in transmittance is made constant by
the time controlling unit, whereby the grayscale to be displayed
can be made constant.
[0016] Moreover, if the response time becomes longer, the length of
the sub-field is controlled to be shorter. However, in this case,
the intensity of the light incident on the pixel is controlled to
be larger since the residual period becomes longer. If so, the
temperature rises, and the response time becomes shorter.
Consequently, by the carrying out of a feedback control, the length
of the sub-field and the intensity of the light incident on the
pixel will converge respectively to certain values. Therefore, it
is possible to perform a stable control.
[0017] Moreover, it is preferable that the light source controls
the intensity of the light irradiated on the basis of a control
signal, and that the light intensity controlling unit provides the
control signal generated on the basis of the temperature signal to
the light source. In this case, the intensity of the irradiated
light of the light source is directly adjusted, thereby making it
possible to simplify the configuration.
[0018] Moreover, it is preferable that the light intensity
controlling unit includes an adjusting unit that is disposed
between the light source and the pixel and that adjusts the
intensity of the light irradiated by the light source in response
to the magnitude of the control signal and output it to the pixel,
and a controlling unit that controls the magnitude of the control
signal provided to the adjusting unit on the basis of the
temperature signal. In this case, it is possible to make the amount
of light from the light source constant. Herein, the adjusting unit
may be a mechanical aperture that changes in response to the
magnitude of the control signal, or an electro-optical panel that
can control the transmittance in response to the magnitude of the
control signal.
[0019] Next, according to a second aspect of the invention, an
electronic apparatus including an electro-optical device is
provided. Such an electronic apparatus includes a projector, a
personal computer, a display and the like.
[0020] Next, according to a third aspect of the invention, there is
provided a method of driving an electro-optical device including a
pixel that has an electro-optical element of which light
transmittance changes in response to an applied voltage, and
response time, for which the light transmittance changes to a
predetermined value after the voltage is applied, changes in
response to the temperature. The method includes: including a
predetermined number of sub-fields having the same length in one
frame, and supplying either an on-voltage indicating light-on or an
off-voltage indicating light-off to the pixel in each of the
predetermined number of the sub-fields in response to the grayscale
to be displayed; controlling the length of one sub-field so that
the ratio of the response time to the length of the one sub-field
is constant in response to the temperature of the pixel; and
controlling, when a residual period is defined as a period in which
the predetermined number of the sub-fields are removed from the one
frame, the intensity of the light incident on the pixel in
compensation for the length of the residual period that changes due
to the temperature of the pixel so that the grayscale to be
displayed is constant.
[0021] According to this invention, the length of one sub-field is
adjusted and the intensity of the light incident on the pixel is
controlled even if the response time of the electro-optical element
changes in response to the temperature, thereby making it possible
to keep the display luminance constant even if the temperature
changes. Specifically, if the response time becomes longer in
response to the temperature, it is possible to make the temporal
alteration in the transmittance constant by making shorter the
length of one sub-field even if the temperature changes. The
sub-field is one unit that can control the lighting and/or the
turning off of the pixel. However, if the length of the sub-field
is changed, the length of the residual period changes during which
it is not possible to control the lighting and/or the turning off,
which can be compensated for by controlling the intensity of the
light incident on the pixel. Specifically, the length of one
sub-field is adjusted so that the temporal alteration in the
transmittance is constant even if the temperature changes. The
disadvantages that may occur due to this adjustment are resolved by
adjusting the intensity of the light incident on the pixel. By
performing a two-step adjustment in this way, it is possible to
prevent a reversion of the grayscale while displaying a number of
grayscales equal to or greater than the number of sub-fields.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] The invention will be described with reference to the
accompanying drawings, wherein like numbers reference like
elements.
[0023] FIG. 1 is a block diagram showing the entire configuration
of a liquid crystal display according to an embodiment of the
invention.
[0024] FIG. 2 shows various types of timing charts for the signals
which are output from a control circuit of the liquid crystal
display.
[0025] FIG. 3 is an electrical configuration diagram of a pixel of
the liquid crystal display.
[0026] FIG. 4 is a schematic view showing a portion of the
structure of an image display region A of the liquid crystal
display 1.
[0027] FIG. 5 is a schematic view showing the contents of a
conversion table of the liquid crystal display.
[0028] FIG. 6 is a view illustrating the response images of a
liquid crystal element with respect to the input signals (applied
voltages) to the liquid crystal element in the pixel.
[0029] FIG. 7 is a view illustrating the response images in a case
where the applied voltages change in the same manner.
[0030] FIG. 8 is a graph showing the relationship between the
change in temperature and the deviation in the grayscales of a
liquid crystal LC of the liquid crystal element.
[0031] FIG. 9 is a view showing the temperature dependence of the
response characteristics of the liquid crystal LC.
[0032] FIG. 10 is a view illustrating the response images of the
liquid crystal element with respect to the applied voltages.
[0033] FIG. 11 is a view showing the contents of adjustments of the
emission intensity of a light source in the liquid crystal
display.
[0034] FIG. 12 is a graph showing the relationship between the
adjustment of the emission intensity of the light source and the
change in temperature of a liquid crystal panel.
[0035] FIG. 13 is a view showing an optical configuration of a
projection display according to the first application of the same
embodiment.
[0036] FIG. 14 is a view showing an optical configuration of a
projection display according to the second application of the same
embodiment.
DESCRIPTION OF EXEMPLARY EMBODIMENTS
1. Embodiments
[0037] A liquid crystal display 1 according to an embodiment of the
invention includes a crystal display panel as a main portion. The
crystal display panel is constituted in such a manner that the
electrode forming surfaces of an element substrate and an opposite
substrate, on which a thin film transistor (hereinafter, referred
to as "TFT") are formed as a switching element, face each other,
and are adhered so that a uniform gap is maintained, with liquid
crystals being interposed in the gap.
[0038] FIG. 1 is a block diagram showing the entire configuration
of the liquid crystal display 1. The liquid crystal display 1
includes a scan line driving circuit 100, a data line driving
circuit 200, a control circuit 300, a temperature sensor 400, a
light source 500, a memory 600, a conversion table 700, and an
image display region A. Among these constituents, the liquid
crystal panel includes at least the image display region A. The
light source 500 is constituted as external circuitry of the liquid
crystal panel. The scan line driving circuit 100, the data line
driving circuit 200, the control circuit 300, the temperature
sensor 400, the memory 600, and a conversion table 700 may be
incorporated into the liquid crystal panel or may be constituted as
external circuitry. In this embodiment, the image display region A,
the scan line driving circuit 100, and the data line driving
circuit 200 are formed on the element substrate of the liquid
crystal panel.
[0039] On the image display region A, n scan lines 10, m data lines
20, and pixels 50 of n columns.times.m rows corresponding to the
intersection of the scan lines 10 and the data lines 20 are formed.
n and m are both natural numbers not less than 2. The light from
the light source 500 is incident on the pixels 50. As described
later, the pixels 50 have liquid crystal elements, and the liquid
crystal display 1 performs the grayscale display through light
modulation by adjusting the light transmittance of the pixels
50.
[0040] The control circuit 300 receives display data Da designating
the grayscale level (grayscale: brightness) of each pixel 50 from a
higher-level device (not shown) to write it into the memory 600,
generates X transfer initiation pulse DX and X clock signal XCK and
supplies them to the data line driving circuit 200, and generates Y
transfer initiation pulse DY and Y clock signal YCK and supplies
them to the scan line driving circuit 100.
[0041] FIG. 2 shows various types of timing charts for the signals
which are output from the control circuit 300. In this embodiment,
a portion or whole of one frame is divided into eighty sub-fields
SF1 to SF80 as shown in FIG. 2. When a portion of one frame is
divided into eighty sub-fields SF1 to SF80, the remaining period is
referred to as a "residual period". One frame represents the period
necessary to form a piece of image (for example, 16.6 ms) and is
synonymous with one frame. Moreover, the lengths of the sub-fields
SF1 to SF80 are equal to each other.
[0042] Each sub-field is divided into n horizontal scan periods
(1H). The X transfer initiation pulse DX and the Y clock signal YCK
each are at an active level at initial 1H in each sub-field, and
are at an non-active level at other periods. The X clock signal XCK
and the Y clock signal YCK are each pulse signals of which the
periods are set to 1H. In addition, this embodiment may be modified
so that the number of sub-fields included in one frame is set to
the arbitrary plural number other than 80.
[0043] The scan line driving circuit 100 performs scanning of
n.times.m pixels 50 for each sub-field. In particular, the scan
line driving circuit 100 has a shift register which sequentially
shifts the Y initiation pulse DY in accordance with the Y clock
signal YCK, and in each sub-field, generates scan signals Y1 to Yn
which sequentially and exclusively select n scan lines 10 and
supplies them to the n scan lines 10, respectively, by the shift of
Y initiation pulse DY. Accordingly, the scan signals Y1 to Yn are
at the active level during the sub-field period, and each scan line
10 is selected at 1H during which the supplied scan lines Y are at
the active level.
[0044] FIG. 3 is an electrical configuration diagram of the pixel
50, and FIG. 4 is a schematic view showing a portion of structure
of the image display region A. As shown in FIG. 3 and FIG. 4, the
pixel 50 includes a liquid crystal element 60, a selection
transistor 51 disposed between a data line 20 and the liquid
crystal element 60, and a retention capacitor 52. The selection
transistor 51 is formed on an element substrate 80, and a pixel
electrode 53 is formed thereon. On the other hand, an opposite
electrode 54 is formed all over the lower surface of an opposite
substrate 70. The pixel electrode 53 and the opposite electrode 54
are disposed so as to face each other via a predetermined gap with
liquid crystals LCs filled therebetween.
[0045] In such a configuration, when the scan signal Y supplied
through the scan line 10 is at an active level, the selection
transistor 51 is in an on-state, and a data potential DAT is
written into the pixel 50 (retention capacitor 52). And then, when
the scan signal Y is at a non-active level, the selection
transistor 51 is in an off-state, and the written data potential
DAT is retained in retention capacitor 52. The retention capacitor
52 is disposed between the pixel electrode 53 and a retention
capacitor line 30. The actual selection transistor 51 does not have
a complete off-state, but generates a constant leak current.
However, it is possible to reduce the effect of the leak current
and improve the retention characteristics of the data potential DAT
by disposing the retention capacitor 52.
[0046] In this embodiment, the liquid crystal 60 is set to a
normally white mode. Therefore, the light transmittance of the
liquid crystal element 60 becomes lower as the effective value of
the difference voltage between the pixel electrode 53 and the
opposite electrode 54 becomes larger, and the grayscale of the
pixel 50 becomes nearly a black color in a state where no voltage
is applied. However, in this embodiment, only either an on-voltage
(for example, 5V) making the difference voltage be equal to or more
than the saturated voltage, or an off-voltage (for example, 0V)
making it be equal to or less than the threshold voltage, is
applied to the pixel electrode 53. Further, it is possible to
obtain the same configuration as this embodiment even in the case
when using liquid crystal elements set to a normally black
mode.
[0047] When the light transmittance in the darkest condition is set
to 0% of the relative light transmittance, and the light
transmittance in the brightest condition is set to 100% of the
relative light transmittance in the normally white mode, among the
voltages applied to the liquid crystal element 60, the voltage
where the relative light transmittance is 10% is referred to as the
optical threshold voltage, and the voltage where the relative light
transmittance is 90% is referred to as the optical saturated
voltage. A voltage modulation method (analog driving), when making
the liquid crystal element 60 become halftone (gray), is designed
so that a voltage equal to or less than the optical saturated
voltage is applied to the liquid crystals LCs. Therefore, the light
transmittance of the liquid crystals LCs has a value nearly
proportional to the applied voltage of the liquid crystals LCs.
[0048] In regard to this, the grayscale display is performed in
this embodiment by using only two voltages applied to the liquid
crystal element 60, the on-voltage and off-voltage. In particular,
this embodiment is constituted so that a portion or the whole of
one frame is divided into eighty sub-fields, a configuration where
the on- or off-voltage is applied to the liquid crystal element 60
is adopted in each sub-field, and the grayscale display is
performed by controlling the temporal placement between the
sub-field to which the on-voltage is applied and the sub-field to
which the off-voltage is applied with respect to each liquid
crystal element 60.
[0049] While a retention capacitor line potential VCOM is supplied
from a power supply circuit (not shown) to the retention capacitor
line 30, an opposite electrode potential LCCOM is supplied from the
power supply circuit to the opposite electrode 54. Although the
retention capacitor line potential VCOM and the opposite electrode
potential LCCOM do not necessarily have to be identical to each
other, both potentials are made to be identical for the purpose of
simplifying the configuration of the power supply circuit in this
embodiment.
[0050] The memory 600 has storage regions corresponding to the
n.times.m pixels 50, and each of storage regions stores display
data Da related to the corresponding pixels 50. Display data Da are
for designating the brightness (grayscale level) of the pixel 50,
and are designated by q stages from 0 to q-1 in this embodiment. q
is a natural number not less than 2. Herein, the grayscale level of
"0" designates black at the lowest grayscale, and the brightness
slowly increases as the grayscale level is higher so that the
grayscale level of "q-1" designates white at the highest
grayscale.
[0051] When the scan line 10 is selected by the scan line driving
circuit 100, the control circuit 300 reads display data Da of m
pixels 50 corresponding to the selected scan line 10 from the
memory 600 and supplies it to the conversion table 700. Further,
the control circuit 300 has a ring counter representing the present
sub-field. Counter values of the ring counter increase by 1 at the
point where the Y initiation pulse DY is at an active level, and
are set to return to 1 after reaching 80.
[0052] FIG. 5 is a schematic view showing the contents of the
conversion table 700. As showed in FIG. 5, the conversion table 700
designates the voltage (on-voltage or off-voltage) applied to the
liquid crystal element 60 in the sub-field so as to achieve the
grayscale level (brightness) for each q grayscale level capable of
being designated by display data Da and for each sub-field.
Further, the conversion table 700 converts display data Da which
are read from the memory 600 into data Db designating the voltage
corresponding to the grayscale level designated by the display data
Da and the present sub-field.
[0053] FIG. 5 is a view showing the case of q>4. The on-voltage
is represented as "1", and the off-voltage is represented as "0" in
this figure. The conversion table 700 designates that the
on-voltage is applied at the sub-field SF1 and SF2 and the
off-voltage is applied at the sub-field SF3 to SF80, for example,
with respect to the liquid crystal element 60 in the pixel 50 in
which the grayscale level is set to "0". Further, permutations of
the voltages corresponding to the sub-field SF1 to SF80 are
different for each grayscale level.
[0054] The data line driving circuit 200 of FIG. 1 converts the
converted m data Db respectively into the m data potentials DAT1 to
DATm using the conversion table 700, and supplies the converted
data potentials DAT1 to DATm respectively to m data lines 20 for
each 1H in accordance with the X transfer initiation pulse DX and
the X clock signal XCK. As shown in FIG. 3, when the opposite
electrode potential LCCOM is applied to the opposite electrode 54,
and then the data potential DAT is applied to the pixel electrode
53, the difference voltage between the data potential DAT and the
opposite electrode potential LCCOM is applied to the liquid crystal
element 60. The data potential DAT is a potential in response to
the corresponding data Db, and is determined so that the difference
voltage and the voltage (on-voltage or off-voltage) designated by
the data Db are identical to each other.
[0055] As is obvious from the above-mentioned description, the
control circuit 300 supplies the Y transfer initiation pulse DY and
the Y clock signal YCK to the scan line driving circuit 100, the
scan line driving circuit 100 generates the scan signals Y1 to Yn
in accordance to these signals, and supplies them to the
corresponding scan lines 10. By doing this, the n scan lines 10 are
sequentially and exclusively selected in 1H unit.
[0056] Further, the control circuit 300, prior to selecting the
first line of the scan lines 10, reads the display data Da of m
pixels 50 corresponding to the scan lines 10 from the memory 600
and supplies them to the conversion table 700. The conversion table
700 converts each of the m display data Da readings into data Db
corresponding to the grayscale level designated by the display data
Da and the first sub-field. The data line driving circuit 200
stores up the m (1 line) data Db converted by the conversion table
700, and then when the scan signal Y1 is at an active level, it
supplies the data potentials DAT1 to DATm in response to the
voltage (on-voltage or off-voltage) designated by the m data Db
respectively to the m data lines 20. In this way, the data
potentials DAT1 to DATm are written into the first m pixels 50,
each of the light transmittances of these pixels 50 depend on the
data potentials DATs written into the pixels 50, and the grayscales
of the pixels 50 ideally have the grayscales designated by the
display data Da. Such a process of one line is repeatedly performed
in 1H units.
[0057] When finishing the process of one line with respect to the
last line, the sub-field SF1 is finished and the sub-field SF2 is
started. The same process as the above is performed in the
sub-field SF2 as well. Such a process is repeatedly performed until
the sub-field SF80 is finished.
[0058] FIG. 6 is a view illustrating the response images of the
liquid crystal element 60 with respect to the input signals
(applied voltages) to the liquid crystal element 60. This
illustration represents the case where the whole of one frame is
divided into eighty sub-fields SF1 to SF80, and the length (T1) of
the sub-field in this illustration is 16.6/80 ms. The upper side of
the drawing shows the response image in the case where 128 is
designated as the grayscale level, and the lower side of the
drawing shows the response image in the case where 129, which is
brighter than 128, is designated as the grayscale level. Further,
this illustration assumes that q=256.
[0059] At the upper side of the drawing, since the voltage of the
input signal is maintained at 0V in the period of the sub-fields
SF1 to SF2, the light transmittance of the liquid crystal LC
reaches 100%, and the grayscale of the pixel 50 is equivalent to
the highest grayscale as shown by the image. Since the voltage of
the input signal is maintained at 5V in the period of the
sub-fields SF3 to SF49, the light transmittance of the liquid
crystal LC is maintained by lowering it from 100% to 0%, and the
grayscale of the pixel 50 is maintained by lowering it from the
highest grayscale to the lowest grayscale. Since the voltage of the
input signal is maintained at 0V in the period of the sub-fields
SF46 to SF48, the light transmittance of the liquid crystal LC
increases from 0%, and the grayscale of the pixel 50 increases from
the lowest grayscale. Since the voltage of the input signal is
maintained at 5V in the period of the sub-field SF49, the light
transmittance of the liquid crystal LC falls, and the grayscale of
the pixel 50 falls. Since the voltage of the input signal is
maintained at 0V in the period of the sub-fields SF50 to SF80, the
light transmittance of the liquid crystal LC is maintained by
increasing to 100%, and the grayscale of the pixel 50 is maintained
by increasing to the highest grayscale. As a result, the integrated
value (mean value) of the light transmittance over one frame is
21.9%.
[0060] At the lower side of the drawing, since the voltage of the
input signal is maintained at 0V in the period of the sub-fields
SF1 to SF2, the light transmittance of the liquid crystal LC
reaches 100%, and the grayscale of the pixel 50 is equivalent to
the highest grayscale. Since the voltage of the input signal is
maintained at 5V in the period of the sub-fields SF3 to SF42, the
light transmittance of the liquid crystal LC is maintained by
falling from 100% to 0%, and the grayscale of the pixel 50 is
maintained by falling from the highest grayscale to the lowest
grayscale. Since the voltage of the input signal is maintained at
0V in the period of the sub-fields SF43 to SF51, the light
transmittance of the liquid crystal LC increases from 0%, and the
grayscale of the pixel 50 increases from the lowest grayscale.
Since the voltage of the input signal is maintained at 5V in the
period of the sub-field SF52, the light transmittance of the liquid
crystal LC falls, and the grayscale of the pixel 50 falls. Since
the voltage of the input signal is maintained at 0V in the period
of the sub-fields SF53 to SF80, the light transmittance of the
liquid crystal LC is maintained by increasing to 100%, and the
grayscale of the pixel 50 is maintained by increasing to the
highest grayscale. As a result, the integrated value (mean value)
of the light transmittance over one frame is 22.3%, the grayscale
(129) of the pixel 50 over one frame is higher than the grayscale
(128) in the case of the upper side of the drawing.
[0061] In this embodiment, the grayscale display is performed in
this way. However, in practice, the grayscale to be displayed is
different in response to the temperature of the liquid crystal LC
as shown in FIG. 7. At the upper side and the lower side of the
drawing, the identical grayscale levels are designated with respect
to the identical pixels 50, and response images are shown for the
case where the voltages of the input signals change in the same
manner. The response image at the upper side corresponds to the
case where the temperature of the liquid crystal LC is 40.degree.
C., and the response image at the lower side corresponds to the
case where the temperature of the liquid crystal LC is 50.degree.
C. As is obvious from comparing to both sides, in spite that the
identical grayscale level is designated, the identical pixel 50
becomes dark in the case of the upper side, and becomes bright in
the case of the lower side. This difference is attributed to the
change in viscosity, that is, the response characteristics of the
liquid crystal LC in accordance to the temperature.
[0062] FIG. 8 is a graph showing the relationship between the
change in temperature and the deviation in grayscale of the liquid
crystal LC. This graph is obtained using the display data Da which
make y equal to 2.2 when the temperature of the liquid crystal LC
is 40.degree. C. In this regard, the grayscale level is 256
(q=256). It has been found that, from this graph, the deviation in
grayscale becomes large as the temperature of the liquid crystal LC
rises, and that a reversion of the grayscale occurs due to the
change in temperature of the liquid crystal LC. This reversion of
the grayscale has particularly occurred in large numbers when the
grayscale is low. One of the aims of this embodiment is to prevent
the grayscale from reversing.
[0063] FIG. 9 is a view showing temperature dependence of the
response characteristics of the liquid crystal LC. As shown in FIG.
9, even when the timing (on-off boundary) of the change from the
on-voltage to the off-voltage is in accord with each other within
one frame when the voltage applied to the liquid crystal element 60
including the liquid crystal LC with the light transmittance of 0%,
the time (on-off time) from the on-off boundary to the point in
time when the light transmittance reaches a value of 90% due to the
temperature of the liquid crystal LC varies to a great extent
depending on the temperature of the liquid crystal LC.
Consequently, in this embodiment, the reversion of the grayscale is
prevented by making the ratio of on-off time to the temperatures
equal to the ratio of the length of the sub-field between the
temperatures.
[0064] The process of making both of the ratios equal to each other
is performed by the control circuit 300 and the temperature sensor
400. The temperature sensor 400 is disposed within or in proximity
to the liquid crystal panel, and measures the temperature of the
liquid crystal panel. The control circuit 300 adjusts the length of
the sub-field on the basis of the temperature measured by the
temperature sensor 400. The control circuit 300 generates the X
transfer initiation pulse DX, the X clock signal XCK, the Y
transfer initiation pulse DY, and Y clock signal YCK in accordance
to the length after adjusting, and reads display data Da from the
memory 600 at the timing in accordance to the length after
adjusting. Of course, in adjusting the length of the sub-field, the
total length of all of the lengths of the eighty sub-fields does
not exceed one frame.
[0065] FIG. 10 is a view illustrating the response images of the
liquid crystal element 60 with respect to the input signals
(applied voltages) to the liquid crystal element 60. In this
illustration, a portion of one frame is divided into eighty fields
by adjusting the length of the sub-field, and the remaining period
is referred to as a residual period. The response image at the
upper side of the drawing corresponds to the case where the
temperature of the liquid crystal panel is 40.degree. C., and the
response image at the lower side corresponds to the case where the
temperature of the liquid crystal panel is 50.degree. C. As shown
in FIG. 10, when the length of the sub-field of the upper side is
T2, and the length of the sub-field of the lower side is T3, the
relation is T2>T3. Since the ratio of T2 and T3 is equal to the
ratio of on-off time at 40.degree. C. and on-off time at 50.degree.
C., it is possible to equally set the temporal placements, in
eighty sub-fields, of the sub-fields where the on-voltage is
applied and the sub-fields where the off-voltage is applied to
liquid crystal element 60 at the upper side and the lower side,
that is, to display the grayscales by using the same transient
response at the upper side and the lower side. Therefore, according
to this embodiment, it is possible to maintain the relationship
(rate of change in brightness between the grayscales) between the
grayscales. In the end, according to this embodiment, it is
possible to prevent the grayscale from reversing.
[0066] However, in this embodiment, the brightness of the pixel 50
increases due to the residual period. As shown in FIG. 10, the
residual period becomes longer at 60.degree. C. and becomes shorter
at 40.degree. C., as compared to the case where the temperature of
the liquid crystal LC is 50.degree. C. Consequently, in this
embodiment, the emission intensity of the light source 500 is
adjusted in response to the temperature of the liquid crystal LC,
so that the grayscale change in the pixel 50 due to the adjusting
of the length of the sub-field is suppressed.
[0067] A process of adjusting the emission intensity of the light
source 500 in response to the temperature of the liquid crystal LC
is performed by the control circuit 300, the temperature sensor 400
and the light source 500. The light source 500 is a light source
capable of adjusting the emission intensity, and the control
circuit 300 adjusts the emission intensity of the light source 500
on the basis of the temperature measured by the temperature sensor
400. In such an adjustment, when the temperature measured by the
temperature sensor 400 is lower than 50.degree. C., the control
circuit 300 raises the emission intensity of the light source 500
by an amount corresponding to the difference between this
temperature and 50.degree. C., as shown in FIG. 11. When the
temperature is higher than 50.degree. C., the control circuit 300
lowers the emission intensity of the light source 500 by an amount
corresponding to the difference between this temperature and
50.degree. C. By doing this, the grayscale change in the pixel 50
due to the adjusting of the length of the sub-field is
suppressed.
[0068] FIG. 12 is a graph showing the relationship between the
temperature of the liquid crystal panel and the emission intensity
of the light source. In this embodiment, since the emission
intensity of the light source 500 is controlled as described above.
For example, if the temperature of the liquid crystal panel rises
(.alpha.), the emission intensity of the light source 500 falls
(.beta.) by controlling the control circuit 300, as shown in FIG.
12. Further, if the temperature of the liquid crystal panel falls
(.alpha.'), the emission intensity of the light source 500 rises
(.beta.') due to the controlling by the control circuit 300.
Therefore, according to this embodiment, it is possible to make the
temperature of the liquid crystal panel converge into a stable
temperature which is balanced between the temperature of
surroundings where the liquid crystal panel is placed and the
ability to cool the liquid crystal panel.
[0069] According to this embodiment as described above, it is
possible to make the temporal alteration in the transmittance
constant by adjusting the length of one sub-field even if the
response time of the liquid crystal LC changes in response to the
temperature. The disadvantages that may occur due to this
adjustment are resolved by adjusting the intensity of the light
incident on the pixel 50. By performing a two-step adjustment in
this way, it is possible to prevent a reversion of the grayscale
while displaying a number of grayscales equal to or greater than
the number of sub-fields.
2. Applications and Modifications
[0070] The invention is not limited to the embodiments described
above, but includes various types of applications and modifications
in the scope thereof. The applications and modifications included
in the invention will be illustrated below.
[0071] FIG. 13 is a view showing an optical configuration of a
projection display 2 according to the first application of this
embodiment. As shown in FIG. 13, the projection display 2 has three
liquid crystal panels 61 (61r, 61g, 61b) that correspond to each of
the colors of red (R), green (G) and blue (B), an illumination
optics 70 that emits light, an optical isolation optics 80 that
isolates the emitted light from this illumination optics 70 into
each of the color lights of red, green and blue and leads each of
them to the liquid crystal panels 61, a dichroic prism 91 that
synthesizes the emitted light from each of the liquid crystal
panels 61, and a projection lens 92 that projects the emitted light
from the dichroic prism 91.
[0072] The illumination optics 70 has a light source 71 (for
example, a halogen lamp) that emits white light. The emitted light
from the light source 71 is converted into a plurality of beams of
light by being transmitted into a first lens array 72 and a second
lens array 73, each of which are formed of a plurality of lens
arranged in a plane, and then reaches a polarization generator 74.
This polarization generator 74 is a plate-like member that converts
the incident light into any one of s polarization and p
polarization and emits it. The emitted light from the polarization
generator 74 (polarized light) is transmitted into a lens 75 and is
incident on the optical isolation optics 80.
[0073] This optical isolation optics 80 is a unit that isolates the
emitted light from the illumination optics 70 into red light, green
light and blue light. A dichroic mirror 81 reflects the red light
and transmits the green light and the blue light. The red light
reflected by this dichroic mirror 81 is reflected by a reflector
82, and then transmitted into a field lens 83, and is incident on
the liquid crystal panel 61r. On the other hand, a dichroic mirror
84 reflects the green light among the light transmitted into the
dichroic mirror 81, and transmits the blue light. The green light
reflected by this dichroic mirror 84 is transmitted into a field
lens 85, and is incident on the liquid crystal panel 61g. Moreover,
the blue light transmitted into a dichroic mirror 84 is incident on
the liquid crystal panel 61b via a relay lens system 86.
Specifically, this blue light is transmitted into a lens 861 on the
incidence side, reflected by a reflector 862, and then transmitted
into a relay lens 863. And furthermore, this blue light is
reflected by a reflector 864, and then transmitted into a lens 865
on the emission side and is incident on the liquid crystal panel
61b.
[0074] The projection display 2 has the same constitution as that
of the liquid crystal display 1 of FIG. 1. The light source 71
corresponds to the light source 500, and each of the liquid crystal
panels 61 (61r, 61g, 61b) corresponds to the liquid crystal panel
in the liquid crystal display 1. Adjustment of the emission
intensity of the light source 71 is performed by adjusting the
luminance of the light source 71. The emitted light from the three
liquid crystal panels 61 is synthesized by a dichroic prism 91, and
then is incident on a projection lens 92. The projection lens 92
projects the emitted light from the dichroic prism 91 onto a
tabular screen R. In other words, the projection display 2 is one
kind of the liquid crystal display performing the grayscale display
through light modulation by adjusting the light transmittance of
the pixels in each of the liquid crystal panels 61 (61r, 61g,
61b).
[0075] FIG. 14 is a view showing an optical configuration of a
projection display 3 according to the second application of this
embodiment. The projection display 3 is obtained by transforming
the projection display 2. The difference of the projection display
3 to the projection display 2 is that it includes a liquid crystal
panel 62 between the lens 75 of the illumination optics 70 and the
dichroic mirror 81 in the optical isolation optics 80. The light
emitted from the light source 71 and transmitted into the light
lens 75 is incident on the liquid crystal panel 62, and the light
transmitted into the liquid crystal panel 62 is incident on the
dichroic mirror 81.
[0076] In the projection display 3, the control circuit
corresponding to the control circuit 300 achieves the same effects
as those obtained through the adjustment of the emission intensity
of the light source by adjusting not the luminance of the light
source 71, but the light transmittance of the liquid crystal panel
62. In addition, a configuration is also possible wherein the
projection display 3 is transformed, an iris (aperture) is adopted
instead of the liquid crystal panel 62, and the aperture ratio of
the iris is adjusted. As is obvious from the above explanation, in
the invention, not only the unit that adjusts the emission
intensity of the light source, but also any unit that adjusts the
intensity (amount of light per unit time) of the light (irradiated
light) incident on the liquid crystal panel from the light source
may be adopted as the unit that suppresses the grayscale change in
the pixel by adjusting the length of the sub-field.
[0077] The applications of the embodiment described above are not
limited to the projection display 2. For example, a liquid crystal
display (for example, HMD (head-mounted display)) provided with
backlight as the light source 500 is also one kind of the liquid
crystal display performing the grayscale display through light
modulation by adjusting the light transmittance of the pixel of the
liquid crystal panel, and may be an application of the embodiment
described above. EVF (electronic viewfinder) may be exemplified as
another application example. Furthermore, it is also possible to be
applied to a personal computer, a handheld terminal, or a cellular
phone and the like.
[0078] A configuration is also possible wherein the embodiment
described above is transformed, and a liquid crystal panel having a
reflection mode is used in the grayscale display. In this case, the
unit that adjusts the intensity of the light incident on the liquid
crystal panel may be omitted. Moreover, a configuration is also
possible wherein the embodiment described above, the application or
the modification are transformed, a measuring unit that measures
the intensity of the light incident on the liquid crystal panel is
provided, and the intensity of the light incident on the liquid
crystal panel is adjusted on the basis of not only the temperature
of the liquid crystal panel, but also the intensity measured by
this measuring unit. Moreover, the light source may include a
plurality of light sources. In this case, a configuration is also
possible wherein the adjustment of the emission intensity is not
performed with respect to at least one light source among a
plurality of the light sources. Moreover, the temperature sensor
may include a plurality of temperature sensors. In this case, a
configuration is also possible wherein a control circuit adjusts
the intensity of the light incident on the liquid crystal panel
from the light source on the basis of a statistical value of the
temperature measured by a plurality of the temperature sensors.
* * * * *