U.S. patent application number 10/468274 was filed with the patent office on 2004-10-14 for liquid-crystal luminous quantity adjuster, method for driving liquid crystal luminous quantity adjuster, and camera comprising liquid crystal luminous quantity adjuster.
Invention is credited to Takaoka, Toshifumi.
Application Number | 20040201559 10/468274 |
Document ID | / |
Family ID | 19187719 |
Filed Date | 2004-10-14 |
United States Patent
Application |
20040201559 |
Kind Code |
A1 |
Takaoka, Toshifumi |
October 14, 2004 |
Liquid-crystal luminous quantity adjuster, method for driving
liquid crystal luminous quantity adjuster, and camera comprising
liquid crystal luminous quantity adjuster
Abstract
A liquid-crystal light-amount adjusting device in which the
effective power of driving pulses for acceleration is optimized to
improve response characteristics of optical transmittance, a method
of driving the apparatus, and a camera including the apparatus. The
liquid-crystal light-amount adjusting device according to the
present invention includes a driving-pulse generating circuit (16)
for generating driving pulses having pulsed voltages, a
liquid-crystal light-amount adjusting unit (12) for changing the
optical transmittance of a liquid crystal according to pulse shapes
of the driving pulses, and a microcomputer (19) for controlling
frequencies and duty ratios of the driving pulses. The
microcomputer (19) exercises control so that the driving-pulse
generating circuit (16) generates driving pulses including driving
pulses for acceleration, having a pulse shape with a first
frequency and a first duty ratio, and driving pulses for dimming,
having a pulse shape with a second frequency that is different from
the first frequency and a second duty ratio that is different from
the first duty ratio.
Inventors: |
Takaoka, Toshifumi; (Tokyo,
JP) |
Correspondence
Address: |
William S Frommer
Frommer Lawrence & Haug
745 Fifth Avenue
New York
NY
10151
US
|
Family ID: |
19187719 |
Appl. No.: |
10/468274 |
Filed: |
August 18, 2003 |
PCT Filed: |
November 15, 2002 |
PCT NO: |
PCT/JP02/11965 |
Current U.S.
Class: |
345/87 ;
348/E5.028; 348/E5.04 |
Current CPC
Class: |
H04N 5/232 20130101;
G02F 1/13306 20130101; G02F 2203/60 20130101; H04N 5/238
20130101 |
Class at
Publication: |
345/087 |
International
Class: |
G09G 003/36 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 18, 2001 |
JP |
2001-384385 |
Claims
1. A liquid-crystal light-amount adjusting device comprising: a
driving-pulse generating circuit for generating driving pulses
having pulsed voltages; a liquid-crystal light-amount adjusting
unit, to which the driving pulses are supplied, for changing an
optical transmittance of a liquid crystal according to pulse shapes
of the driving pulses; and a control circuit for controlling
frequencies and duty ratios of the driving pulses; wherein the
control circuit exercises control so that the driving-pulse
generating circuit generates driving pulses for acceleration,
having a pulse shape with a first frequency and a first duty ratio,
and driving pulses for dimming, having a pulse shape with a second
frequency that is different from the first frequency and a second
duty ratio that is different from the first duty ratio.
2. A liquid-crystal light-amount adjusting device according to
claim 1, further comprising a temperature detecting circuit for
detecting an ambient temperature of the liquid-crystal light-amount
adjusting unit and outputting a result of detection to the
controller, wherein the control circuit changes the frequencies of
the driving pulses in accordance with the ambient temperature.
3. A liquid-crystal light-amount adjusting device according to
claim 1, further comprising a storage circuit for storing an
association table defining association of an optical transmittance
with driving pulses for acceleration and driving pulses for dimming
for achieving the optical transmittance, wherein the control
circuit exercises control so that the driving-pulse generating
circuit, by referring to the association table, generates driving
pulses with which a desired optical transmittance is achieved.
4. A method of driving a liquid-crystal light-amount adjusting
device that changes an optical transmittance thereof in accordance
with pulse shapes of driving pulses supplied to a liquid crystal,
wherein, before driving pulses having a pulse shape in accordance
with a desired optical transmittance is supplied to the liquid
crystal, driving pulses having a frequency that is different from a
frequency of the driving pulses having the pulse shape in
accordance with the desired optical transmittance is supplied to
the liquid crystal.
5. A camera comprising: an optical system for focusing light coming
from a target on an imaging surface; a liquid-crystal light-amount
adjusting unit for adjusting the amount of the light that reaches
the imaging surface, by changing an optical transmittance of a
liquid crystal; a driving-pulse generating circuit for generating
driving pulses having pulsed voltages, the driving pulses being
supplied to the liquid crystal to change the optical transmittance;
and a control circuit for controlling frequencies and duty ratios
of the driving pulses; wherein the control circuit exercises
control so that the driving-pulse generating circuit generates
driving pulses for acceleration, having a first frequency and a
first duty ratio, and driving pulses for dimming, having a second
frequency that is different from the first frequency and a second
duty ratio that is different from the first duty ratio.
6. A camera comprising: an optical system for focusing light coming
from a target on an imaging surface; a liquid-crystal light-amount
adjusting unit for adjusting the amount of the light that reaches
the imaging surface, by changing an optical transmittance of a
liquid crystal; a driving-pulse generating circuit for generating
driving pulses having pulsed voltages, the driving pulses being
supplied to the liquid crystal to change the optical transmittance;
and a control circuit for controlling a frequency and a duty ratio
of the driving pulses; wherein the control circuit changes the
frequency of the driving pulses during-imaging for one screen.
7. A camera according to claim 5, wherein an imaging time for one
screen is a sum of a time during which the driving pulses for
acceleration are supplied and a time during which the driving
pulses for dimming are supplied.
Description
TECHNICAL FIELD
[0001] The present invention relates to a liquid-crystal
light-amount adjusting device that controls the amount of
transmitting light, and to an apparatus for driving the
liquid-crystal light-amount adjusting device. The present invention
also relates to a camera in which the liquid-crystal light-amount
adjusting device is used as a diaphragm.
BACKGROUND ART
[0002] In cameras such as video cameras with built-in cameras,
electronic still cameras, and still cameras, diaphragms are
provided so that aperture values can be changed.
[0003] FIG. 8 is a schematic diagram showing an optical system of a
conventional camera.
[0004] Referring to FIG. 8, light coming from a point 101 of a
target becomes parallel beams 111 through a lens 102, and the
parallel beams are partially blocked by a dual-blade diaphragm 103
and remaining beams 112 are transmitted, forming an image on an
imaging surface of an imaging device 105 through a lens 104. The
amount of light that transmits through the two-blade diaphragm 103
is adjusted by changing the area of a diaphragm aperture 107 by
moving the blades of the two-blade diaphragm 103 by a driving
apparatus 106.
[0005] The method described above, in which the amount of light
that reaches the imaging device 105 from the target is adjusted by
partially blocking light coming from the target by the diaphragm,
causes diffraction at the edges of the blades, allowing diffracted
light 113 to also reach the imaging device 105. The diffracted
light 113 reaches a point that is different from the focus of the
main light 112, causing degradation in resolution. The degree of
degradation in resolution due to the diffraction is determined by
the ratio between the amount of the light 112 that transmits
through the diaphragm aperture 107 and the amount of the diffracted
light 113 that diffracts at the edges of the diaphragm aperture
107, and it therefore becomes larger as the size of the camera is
reduced and the size of the diaphragm is reduced accordingly. Thus,
this problem is significant considering the recent miniaturization
of cameras.
[0006] In order to overcome the problem, as an alternative to a
blade diaphragm, a liquid-crystal light-amount adjusting device
that employs liquid crystal has been proposed. The liquid-crystal
light-amount adjusting device is disposed in place of a blade
diaphragm. Since the optical transmittance is substantially uniform
over the entire surface on which light is incident, and the optical
transmittance can be controlled continuously, the liquid-crystal
light-amount adjusting device allows adjustment of the amount of
light without causing degradation in resolution due to diffraction.
That is, the aperture value can be adjusted.
[0007] In the liquid-crystal light-amount adjusting device, in
order to achieve quick response in changing the optical
transmittance, a voltage that is higher (or lower) than a target
voltage for achieving a specified optical transmittance (a
driving,voltage for acceleration) is applied to a liquid crystal
for a short period. By supplying a certain electric power by the
driving voltage for acceleration, a large driving force is exerted
on the liquid-crystal molecules in the process of changing the
orientation of the liquid crystal.
[0008] When the driving voltage for acceleration is applied by
pulses, the pulses must be applied for an optimal time in order to
achieve optimal response characteristics. For that purpose, the
resolution increases as the frequency of the pulses becomes higher,
allowing delicate control of the time of application.
[0009] However, since the liquid-crystal light-amount adjusting
device is electrically a capacitor having the liquid crystal held
between electrodes, a large amount of power is consumed at pulse
edges if the frequency is high. In order to reduce power
consumption, it is possible to reduce the current by disposing a
resistor in series with the liquid crystal held between the
electrodes. However, according to this method, a low-pass filter is
formed by the liquid crystal and the capacitor, causing degradation
in the response characteristics of the liquid crystal. Thus, from
the viewpoint of reducing power consumption, the frequency of the
pulses is preferably as low as possible.
[0010] As described above, the frequency of the pulses of the
driving voltage for acceleration must satisfy contradicting
requirements.
[0011] Accordingly, it is an object of the present invention to
provide a liquid-crystal light-amount adjusting device and a
liquid-crystal light-amount adjusting method that allow delicate
control of a driving power for acceleration.
[0012] It is another object of the present invention to provide a
camera in which the liquid-crystal light-amount adjusting device is
used as a diaphragm to avoid degradation in resolution due to
diffraction.
DISCLOSURE OF INVENTION
[0013] According to the present invention, the frequency of driving
pulses for acceleration and the frequency of driving pulses for
dimming are allowed to be different. Thus, the effective power of
the driving pulses for acceleration can be optimized in accordance
with driving status of a liquid-crystal light-amount adjusting
device. Accordingly, a liquid-crystal light-amount adjusting device
that exhibits favorable response characteristics can be
implemented. Furthermore, a liquid-crystal light-amount adjusting
device with low power consumption can be implemented. Furthermore,
a liquid-crystal light-amount adjusting device with an extended
lifetime can be implemented since DC components can be
suppressed.
BRIEF DESCRIPTION 0F THE DRAWINGS
[0014] FIG. 1 is a diagram showing the construction of a camera
according to an embodiment of the present invention.
[0015] FIG. 2 is a diagram showing the construction of a
liquid-crystal light-amount adjusting unit.
[0016] FIGS. 3A to 3C are diagrams showing an example of driving
pulses and an example of optical transmittance.
[0017] FIG. 4 is a diagram showing a driving-pulse generating
circuit.
[0018] FIGS. 5A and 5B are (first) diagrams for explaining the
operation of the driving-pulse generating circuit.
[0019] FIG. 6 is a (second) diagram for explaining the operation of
the driving-pulse generating circuit.
[0020] FIG. 7 is a diagram for explaining the operation of the
liquid-crystal light-amount adjusting unit.
[0021] FIG. 8 is a schematic diagram showing an optical system of a
conventional camera.
BEST MODE FOR CARRYING OUT THE INVENTION
[0022] Now, an embodiment of the present invention will be
described with reference to the drawings. The same parts are
denoted by the same numerals throughout the drawings, and
descriptions thereof will be omitted.
[0023] FIG. 1 is a diagram showing the construction of a camera
according to an embodiment of the present invention.
[0024] Referring to FIG. 1, the camera according to this embodiment
includes lenses 11 and 13, a liquid-crystal light-amount adjusting
unit 12, an imaging device 14, a driving circuit 15, a signal
processing circuit 18, a microcomputer 19, a storage circuit 20,
and a temperature detecting circuit 21.
[0025] Light coming from a target (not shown) is adjusted by the
lens 11 to become substantially parallel beams, and the parallel
beams enter the liquid-crystal light-amount adjusting unit 12.
[0026] The incident light is adjusted by the liquid-crystal
light-amount adjusting unit 12 so that an adjusted amount of light
will be transmitted, and the resulting light enters the lens 13.
Since the liquid-crystal light-amount adjusting unit 12 alters
optical transmittance substantially uniformly over the entire
surface on which the light is incident, the light incident on the
liquid-crystal light-amount adjusting unit 12 is dimmed
substantially uniformly when it comes out. That is, the light that
comes out of the lens 11 transmits the liquid-crystal light-amount
adjusting unit 12 by a specified optical transmittance with no
change between an area of the incident surface and an area of the
emitting surface of the liquid-crystal light-amount adjusting unit
12.
[0027] FIG. 2 is a diagram showing the construction of the
liquid-crystal light-amount adjusting unit.
[0028] Referring to FIG. 2, the liquid-crystal light-amount
adjusting unit 12 includes substrates 31a and 31b, electrodes 32a
and 32b, oriented films 33a and 33b, and a liquid crystal 34.
[0029] The electrode 32a is formed on one surface of the substrate
31a, and the oriented film 33a is formed on a surface of the
electrode 32a. Similarly, the electrode 32b is formed on one
surface of the substrate 31b, and the oriented film 33b is formed
on a surface of the electrode 32b. These arrangements are disposed
substantially in parallel with each other at a predetermined
distance, with the oriented films 33a and 33b being opposed to each
other, and the gap between the oriented films 33a and 33b is filled
with the liquid crystal 34.
[0030] The substrates 31a and 31b are formed of transparent parts,
such as glass films or plastic films. The electrodes 32a and 32b
are transparent electrodes formed of, for example, ITO (indium tin
oxide). The oriented films 33a and 33b are formed of a material
selected in accordance with a material of the liquid crystal 34. In
this embodiment, the liquid crystal 34 is a two-color guest-host
liquid crystal.
[0031] When a potential difference does not exist between the
electrodes 32a and 32b, the orientation of the liquid-crystal
molecules is determined by intermolecular forces between the
liquid-crystal molecules and the oriented films 33a and 33b and
between the liquid-crystal molecules themselves. When a potential
difference exists between the electrodes 32a and 32b, the
liquid-crystal molecules are polarized, causing a Coulomb force
with an electric field between the electrodes 32a and 32b. Thus,
when a potential difference exists, the orientation of the
liquid-crystal molecules is determined by the balance between the
Coulomb force and the intermolecular forces mentioned above. Thus,
the orientation of the liquid-crystal molecules can be changed by
changing the potential difference between the electrodes 32a and
32b, and the optical transmittance of the liquid-crystal
light-amount adjusting unit 12 can be changed accordingly. Since
the electrodes 32a and 32b are disposed substantially in parallel
to each other, the electric field generated therebetween is
substantially uniform in a plane that is parallel to the electrodes
32a and 32b. Thus, the optical transmittance of the liquid-crystal
light-amount adjusting unit 12 becomes substantially uniform.
[0032] In the two-colored guest-host liquid crystal, dye molecules
that determine optical characteristics (optical transmittance in
this embodiment) have shapes like thin rods, and move in
association with the liquid-crystal molecules. This movement occurs
so that the potential energy associated with intermolecular forces
with the liquid-crystal molecules becomes smaller. That is, the dye
molecules move so as to become parallel with the liquid-crystal
molecules.
[0033] More specifically, the orientation of the liquid-crystal
molecules changes in accordance with the potential difference
between the electrodes 32a and 32b, and the direction of absorption
axis of the dye molecules changes in accordance with the change in
the orientation of the liquid-crystal molecules, whereby the
optical transmittance of the two-color guest-host liquid-crystal
light-amount adjusting unit 12 changes.
[0034] Since the liquid-crystal molecules used in this embodiment
are not spontaneously polarized and are polarized by an external
electric field, the Coulomb potential is minimized when a
polarization-facilitating axis of the liquid-crystal molecules
coincides with the direction of the external electric field. Thus,
regardless of the direction of the electric field, the orientation
of the liquid-crystal molecules is determined by the intensity of
the electric field alone. Thus, the effective power supplied from
the electrodes 31a and 31b to the liquid crystal 34 is a function
of the optical transmittance of the liquid-crystal light-amount
adjusting unit 12. Since the frequency characteristics of the
orientation of the two-color guest-host liquid crystal is on the
order of several tens Hz, when pulses having a frequency that is
sufficiently higher compared therewith (a frequency at which the
direction of the orientation of the dye molecules does not follow
change in the direction of the electric field) are applied to the
electrodes 31a and 31b, the effective power is given by a duty
ratio of the pulses multiplied by a pulse amplitude voltage.
[0035] FIGS. 3A to 3C are diagrams showing an example of driving
pulses and an example of optical transmittance.
[0036] FIGS. 3A and 3B show driving pulses. When effective powers
in FIGS. 3A and 3B are compared with each other, the effective
power is larger in FIG. 3B since the pulse width is larger. FIG. 3C
shows an example of relationship between effective power and
optical transmittance. As described above, if the frequency of
pulses (1/T0) is sufficiently high, the effective power P is given
by a duty ratio (D=a.times.100/b) multiplied by a pulse amplitude
voltage Vt. As shown in FIG. 3C, the optical transmittance starts
changing from a threshold value Pth, and increases as the effective
power becomes larger. The optical transmittance has a one-to-one
relationship with the effective power in a range of effective power
larger than the threshold value Pth.
[0037] Referring back to FIG. 1, the light incident on the lens 13
is focused on a surface of the imaging device 14 through the lens
13. More specifically, the lens 13 is moved between the
liquid-crystal light-amount adjusting unit 12 and the imaging
device 14 along an optical axis by a moving mechanism not shown,
thereby focusing the incident light on the imaging device 14 within
a range of focal -depth.
[0038] Although an optical system having single lenses 11 and 13 is
shown in FIG. 1 for simplicity of description, lens sets each
including a plurality of lenses may be used in accordance with
required capabilities of viewing-angle adjustment and focusing.
Furthermore, the optical system may include a plurality of lens
sets, e.g., three lens sets or four lens sets, for viewing-angle
adjustment and focusing. In that case, the liquid-crystal
light-amount adjusting unit 12 serves to change the aperture value
similarly to a conventional diaphragm, and is disposed between a
pair of lens sets among the plurality of lens sets.
[0039] The imaging device 14 performs photoelectric conversion for
converting an incident light into an amount of electricity in
accordance with the brightness of the incident light, signal
accumulation for holding and integrating an amount of electricity
for a certain period in order to improve sensitivity, and scanning
for yielding time-series image signals from a spatial distribution
of the amount of electricity obtained in relation to an image
corresponding to the incident light. The imaging device 14 can be
implemented by a solid-state imaging device such as a
charge-coupled device (CCD).
[0040] An output from the imaging device 14 is processed by the
signal processing circuit 18 in accordance with a desired video
signal format, and the result is output from the signal processing
circuit 18 to the microcomputer 19 or to a display or a recorder
not shown. The temperature detecting circuit 21 detects an ambient
temperature of the liquid-crystal light-amount adjusting unit 12,
and outputs a result of detection to the microcomputer 19. The
temperature detecting circuit 21 can be implemented, for example,
by a thermistor. The microcomputer 19 is also connected to the
storage circuit 20 that stores a comparison-value association table
and various data, as will be described later.
[0041] The liquid-crystal light-amount adjusting unit 12 is driven
by the driving circuit 15. The driving circuit 15 includes a
driving-pulse generating circuit 16, and a plurality
of-comparison-value supplying circuits 17 for respectively
supplying comparison values to the driving-pulse generating circuit
16. In this embodiment, the comparison-value supplying circuits 17
include four comparison-value supplying circuits 17a to 17d. The
plurality of comparison-value supplying circuits 17a to 17d are
controlled by the microcomputer 19, as will be described later, to
respectively supply comparison values to the driving-pulse
generating circuit 16.
[0042] Next, the driving-pulse generating circuit 16 will be
described.
[0043] FIG. 4 is a diagram showing the driving-pulse generating
circuit.
[0044] Referring to FIG. 4, the driving-pulse generating circuit 16
includes a binary counter 51, comparators 53, 57, 58, and 59, an
AND circuit 60, inverters 62 and 65, and field-effect transistors
(FETs) 61, 63, 64, and 66.
[0045] The binary counter 51 has a clock input to which a clock
signal having a predetermined cycle is input, and a reset input to
which an output of the comparator 53 is input. An output of the
binary counter 51 is input to the comparator 53, the comparator 57,
the comparator 58, and to the comparator 59.
[0046] The comparator 53 also receives input of a comparison value
1 supplied from the comparison-value supplying circuit 17a. The
comparator 53 compares the comparison value 1 with the output of
the binary counter 51, and outputs an output signal if these values
coincide with each other.
[0047] The comparator 57 also receives input of a comparison value
2 supplied from the comparison-value supplying circuit 17b. The
comparator 57 compares the comparison value 2 with the output of
the binary counter 51, and outputs an output signal if these values
coincide with each other.
[0048] The comparator 58 also receives input of a comparison value
3 supplied from the comparison-value supplying circuit 17c. The
comparator 58 compares the comparison value 3 with the output of
the binary counter 51, and outputs an output signal if these values
coincide with each other.
[0049] The comparator 59 also receives input of a comparison value
4 supplied from the comparison-value supplying circuit 17d. The
comparator 59 compares the comparison value 4 with the output of
the binary counter 51, and outputs an output signal if these values
coincide with each other.
[0050] The output signal of the comparator 57 is supplied to the
gate of the FET 61, and is also supplied, after being inverted by
the inverter 62, to the gate of the FET 63. The source of the FET
61 is connected to a power source Vcc, and the drain of the FET 61
is connected to the source of the FET 63. A node between the drain
of the FET 61 and the source of the FET 63 is connected tc a first
one of the electrodes 32a and 32b of the liquid-crystal
light-amount adjusting unit 12. The drain of the FET 63 is
grounded.
[0051] The FET 61 and the FET 63 are implemented by a pair of FETs
of mutually different channel types. In this embodiment, the FET 61
is a p-channel FET and the FET 63 is an n-channel FET.
[0052] The output signals of the comparators 58 and 59 are input to
the AND circuit 60. An output signal of the AND circuit 60 is
supplied to the gate of the p-channel FET 64, and is also supplied,
after being inverted by the inverter 65, to the gate of the
n-channel FET 66. The source of the FET 64 is connected to the
power source Vcc, and the drain of the FET 64 is connected to the
source of the FET 66. A node between the drain of the FET 64 and
the source of the FET 66 is connected to a second one of the
electrodes 32a and 32b of the liquid-crystal light-amount adjusting
unit 12. The drain of the FET 66 is grounded.
[0053] Next, the operation of the driving-pulse generating circuit
will be described.
[0054] FIGS. 5A and 5B are (first) diagrams for explaining the
operation of the driving-pulse generating circuit.
[0055] FIG. 6 is a (second) diagram for explaining the operation of
the driving-pulse generating circuit.
[0056] In FIGS. 5A and 5B, the horizontal axis represents time and
the vertical axis represents voltage. In parts (A), (B), and (C) of
FIG. 6, although axes are not shown, the horizontal direction
represents elapse of time and the vertical direction represents
voltage magnitude.
[0057] In FIGS. 5A and 5B, an output of the binary counter 51 is
used as an input to the comparator 53, and an output of the
comparator 53 is used as a reset input of the binary counter 51.
Thus, the output of the binary counter 51 forms a triangular wave
that is reset at the comparison value 1, as shown in FIGS. 5A and
5B. The triangular wave is input to each of the comparators 57, 58,
and 59.
[0058] Thus, the output of the comparator 57 form pulses that rise
at timings when the voltage value of the triangular wave coincides
with the comparison value 2 and fall at timings when the voltage
value of the triangular wave coincides with the comparison value 1,
as shown in FIG. 5A. The output of the AND circuit 60, to which the
outputs of the comparators 58 and 59 are input, forms pulses that
rise at timings when the voltage value of the triangular wave
coincides with the comparison value 3 and fall at timings when the
voltage value of the triangular wave coincides with the comparison
value 4, as shown in FIG. 5B.
[0059] The output of the comparator 57 is applied to the gate of
the p-channel FET 61, and is also applied, via the inverter 62, to
the gate of the n-channel FET 63. Thus, the voltage applied to the
electrode 32a are pulses whose pulse widths are controlled in
accordance with the magnitude of the comparison value 2, as shown
in part (A) of FIG. 6. Furthermore, the output of the AND circuit
60 is applied to the gate of the p-channel FET 61, and is also
applied, via the inverter 62, to the gate of the n-channel FET 63.
Thus, the voltage applied to the electrode 32b are pulses whose
pulse widths are controlled in accordance with the difference
between the comparison values 3 and 4, as shown in part (B) of FIG.
6.
[0060] The pulses shown in parts (A) and (B) of FIG. 6 are applied
to the electrodes 32a and 32b of the liquid-crystal light-amount
adjusting unit 12, respectively. Thus, the potential difference
between the electrodes 32a and 32b form pulses that fall to a
predetermined pulse amplitude voltage -Vt when the voltage applied
to the electrode 32b rises, that return to zero voltage when the
voltage applied to the electrode 32a rises, that rise to a
predetermined pulse amplitude voltage +Vt when the voltage applied
to the electrode 32b falls, and that return to zero voltage when
the voltage applied to the electrode 32a falls, as shown in part
(C) of FIG. 6.
[0061] As described above, the period TO of the pulses applied
between the electrodes 32a and 32b is controlled in accordance with
the comparison value 1, and the duty ratio D thereof is controlled
in accordance with the comparison values 2, 3, and 4.
[0062] Next, a method of driving the liquid-crystal light-amount
adjusting unit will be described.
[0063] FIG. 7 is a diagram for explaining a method of driving the
liquid-crystal light-amount adjusting unit.
[0064] In FIG. 7, the horizontal direction represents elapse of
time and the vertical direction represents voltage magnitude.
[0065] When an aperture value has been determined, the
microcomputer 19, with consideration of a result of detection by
the temperature detecting circuit 21, supplies driving pulses as
shown in FIG. 7 from the driving circuit 15 to the liquid-crystal
light-amount adjusting unit 12 so that an optical transmittance in
accordance with the aperture value will be achieved.
[0066] For example, in the case of a video camera with a built-in
camera, since a capturing time for one screen is predetermined, an
optimal aperture value is calculated from the amount of light
received by the imaging device 14. As another example, in the case
of an electronic still camera, an aperture value is determined
according to a program stored in the storage circuit 20 in the case
of automatic exposure, and an aperture value is determined
according to an input value input by a user via an input device not
shown in the case of manual exposure.
[0067] The driving pulses include driving pulses for acceleration
and driving pulses for dimming. The driving pulses for dimming are
used to drive the liquid-crystal light-amount adjusting unit 12 so
that an optical transmittance in accordance with the aperture value
will be achieved. The frequency fd, the pulse amplitude Vt, and the
duty ratio D of the driving pulses for dimming are determined with
consideration of the areas of the electrodes 32a and 32b, the
distance (gap) between the electrodes 32a and 32b, an ambient
temperature, the type of material of the liquid crystal 34, etc. in
the liquid-crystal light-amount adjusting unit 12. The driving
pulses for acceleration are used to speed up response in changing
the optical transmittance of the liquid-crystal light-amount
adjusting unit 12. The frequency fa, the pulse amplitude Vt, the
duty ratio D, and the applying time Ta of the driving pulses for
acceleration are determined with consideration of relationship
between current driving pulses for dimming and target driving
pulses for dimming, in addition to the areas of the electrodes 32a
and 32b, the distance (gap) between the electrodes 32a and 32b, an
ambient temperature, the type of material of the liquid crystal 34,
etc. in the liquid-crystal light-amount adjusting unit 12. More
specifically, in order to reduce brightness (decrease optical
transmittance) relative to an optical transmittance that is
achieved by driving with the current driving pulses for dimming,
the target driving pulses for dimming are adjusted so that the
effective power thereof will be smaller than the effective power of
the current driving pulses for dimming. On the other hand, in order
to increase brightness (increase optical transmittance) relative to
an optical transmittance that is achieved by driving with the
current driving pulses for dimming, the target driving pulses for
dimming are adjusted so that the effective power thereof will be
larger than the effective power of the current driving pulses for
dimming. If the pulse amplitude Vt is constant, the effective power
is adjusted by adjusting the pulse width (duty ratio D).
[0068] Furthermore, the frequencies fd and fa of the driving pulses
must be high enough not to cause flicker. If the frequency is low,
polarized liquid-crystal molecules is able to follow the driving
pulses alternating between +Vt and -Vt, so that flicker occurs due
to the optical transmittance changing in synchronization with the
pulse cycle.
[0069] Furthermore, the frequencies fd and fa of the driving pulses
must be low enough so that the impedance of the liquid-crystal
light-amount adjusting unit 12 remains. This is because adjustment
of light amount is not allowed if the frequencies of the driving
pulses are high and if the impedance of the liquid-crystal
light-amount adjusting unit 12 is thus eliminated.
[0070] Furthermore, the driving pulses must avoid generating DC
components. More specifically, if pulses having a 1/4 period or
pulses having a 1/2 period exist during the applying time Ta or the
applying time Td, a DC component is generated. Thus, the number of
pulses in the driving pulses must be an integer multiple of one
period in each of the applying times Ta and Td. If a DC component
is generated, the liquid crystal 34 is electrolyzed, and metallic
ions, etc. in the liquid crystal 34 precipitates on the electrodes
32a and 32b. This inhibits transmittance of light, degrading
performance of the liquid-crystal light-amount adjusting unit
12.
[0071] Since the imaging time Tp for one screen is predetermined,
the applying time Td of the driving pulses for dimming is
determined when the applying time Ta of the driving pulses for
acceleration is determined. For example, if the liquid-crystal
light-amount adjusting unit 12 is used in a video camera with a
built-in camera, the imaging time Tp for one screen is
approximately 16.7 ms corresponding to a field (a period of one
cycle at 60 Hz) if imaging is performed on a field-by-field basis,
or approximately 33.3 ms corresponding to a frame (a period of one
cycle at 30 Hz) if imaging is performed on a frame-by-frame basis.
As another example, if the liquid-crystal light-amount adjusting
unit 12 is used in an electronic still camera or a still camera,
the imaging time Tp for one screen is determined according to a
shutter speed.
[0072] The driving pulses for acceleration are needed to be applied
when the optical transmittance of the liquid-crystal light-amount
adjusting unit 12 is to be changed. Thus, if the aperture value is
not to be changed when capturing an image of a next screen, the
driving pulses for acceleration are not needed, and the driving
pulses for dimming are used over the entire imaging time Tp for one
screen. That is, the applying time Td of the driving pulses for
dimming is equal to the imaging time Tp for one screen.
[0073] Furthermore, in this embodiment, the driving pulses for
dimming and the driving pulses for acceleration are determined by
actual measurements, and stored in the storage circuit 20 in the
form of a comparison-value association table. As will be understood
from the description given above, the comparison-value association
table includes a table of association between aperture values and
driving pulses for dimming, defining association between a
plurality of aperture values and driving pulses for dimming for
achieving the respective aperture values, and a table of
association between driving pulses for dimming and driving pulses
for acceleration, defining association between a plurality of
relationships between target driving pulses for dimming and
respective driving pulses for acceleration. The comparison-value
association table is created on a temperature-by-temperature basis
at a predetermined temperature interval. The fields for driving
pulses for dimming in the table of association between aperture
values and driving pulses for dimming store comparison values 1 to
4 for generating the driving pulses for dimming, and the fields of
driving pulses for acceleration in the table of association between
driving pulses for dimming and driving pulses for acceleration
store comparison values 1 to 4 and an applying time Ta of driving
pulses for acceleration for generating the driving pulses for
acceleration.
[0074] Thus, the microcomputer 19, when an aperture value is
specified and a need is raised to change the aperture value,
obtains the comparison values 1 to 4 for the driving pulses for
acceleration and the applying time Ta of the driving pulses for
acceleration, and the comparison values 1 to 4 for the driving
pulses for dimming, from the comparison-value association table at
a relevant temperature.
[0075] The microcomputer 19 first outputs the comparison values 1
to 4 for the driving pulses for acceleration to the
comparison-value supplying circuits 17a to 17d of the driving
circuit 15, and exercises control so that the comparison-value
supplying circuits 17a to 17d will generate the associated
comparison values 1 to 4.
[0076] The comparison-value supplying circuits 17a to 17d generate
specified comparison values, and supplies the comparison values
generated to the comparators 53, 57, 58, and 59, respectively.
[0077] The driving-pulse generating circuit 16 generates driving
pulses for acceleration according to the comparison values, thereby
driving the liquid-crystal light-amount adjusting unit 12.
[0078] When the applying time Ta of the driving pulses for
acceleration elapses, the microcomputer 19 outputs the comparison
values 1 to 4 for the driving pulses for dimming to the
comparison-value supplying circuits 17a to 17d of the driving
circuit 15, and exercises control so that the comparison-value
supplying circuits 17a to 17d will generate the associated
comparison values 1 to 4, respectively.
[0079] The comparison-value supplying circuits 17a to 17d generate
the specified comparison values, and output the comparison values
generated to the comparators 53, 57, 58, and 59, respectively.
[0080] The driving-pulse generating circuit 16 generates driving
pulses for dimming in accordance with the comparison values,
thereby driving the liquid-crystal light-amount adjusting unit
12.
[0081] The microcomputer 19 controls the comparison-value supplying
circuits 17a to 17d so that the liquid-crystal light-amount
adjusting unit 12 will be driven by the driving pulses for dimming
until the imaging time Tp for one screen is reached.
[0082] As described above, the microcomputer 19 controls the
driving circuit 15 to supply driving pulses to the liquid-crystal
light-amount adjusting unit 12, thereby adjusting the amount of
light to achieve a specified aperture value.
[0083] As described above, according to the present invention, the
frequency fa of the driving pulses for acceleration and the
frequency fd of the driving pulses for dimming are allowed to be
different from each other.
[0084] Accordingly, the driving pulses for acceleration can be
controlled independently of the driving pulses for dimming. Thus,
the effective power of the driving pulses for acceleration can be
optimized in accordance with the difference between the current
optical transmittance and the target optical transmittance, the
ambient temperature, the gap between the electrodes 32a and 32b,
dye concentration, etc. in the liquid-crystal light-amount
adjusting unit 12.
Industrial Applicability
[0085] As described above, according to the present invention,
before driving pulses for dimming with which a desired optical
transmittance will be achieved are supplied to liquid crystal,
predetermined driving pulses for acceleration are supplied,
speeding up change in optical transmittance. Furthermore, the pulse
shape of the predetermined driving pulses for acceleration and the
pulse shape of the driving pulses for dimming can be controlled
independently. Thus, the pulse shape of the driving pulses for
acceleration can be controlled without being constrained by the
pulse shape of the driving pulses for dimming. Accordingly, the
predetermined driving pulses for acceleration are reliably
obtained, serving to further enhance speed. Furthermore, in a
liquid-crystal light-amount adjusting device, power consumption is
reduced and lifetime is extended.
* * * * *