JP2010148122A - Dimmer and imaging apparatus and method for driving both - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a dimmer for stably controlling a transmittance without causing liquid crystal orientation fluctuations, an imaging apparatus using the dimmer, and a method for driving both. <P>SOLUTION: The dimmer includes a GH cell 12 and a polarizing plate 11 and equips a pulse control part 64 that changes the transmittance of the light incident to the GH cell 12 to a target transmittance from a current transmittance by the drive pulse controlled at least in two steps. The imaging apparatus includes this dimmer in the optical path of an imaging system. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a light control device for adjusting and outputting the amount of incident light, an imaging device using the same, and a driving method thereof.

  Usually, a polarizing plate is used for a light control device using a liquid crystal cell. As this liquid crystal cell, for example, a TN (Twisted Nematic) type liquid crystal cell or a guest-host type liquid crystal cell (GH (Guest Host) cell) is used.

  FIG. 28 is a schematic view showing the operating principle of a conventional light control device. This light control device is mainly composed of a polarizing plate 1 and a GH cell 2. The GH cell 2 is enclosed between two glass substrates (not shown), and the operation electrode and the liquid crystal alignment film are not shown (the same applies hereinafter). Liquid crystal molecules 3 and dichroic dye molecules 4 are enclosed in the GH cell 2. The dichroic dye molecule 4 is a positive (p-type) dye molecule having anisotropy in light absorption, for example, absorbing light in the molecular long axis direction. Further, the liquid crystal molecule 3 is a positive type (positive type) having a positive dielectric anisotropy.

  FIG. 28A shows the state of the GH cell 2 when no voltage is applied (no voltage is applied). Incident light 5 is converted into linearly polarized light by passing through polarizing plate 1. In FIG. 20A, since the polarization direction and the molecular long axis direction of the dichroic dye molecule 4 coincide with each other, light is absorbed by the dichroic dye molecule 4 and the transmittance of the GH cell 2 decreases. .

  As shown in FIG. 28B, when a voltage is applied to the GH cell 2, the molecular long axis direction of the dichroic dye molecule 4 becomes perpendicular to the polarization direction of linearly polarized light. For this reason, the light is transmitted by the GH cell 2 with almost no absorption.

  The use of a negative (n-type) dichroic dye molecule that absorbs light in the direction of the minor axis of the molecule is the reverse of the case of the positive dichroic dye molecule 4, and light is applied when no voltage is applied. Is not absorbed, and light is absorbed when a voltage is applied.

  In the light control device shown in FIG. 28, the ratio of absorbance between when voltage is applied and when no voltage is applied, that is, the ratio of optical density is about 10. This has an optical density ratio that is approximately twice that of a light control device that includes only the GH cell without using the polarizing plate 1.

  In the conventional light control device described above, particularly in the driving of the negative type liquid crystal having a negative dielectric anisotropy, the transmittance is changed in a large step, but such a step response (especially transparent) From time to time, a large step response at the time of light shielding) disturbs the liquid crystal alignment, which makes the optical characteristics unstable. As will be described later, this appears as in-plane transmission nonuniformity.

  In other words, when the step width of the voltage change is large, a state in which the orientation of the liquid crystal molecules is different occurs as a transient response, and when this time enters the practical time range, it appears as in-plane transmittance non-uniformity. After a certain period of time, it recovers due to realignment of the liquid crystal and the dye, but may not recover partially.

  The present invention has been made in consideration of the above-described circumstances, and an object thereof is a light control device that enables stable transmittance control without causing disturbance in liquid crystal alignment, and an imaging device using the same. And providing these driving methods.

  That is, the present invention includes a liquid crystal element and a pulse control unit that changes the transmittance of light incident on the liquid crystal element from the current transmittance to the target transmittance by a drive pulse controlled in at least two stages. The present invention relates to an optical device and an imaging device in which the light control device is arranged in an optical path of an imaging system.

  According to the light control device and the imaging device of the present invention, the drive pulse of the liquid crystal element for light control is controlled in at least two stages (for example, in two stages from a low voltage to a high voltage). Compared with the conventional example to be changed, a pulse for achieving the desired transmittance can be applied after inclining the liquid crystal to some extent by applying a pulse that does not cause disorder in the alignment of the liquid crystal as a preparatory stage of response. The transmittance can be controlled in a uniform state.

  Further, according to the present invention, as a method of driving the light control device and the imaging device of the present invention with good controllability, when driving the liquid crystal element, the transmittance of light incident on the liquid crystal element is changed from the current transmittance to the target transmittance. The present invention also provides a method of driving the light control device and the imaging device, which is changed by a drive pulse controlled in at least two stages.

  According to the light control device and the imaging device of the present invention, the drive pulse of the liquid crystal element for light control is controlled in at least two stages (for example, in two stages from a low voltage to a high voltage). Compared with the conventional example to change, after applying a pulse that does not cause disorder in the alignment of the liquid crystal as a preparatory stage for response, the liquid crystal is tilted to some extent, and then a pulse for achieving the desired transmittance can be applied, and it is uniform in the plane The transmittance can be controlled in a stable state.

FIG. 6 is a graph showing the relationship between the light transmittance of the light control device according to the embodiment of the present invention and the pulse voltage of the drive pulse, and a drive pulse waveform diagram. FIG. 6 is a graph showing the relationship between the light transmittance of the light control device and the pulse voltage of the drive pulse, and a drive waveform diagram. It is a schematic diagram which compares and shows the alignment process of the liquid crystal director of a light control apparatus. It is a graph which shows the relationship between the light transmittance of the light control apparatus for a comparison, and an applied voltage. It is a graph for demonstrating the disorder of orientation in the relationship between the transmittance | permeability of a light modulation apparatus and an applied voltage similarly. It is the schematic which shows the principle of operation of the light modulation apparatus by embodiment of this invention. It is a graph which shows the relationship between the light transmittance of the light modulation apparatus using negative type liquid crystal, and an applied voltage in the range of 0-10V (A) and 0-20V (B). It is a graph which shows the relationship between the light transmittance of a light control apparatus using positive type liquid crystal, and an applied voltage in the range of 0-10V (A) and 0-20V (B). FIG. 6 is a waveform diagram showing various drive pulses of the light control device. FIG. 6 is a graph showing the relationship between the light transmittance of the light control device and the pulse width of the drive pulse, and a drive pulse waveform diagram. It is a graph which shows the relationship between the light transmittance of the light control apparatus for a comparison, and the pulse voltage at the time of pulse width modulation drive. It is a graph which shows the relationship between the light transmittance of the light modulation apparatus by embodiment of this invention, and the pulse width of a drive pulse, and a drive pulse waveform diagram. FIG. 6 is a waveform diagram showing the relationship between the waveform of the drive pulse of the light control device and the transmittance flicker for each of various pulse pause times. FIG. 6 is a waveform diagram by various modulation of drive pulses of the light control device. It is a graph and a table | surface for demonstrating the relaxation process of the negative type liquid crystal type | system | group of a light control apparatus. It is a graph which shows the response characteristic of the light modulation apparatus using a negative type liquid crystal. FIG. 6 is a graph showing a comparison between the light transmittance and voltage in pulse width modulation and pulse voltage modulation of a drive pulse of the light control device. FIG. 7 is a graph showing a comparison between the relationship between the light transmittance and the duty ratio in pulse width modulation and pulse density modulation of a drive pulse of the light control device. FIG. 6 is a diagram of various pulse waveforms and its light transmittance characteristic diagram in the pulse width modulation of the drive pulse of the light control device. FIG. 6 is a light transmittance characteristic diagram according to the number of positive and negative drive pulses of the light control device. 3 is a graph showing a comparison of changes in transmitted light intensity in pulse voltage modulation and pulse width modulation of a drive pulse of a light control device using positive liquid crystal. It is a schematic side view of the light control device. It is a front view of the mechanical iris of the light control device. It is a schematic partial enlarged view showing the operation of the mechanical iris in the vicinity of the effective light path of the light control device. It is a schematic sectional drawing of the camera system incorporating a light control device. This is an algorithm for controlling light transmittance in the camera system. 2 is a block diagram of a camera system including a drive circuit. FIG. It is the schematic which shows the operation principle of the conventional light modulation apparatus.

  In the light control device, the imaging device, and the driving methods thereof according to the present invention, it is desirable that the pulse voltage or the pulse width of the driving pulse is controlled in at least two stages.

  The drive pulse is preferably taken out in synchronization with the clock of the drive circuit section.

  The luminance information of the emitted light is fed back to the control circuit unit, and the drive pulse is obtained in synchronization with the clock of the drive circuit unit by the control signal from the control circuit unit, or the drive circuit Is a drive circuit unit of an image sensor disposed on the light emitting side of the light control device, and an output signal of the image sensor is fed back to the control circuit unit as luminance information, and a control signal from the control circuit unit The drive pulse may be obtained in synchronization with the clock of the drive circuit unit.

  The light control device and the imaging device of the present invention are suitable when the drive electrode of the liquid crystal element is formed at least over the entire effective light transmitting portion, and the drive electrode thus formed is applied to the drive electrode. By controlling the driving pulse, the collective control of the transmittance can be performed with high accuracy over the entire effective optical path width.

  The liquid crystal element is a guest-host type liquid crystal element, that is, the host material of the liquid crystal element is a negative or positive type liquid crystal having a negative or positive dielectric anisotropy, and the liquid crystal element The guest material may comprise a positive or negative dichroic dye molecule having positive or negative light absorption anisotropy.

  In particular, when a negative type liquid crystal (that is, negative dielectric anisotropy (Δε) is negative) is used as the host material, light transmission (especially transparent) is greater than when a positive type (that is, Δε is positive) liquid crystal is used. The light transmittance is improved, and the position can be fixed in the imaging optical system as it is.

  If a polarizing plate is arranged in the optical path of the incident light to the liquid crystal element, the ratio of absorbance when no voltage is applied to that when voltage is applied (that is, the ratio of optical density) is improved, and the contrast ratio of the light control device is increased. Dimming can be performed normally from a bright place to a dark place.

  And it is desirable that the polarizing plate can be taken in and out of the optical path by being disposed on a movable part of a mechanical iris.

  A preferred embodiment in which the present invention is applied to a liquid crystal ND (neutral deusity) filter and a camera system will be described below with reference to the drawings.

[Light transmittance of the light control device and its optical arrangement]
First, in the guest-host type liquid crystal cell (GH cell) 2 shown in FIG. 28, MLC-6849 manufactured by Merck Co., which is a positive type general-purpose liquid crystal having positive dielectric anisotropy (Δε) as the host material 3. The guest material 4 uses DDH made by BDH, which is a positive dye having positive dichroic light absorption anisotropy (ΔA), and the polarizing plate 1 is arranged on the incident side of the GH cell 2. The change in light transmittance when an operating voltage was applied was measured using a rectangular wave as a driving waveform.

  As a result, as shown in FIG. 8, with the application of the operating voltage, the average light transmittance of visible light (refer to the transmittance when a polarizing plate is added in addition to the liquid crystal cell in the air (= 100%)). However, when the voltage is increased to 20 V, the maximum transmittance is about 60%, and the change in the light transmittance is gradual.

  This is because, when a positive type host material is used, the direction of the director changes even when a voltage is applied because the interaction of liquid crystal molecules at the interface with the liquid crystal alignment film of the liquid crystal cell is strong when no voltage is applied. This is thought to be because liquid crystal molecules that do not (or hardly change) remain.

  On the other hand, as shown in FIG. 6, in the guest-host type liquid crystal cell (GH cell) 12, the host material 13 is a negative type liquid crystal having a negative dielectric anisotropy (Δε). Using MLC-6608, the guest material 4 is DDH made by BDH, which is the same positive dye having the above dichroism, and the polarizing plate 11 is arranged on the incident side of the GH cell 12 to drive a rectangular wave. The change in light transmittance when operating voltage was applied was measured with a waveform. The light transmittance in this case is opposite to that in FIG. 28, and is transmitted when no voltage is applied, but becomes non-transmissive as the voltage is applied.

  As a result, as shown in FIG. 7, with the application of the operating voltage, the average light transmittance (in the air) of visible light decreases from the maximum transmittance of about 75% to several percent, and the light transmittance changes. Was found to be steep.

  This is because when a negative type host material is used, the interaction of liquid crystal molecules at the interface with the liquid crystal alignment film of the liquid crystal cell is very weak when no voltage is applied, so that light is easily transmitted when no voltage is applied. Also, it is considered that the direction of the director of the liquid crystal molecules easily changes with the application of voltage.

  As described above, in the GH cell 12 using the negative type host material, the light transmittance is improved, and the design in a region having a high light transmittance is possible. In addition, since the change in the light transmittance is steep, it is possible to provide a light control device that can specifically control the light transmittance by the operating voltage.

  In the above GH cell, the combination of the host material and the guest material may be various. Negative type (Δε <0) -positive type (ΔA> 0), negative type (Δε <0) -negative type (ΔA < 0), positive type (Δε> 0) -positive type (ΔA> 0), and positive type (Δε> 0) -negative type (ΔA <0).

  In the above GH cell, the drive electrode, for example, an ITO (indium tin oxide: tin doped indium oxide) electrode is attached on the substrate surface. It is good also as a system.

  In addition, in the light control device based on this invention, the negative type host material which can use dielectric constant anisotropy ((DELTA) epsilon) can be illustrated below. However, in actual use, a blended composition selected from the following compounds is used so as to exhibit nematic properties in the actual use temperature range (hereinafter the same).

  Moreover, in the light control device based on this invention, the positive type host material with positive dielectric constant anisotropy which can be used can be illustrated below.

<Exemplary compound>

<Example 1 under the trade name> (both made by Merck)

  Moreover, in the light modulation apparatus based on this invention, the dichroic dye molecule which can be used can be illustrated below.

[Effects of rubbing]
As the light control element, the one shown in FIG. 6 is used, a transparent electrode is attached to the surface of the glass substrate, and both glass substrates provided with a liquid crystal alignment layer on the upper surface are opposed to each other, and a cell is produced with a predetermined gap. Guest-host liquid crystal was injected under reduced pressure. Here, an alternating rectangular wave was used as a drive pulse for the element.

  If the liquid crystal alignment layer (not shown) is not rubbed, as shown in FIG. 3A, the liquid crystal is inclined with respect to the molecular substrate surface when a voltage is applied, and at the same time, the liquid crystal and the dye in a plane parallel to the glass substrate surface Is disturbed, resulting in non-uniform transmittance in the substrate surface. In order to avoid this, as a known technique, as shown in FIG. 3B, the liquid crystal alignment layer is rubbed so that the direction in which the liquid crystal is tilted is preliminarily defined and uniformly inclined to improve in-plane uniformity. Yes.

  However, when the step width of the voltage change is large, as shown in FIG. 3C, a state with a different orientation occurs as a transient response. When this time falls within the practical time range, it appears as in-plane non-uniformity in transmittance. After a certain period of time, the liquid crystal and the dye recover by realignment, but some may not recover.

  In order to solve this problem, as shown in FIG. 3D, a high voltage for achieving a desired transmittance is obtained after applying a low voltage that does not cause liquid crystal alignment disorder and tilting the liquid crystal to some extent. It was found that the transmittance can be controlled in a uniform state in the plane by applying.

[Transmittance and orientation disorder during single-stage driving (1)]
For example, as shown in FIG. 5, in a pulse width modulation (PWM) or pulse voltage modulation (PHM) described later, if an attempt is made to achieve a transmittance of 15% or less in one step from an equivalent voltage of 0 V, FIG. As described, it was found that the alignment disorder of the liquid crystal and the dye occurs (however, in this case, the alignment disorder does not occur even in one step at a transmittance of 15% or more).

  This is specifically shown in FIG. 4 by taking the case of pulse voltage modulation as an example. However, the state of transmittance change varies depending on the magnitude of the pulse voltage, and the orientation is more likely to be disturbed as the voltage is higher. It can be seen that this is likely to occur when the transmittance is 15% or less.

[Transmissivity change during two-stage drive (1)]
Therefore, an example of two-stage driving by voltage modulation of an AC pulse based on the present invention will be shown. FIG. 1 (A) shows a case where 90 V is applied for 90 ms as a preliminary stage when a step change from 0 V to 10 V is performed, and then 10 V is applied for a response. According to this, an unstable transmittance change as shown in FIG. 4 is not seen, and a stable transmittance change is obtained. FIG. 1B shows a case where a response is made by applying 15 V at 4 V, and then applying 10 V. Also in this case, the transmittance change is stabilized. In this way, the pulse time width of each stage can be freely selected, but FIG. 1B is better for quick response.

  FIG. 2 shows in detail a drive waveform similar to the drive waveform of FIG. In each of the two-stage driving pulses, positive voltage and negative voltage are alternately repeated with a constant pulse width of 500 μs, but the first 4.5V pulse is repeated 15 times and the next 10V pulse is repeated 75 times. To do. As a result, it was found that if a voltage of 500 μs and ± 4.5 V was applied in advance for 15 ms or longer in driving, alignment disturbance would not occur even if a voltage exceeding 5 V was applied.

  Thus, as a result of driving by applying the two-stage pulse according to the present invention, the change in light transmittance becomes a desired profile, the accuracy of transmittance control is improved, and the in-plane uniformity of the transmittance of the light control element is improved. improves.

  In this two-stage driving, the time width and voltage of preliminary voltage application can be freely defined, and for example, various patterns shown in FIGS. 9A to 9E can be selected. The number of steps is not limited to two, and it is possible to have two or more steps depending on the system. Further, as will be described later, a method of changing only the pulse width with a constant voltage, such as pulse width modulation, can be applied.

[Transmissivity and orientation disorder during single-stage driving (2)]
For example, as shown in FIG. 11, when an attempt is made to achieve a transmittance of 15% or less in one step from an equivalent voltage of 0 V by pulse width modulation, the transmittance varies depending on the magnitude of the pulse voltage as described in FIG. The state of change is different, and when the voltage is 5V as shown in FIG. 11A, the change in transmittance due to the disorder of orientation does not become unstable, but when it is 10V, the transmittance becomes unstable as shown in FIG. 11B. It can be seen that (abnormal response) is likely to occur.

[Transmissivity change during two-stage drive (2)]
Therefore, in order to prevent the above-mentioned alignment disorder, the driving under a high applied voltage is a two-step driving by pulse width modulation, based on the present invention. For example, when changing from 0V to 10V, as shown in FIG. 10, first, as a preliminary step, a positive and negative pulse of 20 μs at 10 V is applied for 15 ms, and then a positive pulse width of 10 μs at 10 V is applied. A negative pulse was applied to respond. In this case, the transmittance change became stable as shown in the figure. Such combinations of pulses having different pulse widths may be other than the above, and may be two or more stages, and the pulse voltages thereof may be various.

[About pulse width modulation]
(1) Rectangular wave basic drive waveform and dimmer flicker As shown in FIG. 6, for example, the drive waveform of the GH cell 12 is a rectangular wave, but it can be driven by either a trapezoidal wave or a sine wave. The inclination of the director of the liquid crystal changes according to the potential difference between the electrodes, and the light transmittance is controlled. Therefore, the transmittance is normally controlled by this peak value (pulse voltage).

  However, such peak value control basically requires D / A conversion performed in place of an analog signal, and it is difficult to control the voltage with high accuracy, which causes an increase in circuit cost.

  On the other hand, the electro-optic response of a nematic liquid crystal material is as fast as milliseconds, and as slow as several hundreds of milliseconds. Here, the present inventor has examined what is appropriate as a frequency when the drive pulse width modulation method is applied to a material system having such response characteristics.

  For example, as shown in FIG. 13, the drive pulses are applied in the order of 0V → 5V → 0V → −5V → 0V. The transmittance variation due to changing the width of the pulse (particularly the pause pulse 0V) was observed.

  As a result, it was found that when the pause pulse period was 300 μs or more, flicker was observed in the transmittance, and optical stability was lacking. It was found that no flickering of transmittance was observed in the rest period of 200 μs or less.

  Therefore, it can be seen that in the case of this light control device, it is sufficient to set the pause period not to be 200 μs or more when driven by pulse width modulation. Since the response speed of the liquid crystal changes depending on the type of liquid crystal or the environmental temperature, it is necessary to select a setting without flicker within the usage conditions. In addition, feedback of the environmental temperature to control the pulse width is also effective for obtaining stable element optical characteristics.

(2) Pulse width modulation From the above examination results, the basic pulse generation period was set to 100 μs, and the pulse width (PW) was controlled within this basic period. FIG. 12 shows a case where the pulse peak values are constant at 5V and 10V.

  In either case, the transmittance could be easily controlled by controlling the pulse width (the transmittance when the polarizing plate when the applied voltage is zero is included is 100%). This is because the orientation of the director of the liquid crystal molecules is changed by the electric field energy corresponding to the width of the pulse voltage, and the orientation is controlled. It can also be seen that the transmittance can be freely controlled by the combination of the pulse peak value and the pulse width. This means that the gradation control due to the minimum clock limit also increases the resolution of gradation control by controlling the voltage digitally to the lower bits and adding pulse width modulation as the upper bits (upper and lower bits). The setting can be reversed.) Further, from the viewpoint of cost, there is an advantage in using a waveform generated by processing a clock of a peripheral circuit on which the dimming element is mounted.

  FIG. 14 shows two forms (A) and (B) of the waveform at the time of pulse width modulation. The waveform of (A) enters a pulse at the start of the basic period, whereas in (B), the basic period is changed. This is an example in which a pulse is applied after a certain delay time from the start, and the effect is the same as in the case of (A). The pulse application after such a delay time may be performed every basic period, or may be arbitrarily combined with the waveform as shown in (A). The required number of drive pulses may be applied within the basic period. (C) shows pulse density modulation for modulating the number density of pulses.

  FIG. 15 shows a liquid crystal relaxation process when a negative liquid crystal is used as a host material as in the GH cell shown in FIG. According to this, it can be seen that the relaxation time after voltage application → off changes depending on the magnitude of the pulse voltage, and the relaxation time becomes longer as the high voltage is applied.

  FIG. 16 shows the transmitted light intensity (A) and the fluctuations (B) and (C) of the light control element in the same relaxation process as in FIG. 15, but in a region where the relaxation occurs linearly (0 to 5 ms). Comparison of fluctuations in the following revealed the following.

An off time of 300 μs or less is required to achieve fluctuations within 1%.
An off time of 420 μs or less is required to achieve fluctuations within 2%.

  This fact corresponds to the condition for eliminating the transmittance flicker as shown in FIG. 13, and the basic period is set under the condition that the fluctuation of 2% or less is allowed (for example, 100 μs). Can do.

  The meaning of the fluctuation within 2% is based on the fact that it is equivalent to the imaging specification of a CCD which is currently used later.

  In actuality, CCD imaging is the average of the amount of light received accumulated in the field period, so it is expected that it will not appear as flicker during normal operation, but the dynamic range of transmittance control is reduced. End up. Further, when the shutter function or the like is used, the relationship between the shutter opening time and the light amount is not linear, which causes a problem in control. Therefore, the above-mentioned fluctuation amount within 2% is considered desirable.

  If a basic period exceeding the field period is used, the image is captured as flicker. Therefore, setting the basic period within the field period is a limit basic period in pulse width control.

(3) Comparison of Pulse Width Modulation and Pulse Voltage Modulation FIG. 17 compares the characteristics of conventional pulse voltage modulation (PHM) and pulse width modulation (PWM) (the horizontal axis in this figure is the distance between electrodes). Is the time average of the absolute value of the potential difference applied to, and is expressed as an equivalent voltage).

  As a result, the pulse width modulation has a lower threshold voltage and shifts to a lower voltage as a whole, so that low voltage control is possible and power consumption can be reduced. Further, since the change in transmittance is relatively gradual, the transmittance can be easily controlled by voltage, and the gradation is improved.

Thus, the pulse width modulation has the following advantages as a whole.
(1) Low threshold characteristics can be obtained by driving pulse width modulation.
(2) The number of gradations of the transmittance level can be increased, and the transmittance can be controlled with high accuracy.
(3) Since D / A conversion is not performed, the circuit cost can be reduced.

(4) Pulse width modulation and pulse density modulation Instead of modulating the peak value (voltage) of the pulse, the pulse density was modulated (PDM) and compared with pulse width modulation (PWM). The pulse density modulation is to change the number of pulses generated within a unit time, and one pulse width is very short and includes a lot of high frequency components.

  As shown in FIG. 18, the drive characteristics of PWM and PDM are similar to each other. However, in view of low power consumption, it can be seen that in pulse density modulation, the amount of charge / discharge current to the liquid crystal cell per unit time is large, and pulse width modulation is more advantageous. From the viewpoint of impedance matching, pulse width modulation is more advantageous.

(5) Influence by the number of pulses When driving using a pulse width modulation waveform, driving is performed so that the time average of the potential difference applied between the electrodes is substantially zero (in other words, the DC component is 0 V). As a result, polarization deviation of ions and the like in the light control element can be eliminated, and the transmittance can be controlled with high accuracy.

  That is, as shown in FIG. 19, when the positive polarity and the negative polarity are alternately applied to the drive waveform of (A) by two pulses each as shown in (B), the time average of the number of positive and negative pulses is obtained. The same transmittance driving characteristics can always be obtained in the case of the same.

  Further, as shown in (C), the number of positive and negative pulses was changed with positive integers such as m = 1, 2,..., But there was no change in the illustrated transmittance vs. pulse width characteristics.

  Further, even when the pulse generation order is different as shown in (D), the same characteristics are exhibited as long as the number of positive and negative pulses is the same. Furthermore, even if the time widths of the pulses are individually modulated, it can be expected that there is no problem as long as the time average is equivalent.

  On the other hand, when the number of positive and negative pulses is different (when the negative pulse is k times larger than the positive pulse), k = 1 is the transmittance at the time of symmetrical driving, but k is large. When the asymmetry increases, the transmittance becomes larger than the predetermined transmittance as shown in FIG. 12, making controllability difficult. This is independent of the value of m and shows a similar variation with respect to k.

  Furthermore, when the asymmetric polarity is instantaneously reversed, the transmittance temporarily decreases and returns to the previous transmittance in a few seconds (observed as long-period flicker). This excessive variation in the order of seconds is considered to be caused by the bias of mobile ions in the liquid crystal cell caused by the bias voltage on the time average.

  For the above reasons, the symmetry of the number of pulses (the same number of positive and negative bipolar pulses) is desirable for stable driving rather than the asymmetry of the number of pulses.

[When using positive liquid crystal]
Unlike the example described above, in FIG. 6, a positive type liquid crystal (MLC-6849 manufactured by Merck) was used as a host material, and a GH cell was manufactured in the same manner as described above.

  As a result, as shown in FIG. 21, the transmitted light intensity change is relatively gradual when pulse width modulation (B) is applied, compared to when voltage modulation is applied to the same rectangular wave as the fundamental frequency (A). Therefore, controllability and gradation are improved, and equivalent transmitted light intensity can be obtained with a lower threshold from the comparison of (C).

[Specific example of light control device]
Next, an example of a light control device using, for example, the GH cell 12 among the above-described GH cells will be described with reference to FIGS.

  As shown in FIG. 22, the light control device 23 includes a GH cell 12 and a polarizing plate 11a. The GH cell 12 is sealed between two glass substrates (not shown). In the GH cell 12, negative liquid crystal molecules (host material) and positive or negative dichroic dye molecules (guest material) are encapsulated. The liquid crystal molecules have a negative dielectric anisotropy, for example, and the dichroic dye molecules have anisotropy in light absorption, for example, p-type that absorbs light in the molecular long axis direction. The light absorption axis of the polarizing plate 11 was orthogonal to the light absorption axis when a voltage was applied to the GH cell.

  The light control device 23 is disposed between a lens front group 15 and a lens rear group 16 each composed of a plurality of lenses such as a zoom lens. The light transmitted through the lens front group 15 is linearly polarized through the polarizing plate 11 and then enters the GH cell 12. The light transmitted through the GH cell 12 is collected by the rear lens group 16 and displayed on the imaging surface 17 as an image.

  The polarizing plate 11 constituting the light control device 23 can be taken in and out of the effective optical path of light incident on the GH cell 12. Specifically, the polarizing plate 11 can be moved out of the effective optical path of light by moving it to a position indicated by a virtual line. As a means for taking in and out the polarizing plate 11, a mechanical iris shown in FIG. 23 may be used.

  This mechanical iris is a mechanical diaphragm device that is generally used for a digital still camera, a video camera, and the like, and mainly includes two iris blades 18 and 19 and a polarizing plate 11 attached to the iris blade 18. The iris blades 18 and 19 can be moved in the vertical direction. The iris blades 18 and 19 are relatively moved in the direction indicated by the arrow 21 using a drive motor (not shown).

  As a result, as shown in FIG. 23, the iris blades 18 and 19 are partially overlapped. When the overlap is increased, the opening 22 on the effective optical path 20 located near the center of the iris blades 18 and 19 is polarized. 11 is covered.

  FIG. 24 is a partially enlarged view showing a mechanical iris near the effective optical path 20. At the same time that the iris blade 18 moves downward, the iris blade 19 moves upward. Along with this, as shown in FIG. 24A, the polarizing plate 11 attached to the iris blade 18 also moves out of the effective optical path 20. Conversely, the iris blades 18 and 19 overlap each other by moving the iris blade 18 upward and the iris blade 19 downward. Accordingly, as shown in FIG. 24B, the polarizing plate 11 moves on the effective optical path 20 and gradually covers the opening 22. When the overlapping of the iris blades 18 and 19 increases, the polarizing plate 11 covers the opening 20 as shown in FIG.

  Next, the light control operation of the light control device 23 using this mechanical iris will be described.

  As the object (not shown) becomes brighter, as shown in FIG. 24 (a), the iris blades 18 and 19 opened upward are driven by a motor (not shown) and start to overlap. Thereby, the polarizing plate 11 attached to the iris blade 18 starts to enter the effective optical path 20 and covers a part of the opening 22 (FIG. 24B).

  At this time, the GH cell 12 does not absorb light (note that there is some absorption by the GH cell 12 due to thermal fluctuation or surface reflection). For this reason, the light having passed through the polarizing plate 11 and the light having passed through the opening 22 have substantially the same intensity distribution.

  Thereafter, the polarizing plate 11 completely covers the opening 22 (FIG. 24C). Further, when the brightness of the subject increases, the voltage to the GH cell 12 is increased, and light adjustment is performed by absorbing light in the GH cell 12.

  On the contrary, when the subject becomes dark, the voltage absorption to the GH cell 12 is first reduced or not applied, thereby eliminating the light absorption effect by the GH cell 12. Further, when the subject becomes dark, the iris blade 18 is moved downward and the iris blade 19 is moved upward by driving a motor (not shown). Thus, the polarizing plate 11 is moved out of the effective optical path 20 (FIG. 24A).

  According to this embodiment, since the polarizing plate 11 (transmittance, for example, 40% to 50%) can be taken out from the effective light path 20 of light, the polarizing plate 11 does not absorb light. Therefore, the maximum transmittance of the light control device can be increased by, for example, twice or more. Specifically, when this light control device is compared with a conventional light control device composed of a fixed polarizing plate and a GH cell, the maximum transmittance is about doubled, for example. The minimum transmittance is the same for both.

  Moreover, since the polarizing plate 11 is put in and out using a mechanical iris that has been put to practical use in a digital still camera or the like, the light control device can be easily realized.

  Further, since the GH cell 12 is used, in addition to the dimming by the polarizing plate 11, the GH cell 12 itself can absorb the light to perform dimming.

  As described above, the light control device according to the present invention can increase the contrast ratio between light and dark and can keep the light amount distribution substantially uniform.

  As the GH cell 12 used in this embodiment, a negative (n-type) dichroic dye molecule may be used under the condition that the dielectric anisotropy of liquid crystal molecules is negative. .

  In the conventional light control device as shown in FIG. 28, the polarizing plate 1 is always fixed and installed in the effective light path. Therefore, for example, 50% of light is always absorbed by the polarizing plate, and there is an influence such as surface reflection of the polarizing plate. For this reason, the maximum transmittance of the light transmitted through the polarizing plate cannot exceed, for example, 50%, and the light amount is significantly reduced. This decrease in the amount of light is one of the factors that make it difficult to put a light control device using a liquid crystal cell into practical use.

  On the other hand, various light control devices that do not use a polarizing plate have been proposed. As an example of a light control device that does not use a polarizing plate, a two-layer GH cell may be used. In this GH cell, the first layer absorbs a polarized light component in the same direction as a certain polarized light, and the second layer absorbs a polarized light component in a direction perpendicular to the polarized light. In addition, there is one that utilizes a phase transition of a cholesteric-nematic liquid crystal cell. Furthermore, there is a polymer scattering type that utilizes scattering of liquid crystal.

  However, in these light control devices that do not use a polarizing plate, the ratio of the absorbance when no voltage is applied to that when the voltage is applied, that is, the ratio of the optical density, is only about 5 as described above. For this reason, the contrast ratio of the light control device is small, which is insufficient to perform light control normally from a bright place to a dark place. Further, in the polymer scattering type light control device, the imaging performance of the imaging optical system is greatly deteriorated.

  In addition, depending on the liquid crystal system to be used, the light transmittance at the time of transparency may become dark. Therefore, when imaging with such a light amount, it is necessary to remove the light control device from the imaging optical system.

  On the other hand, in the present embodiment, since the polarizing plate 11 is used and can be taken in and out of the effective optical path, the light quantity can be increased, the contrast ratio can be increased, and the light quantity can be kept uniform. To do.

[Specific examples of camera system]
FIG. 25 shows an example in which the light control device 23 according to the embodiment is incorporated in a CCD (Charge coupled device) camera.

  That is, in the CCD camera 50, along the optical axis indicated by the alternate long and short dash line, the first group lens 51 and the second group lens (for zooming) 52 corresponding to the lens front group 15 and the third group lens corresponding to the lens rear group 16 are arranged. 53 and a fourth group lens (for focusing) 54 and a CCD package 55 are arranged in this order at an appropriate interval. The CCD package 55 includes an infrared cut filter 55a, an optical low-pass filter system 55b, and a CCD image pickup device 55c. Is stored. Between the second group lens 52 and the third group lens 53, a dimmer 23 comprising the GH cell 12 and the polarizing plate 11 according to the present invention is disposed near the third group lens 53 for light amount adjustment (light amount diaphragm). Installed on the same optical path. The focusing fourth group lens 54 is arranged to be movable between the third group lens 53 and the CCD package 55 along the optical path by a linear motor 57, and the zoom second group lens 52 is arranged in the optical path. Along the first group lens 51 and the light control device 23, the first group lens 51 and the light control device 23 are movably disposed.

  FIG. 26 shows a sequence algorithm of light transmittance control by the light control device 23 in this camera system.

  According to this embodiment, the dimming device 23 according to the present invention set between the second group lens 52 and the third group lens 53 can adjust the amount of light by applying an electric field as described above. The size can be reduced to substantially the size of the effective range of the optical path. Accordingly, it is possible to reduce the size of the CCD camera. Further, since the amount of light can be appropriately controlled according to the magnitude of the voltage applied to the patterned electrode, the conventional diffraction phenomenon can be prevented, and a sufficient amount of light can be incident on the image sensor to eliminate image blurring.

[Camera system drive circuit]
FIG. 27 is a drive circuit block diagram of the CCD camera. According to this, it has the drive circuit part 60 of the CCD image pick-up element 55c arranged on the light emission side of the light control device 23, the output signal of the CCD image pick-up element 55c is processed by the Y / C signal processing part 61, and the luminance Information (Y signal) is fed back to the GH cell drive control circuit unit 62, and the pulse voltage or pulse width is set as described above in synchronization with the basic clock of the drive circuit unit 60 by the control signal from this control circuit unit. A driving pulse controlled in two stages is obtained from the pulse generation circuit unit 63. The control circuit unit 62 and the pulse generation circuit unit 63 constitute a GH liquid crystal drive control unit 64 for stepwise control of the pulse voltage or pulse width.

  Even in a system different from this camera system, the light emitted from the light control device 23 is received by a photodetector (or photomultiplier), and the luminance information of the emitted light is fed back to the control circuit unit 62 from this, and the GH cell drive circuit In synchronization with the clock of the unit (not shown), it is possible to obtain a drive pulse in which the pulse voltage or pulse width is controlled stepwise from the pulse generation circuit unit.

  As described above, the present invention has been described according to the preferred embodiment. However, the above-described embodiment can be variously modified based on the technical idea of the present invention.

  For example, the structure and material of the liquid crystal element and the polarizing plate described above, the driving mechanism, the configuration of the driving circuit and the control circuit, and the like can be variously changed. Further, the drive waveform can be driven by any of a rectangular wave, a trapezoidal wave, and a sine wave, and the inclination of the liquid crystal changes according to the potential difference between both electrodes, and the light transmittance is controlled.

  In addition to the GH cell described above, a GH cell having a two-layer structure or the like can also be used. The position of the polarizing plate 11 with respect to the GH cell 12 is between the lens front group 15 and the lens rear group 16, but is not limited to this arrangement, and may be arranged at an optimum position based on the setting conditions of the imaging lens. That is, unless an optical element that changes the polarization state, such as a retardation film, is used, the polarizing plate 11 is placed at an arbitrary position on the subject side or the imaging element side, for example, between the imaging surface 17 and the rear lens group 16. be able to. Furthermore, the polarizing plate 11 may be disposed before or after a single lens (single lens) instead of the lens front group 15 or the lens rear group 16.

  Further, the number of iris blades 18 and 19 is not limited to two, and a larger number may be used, or conversely one. Further, although the iris blades 18 and 19 are overlapped by moving in the vertical direction, they may be moved in other directions and may be narrowed from the periphery toward the center.

  The polarizing plate 11 is attached to the iris blade 18, but may be attached to the iris blade 19.

  In addition, as the subject becomes brighter, the light is absorbed by the GH cell 12 after the light is adjusted by inserting and removing the polarizing plate 11, but conversely, the light adjustment by the light absorption of the GH cell 12 is first performed. You may do it. In this case, after the transmittance of the GH cell 12 is lowered to a predetermined value, light control is performed by inserting and removing the polarizing plate 11.

  Further, the mechanical iris is used as means for taking the polarizing plate 11 in and out of the effective optical path 20, but the invention is not limited to this. For example, the polarizing plate 11 may be taken in and out by directly installing a film with the polarizing plate 11 on the drive motor.

  In the above example, the polarizing plate 11 is taken in and out of the effective optical path 20, but it is of course possible to fix the position in the effective optical path.

  The light control device of the present invention can also be used in combination with other known filter materials (for example, organic electrochromic materials, liquid crystals, electroluminescence materials, etc.).

  Furthermore, the light control device of the present invention can be widely applied to various optical systems such as an electrophotographic copying machine and an optical communication device in addition to the optical aperture of the imaging device such as the CCD camera described above. Is possible. Furthermore, the light control device of the present invention can be applied to various image display elements that display characters and images in addition to the optical filter.

  The present invention is suitable for a light control device for adjusting the amount of incident light to be emitted, an imaging device using the same, and a driving method thereof.

DESCRIPTION OF SYMBOLS 1, 11 ... Polarizing plate 2, 12 ... GH cell, 3 ... Positive type liquid crystal, 4 ... Positive type dye molecule,
5 ... incident light, 13 ... negative type liquid crystal, 15, 16 ... lens group, 17 ... imaging surface,
18, 19 ... Iris blades, 20 ... Effective optical path, 22 ... Opening, 23 ... Light control device,
50 ... CCD camera, 51 ... 1 group lens, 52 ... 2 group lens, 53 ... 3 group lens,
54 ... 4 group lens, 55 ... CCD package, 55b ... Optical low-pass filter,
55c ... CCD image sensor, 60 ... CCD drive circuit unit, 61 ... Y / C signal processing unit,
62 ... control circuit section, 63 ... pulse generation circuit section,
64... Stepwise control unit for pulse voltage or pulse width (GH liquid crystal drive controller)

Claims (20)

  A light control device comprising: a liquid crystal element; and a pulse control unit that changes the transmittance of light incident on the liquid crystal element from a current transmittance to a target transmittance by a driving pulse controlled in at least two stages.   The light control device according to claim 1, wherein a pulse voltage of the drive pulse is controlled in at least two stages.   The light control device according to claim 1, wherein a pulse width of the drive pulse is controlled in at least two stages.   The light control device according to claim 1, wherein the drive pulse is extracted in synchronization with a clock of a drive circuit unit.   The luminance information of the emitted light is fed back to the control circuit unit, and the drive pulse is obtained in synchronization with the clock of the drive circuit unit by a control signal from the control circuit unit. Dimming device.   The light control device according to claim 1, wherein the liquid crystal element is a guest-host type liquid crystal element.   The light control device according to claim 6, wherein a host material of the liquid crystal element is a negative or positive liquid crystal having a negative or positive dielectric anisotropy.   The light control device according to claim 6, wherein the guest material of the liquid crystal element is composed of a positive or negative dichroic dye molecule having positive or negative light absorption anisotropy.   A light control device including a liquid crystal element and a pulse control unit that changes the transmittance of light incident on the liquid crystal element from a current transmittance to a target transmittance by a driving pulse controlled in at least two stages is an imaging system. An imaging device arranged in the optical path.   The imaging apparatus according to claim 9, wherein a pulse voltage of the drive pulse is controlled in at least two stages.   The imaging device according to claim 9, wherein a pulse width of the drive pulse is controlled in at least two stages.   The imaging device according to claim 9, wherein the drive pulse is extracted in synchronization with a clock of a drive circuit unit.   The drive circuit unit is a drive circuit unit of an image sensor arranged on the light emitting side of the light control device, and an output signal of the image sensor is fed back to the control circuit unit as luminance information, The imaging apparatus according to claim 12, wherein the drive pulse is obtained in synchronization with a clock of the drive circuit unit by a control signal.   The imaging apparatus according to claim 9, wherein the liquid crystal element is a guest-host type liquid crystal element.   The imaging apparatus according to claim 14, wherein a host material of the liquid crystal element is a negative or positive liquid crystal having a negative or positive dielectric anisotropy.   The imaging device according to claim 14, wherein the guest material of the liquid crystal element is composed of a positive or negative dichroic dye molecule having positive or negative light absorption anisotropy.   A driving method for a light control device, wherein when a liquid crystal element is driven, the transmittance of light incident on the liquid crystal element is changed from a current transmittance to a target transmittance by a driving pulse controlled in at least two stages.   The method for driving a light control device according to claim 17, wherein a pulse width of the drive pulse is controlled in at least two stages.   When driving an imaging apparatus in which a liquid crystal element is arranged in the optical path of an imaging system, the transmittance of light incident on the liquid crystal element is changed from the current transmittance to a target transmittance by a driving pulse controlled in at least two stages. The driving method of the imaging apparatus.   The driving method of the imaging apparatus according to claim 19, wherein a pulse width of the driving pulse is controlled in at least two stages.

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