JP2545114B2 - Imaging device - Google Patents

Description

The present invention relates to an image pickup apparatus for detecting an image or the like using a solid-state image pickup element and obtaining an image signal.

[Prior Art] Conventionally, a charge transfer device (hereinafter referred to as CCD) has been put into practical use as a solid-state image pickup device used for an image pickup device such as a video camera, an electronic still camera, or an image detection device such as a facsimile.

This CCD is composed of an image pickup section in which a plurality of light receiving elements for accumulating signal charges corresponding to the intensity of the emitted light are arranged, and a transfer section for accumulating and transferring the charges of the image pickup section,
An interline transfer type CCD (hereinafter referred to as IT type CCD) and a frame transfer type CCD (hereinafter referred to as FT type CCD) are known due to the difference in the charge transfer method. The IT-type CCD consists of parallel rows of light receiving elements in a line and registers in the non-irradiation area.The charges accumulated in the light receiving elements are transferred by the registers in the vertical rows and read out as image signals by the registers in the horizontal rows. Be done. Also FT method CC
In D, the image pickup section in which a plurality of light receiving elements are arranged and the transfer section shielded from light are separately arranged, and the signal charges accumulated in the light receiving element are stored in the storage section of the transfer section, and the horizontal transfer section stores the storage section. The signal charges temporarily stored in the memory are transferred line by line to the output unit at high speed, and the signal charges are read from the output unit.

An image pickup device using such a CCD has excellent characteristics such as less noise and a smaller size than an image pickup tube. However, in these CCDs, there is a phenomenon called vertical smear in which other image charges are mixed during charge transfer, and band-shaped bright parts appear above and below the spot, which is a strong image light.
This vertical smear is a phenomenon that occurs because light is irradiated during the charge transfer period.In the IT type CCD, this vertical smear is caused by light sneaking into the lower side of the vertical register, and also in the FT type CCD. Occurs when the signal charge is transferred to the storage section, that is, because the image pickup section is irradiated with light during the frame transfer period.

A method of reducing the vertical smear described above is disclosed in Technical Report of the Television Society of Japan, TEBS94-5, ED774 (1984.2) and Electronic Materials, pp. 123 to 127, "1/2 type CCD solid-state image sensor by FT method". As described above, in the FT CCD, a method of increasing the frame transfer frequency, or
A method has been proposed in which a vertical smear component is detected and the video signal is corrected by this.

However, the former requires a drive circuit that operates at a high speed of several MHz and has a large drive capacity.
Since the method is to detect the vertical smear amount before the field and remove this vertical smear amount in the next field, it is not possible to follow an image with fast movement. Both of them have a new problem that the electronic circuit becomes complicated and large, and the cost becomes high.

Therefore, a method of blocking the light irradiated to the image pickup unit of the CCD during the frame transfer period has also been proposed. As a method using this method, as disclosed in JP-A-58-116879, a mechanical or electronic shutter means is arranged in front of the image pickup section of the CCD, and this shutter means is used for the transfer period. There is known an image recording apparatus that controls a light blocking state.

[Problems to be Solved by the Invention] However, as described above, in the case where the shutter unit is provided on the front surface of the image pickup unit of the CCD, the operation speed is slow when the mechanical shutter is used as the shutter unit, and However, it is difficult to control the opening and closing in synchronization with the frame transfer period, and there is a drawback that mechanical noise is generated and noise is generated by a driving device or the like. on the other hand,
An electronic shutter having high-speed response characteristics required for the shutter means and having high contrast has not been put into practical use, and it is extremely difficult to prevent vertical smear by using the electronic shutter. .

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an image pickup apparatus in which vertical smear is prevented by a simple configuration without complicating a CCD drive circuit.

[Means for Solving the Problems] In order to solve the problems described above, the present invention provides an imaging unit in which a plurality of light receiving elements that store charges according to incident light are arranged, and signal charges stored in the light receiving elements. And a liquid crystal shutter disposed in the optical path of the light incident on the solid-state image sensor,
The liquid crystal shutter is arranged so as to face each other with the electrode formation surface, which has been subjected to an alignment treatment in a predetermined direction inside, and the liquid crystal enclosed between the pair of substrates, and arranged outside the liquid crystal. The liquid crystal shutter is optically opened / closed by applying a voltage between the electrodes formed on the pair of substrates of the liquid crystal shutter by a driving means.

[Operation] According to the present invention, a birefringent liquid crystal shutter is arranged in the optical path of light incident on a solid-state image sensor, and a drive signal is supplied to the liquid crystal shutter to obtain the effect of liquid crystal birefringence. A pair of polarizing plates sandwiching the liquid crystal is used to open and close as an optical shutter. Since the liquid crystal shutter operates at a high speed and the change in optical characteristics due to the electro-optical effect of liquid crystal is large, the contrast between the open state and the closed state of the shutter is large, and the leakage light in the closed state is extremely small. . Therefore, it is possible to reliably block the incident light on the solid-state image sensor in the shutter closed state,
Also, since the transmittance of incident light is high when the shutter is open,
Vertical smear noise can be sufficiently reduced without affecting the video signal.

EXAMPLES Specific examples of the present invention will be described in detail below with reference to the drawings. As shown in FIG. 1, the image pickup device of the present invention comprises, in front of the image pickup portion of a solid-state image pickup device 1 made of an IT type CCD or an FT type CCD, a lens and a diaphragm for forming an image on the image pickup portion. An optical system 2 is arranged, and a liquid crystal shutter 3 driven at two frequencies is arranged between the solid-state imaging device 1 and the optical system 2. Solid-state image sensor 1
Are driven by a CCD drive circuit 4 which supplies various clock signals, and the liquid crystal shutter 3 is driven by a shutter drive circuit 5 which selectively outputs a high frequency signal f _H and a low frequency signal f _L. The CCD drive timing signal φ ₁ output from the CCD drive circuit 4 is supplied to the shutter drive circuit 5 as a shutter synchronization signal φ ₂ whose phase has been advanced by the phase shift circuit 6, and the shutter drive circuit 5 uses the shutter drive circuit 5. The high frequency signal f _H and the low frequency signal f _L are selectively output according to the synchronization signal φ ₂ .

The liquid crystal shutter 3 is, as shown in FIG. 2 and FIG.
A pair of transparent substrates 11 and 12 are arranged to face each other, and transparent electrodes 13 and 14 formed on substantially the entire inner surfaces of the pair of transparent substrates 11 and 12 facing each other, and the transparent electrodes 13 and 14, respectively.
Opaque metal electrodes 15 and 16 are deposited on the upper part of the transparent substrate 11 and 12 except the shutter effective area A for controlling transmission and blocking of light incident on the solid-state image sensor 3. Part of the extended transparent electrodes 13 and 14 and the metal electrodes 15 and 16 form electrode terminal portions 15a and 16a. The transparent electrode 1
An alignment film 1 is formed on each of the shutter effective areas A of 3 and 14.
7 and 18 are formed, and the alignment films 17 and 18 are each the second
As shown by arrows a and a ₁ in the figure, rubbing treatments are performed in parallel and in opposite directions. Around the pair of transparent substrates 11 and 12 facing each other, a sealing material 19 is formed along the edges where they face each other, and the pair of transparent substrates 11 and 12,
The liquid crystal 20 is sealed between the pair of transparent substrates 11 and 12 while bonding 12 together. A pair of polarizing plates 21 and 22 are arranged outside the pair of bonded transparent substrates 11 and 12, and the polarizing axes (transmission axis or absorption axis) of these polarizing plates 21 and 22 are respectively. As shown by arrows b and b ₁ in FIG. 2, they are parallel to each other and intersect at about 45 ° with respect to the rubbing direction of the alignment films 17 and 18.

The liquid crystal 20 has physical properties shown in Table 1 below.
Moreover, as shown in FIG. 4, the crossover frequency f _{C at} which the permittivity becomes 0
It has positive dielectric anisotropy for low frequency electric field F _L of lower frequency and negative dielectric anisotropy for high frequency electric field F _{H of} higher frequency than crossing frequency f _C. It is a liquid crystal for driving two frequencies.

Then, the liquid crystal 20 sealed between the transparent substrates 11 and 12 is formed.
Are homogeneously aligned in the initial state where no voltage is applied between the transparent electrodes 13 and 14 facing each other, and are selectively supplied between the transparent electrodes 13 and 14 facing each other from the shutter drive circuit 5 described above. According to the low frequency voltage f _L and the high frequency voltage f _H , the orientation of the liquid crystal molecules is the orientation film 1 on the transparent substrates 11 and 12.
The orientation is controlled to be substantially perpendicular to 7 and 18, and the orientation is oblique. That is, when a voltage is applied between the transparent electrodes 13 and 14 facing each other and a low-frequency electric field _FL is applied to the liquid crystal interposed between them, the liquid crystal molecules have their long axes in the transparent substrates 11 and 12 due to the electric field. Arrange in a direction almost perpendicular to the surface of. In this case, since the linearly polarized light that has passed through one of the polarizing plates 21 is incident substantially parallel to the direction of the optical axis of the liquid crystal 20, the state of the linearly polarized light is transmitted through the liquid crystal layer without any change, and the outgoing side Through the other polarizing plate 22. Therefore, when the low frequency electric field _FL is applied to the liquid crystal 20, the liquid crystal shutter 3 is in the light transmitting state (ON). The liquid crystal 20
When a high frequency electric field F _H is applied to the liquid crystal molecules, the liquid crystal molecules are obliquely oriented, and the value of the birefringence Δn changes according to the inclination of the liquid crystal molecules. The linearly polarized light that has passed through the incident-side polarization plate 21 is incident on the liquid crystal layer from a direction intersecting the optical axis of the liquid crystal 20, so that it becomes elliptically polarized light due to birefringence, and the direction of the major axis of the ellipse rotates. . Since the transmittance of the liquid crystal shutter 3 at this time depends on the value of the seat Δn · d of the birefringence Δn and the layer thickness d, the application time of the high frequency electric field F _H is adjusted to tilt the liquid crystal distribution. The transmittance is changed by controlling the inclination angle of. In this way, the liquid crystal shutter 3 applies a high frequency electric field F _H, the inclination angle of the liquid crystal molecules transmittance shows the first minimum value is a resultant state light blocking state. Therefore,
The liquid crystal shutter 3 is controlled to a light blocking state (OFF) when the high frequency electric field F _H is applied to the liquid crystal 20 for a predetermined time.

The operation of the above embodiment when an FT CCD is used as the solid-state image sensor 1 will be described below.

As shown in FIG. 5, the FT CCD has an image pickup section 1a, an image pickup section 1a, a storage section 1b for temporarily storing the charge that is photoelectrically converted by the image pickup section 1a, and an accumulation section 1b. The horizontal transfer section 1c outputs the accumulated charges as an image signal. This FT type CCD operates according to the CCD drive timing signal φ _{1 as} shown in the timing chart of FIG. In the CCD drive timing signal φ ₁ shown in the figure, the image formed by the optical system on the image pickup unit 1a of the CCD 1
Video signal of the screen, the video photoelectric conversion period t _a for photoelectric conversion, and in every field t _f consisting frame transfer period t _s to transfer the converted charge to the storage section 1b
Output from CCD1. A drive signal is supplied to the CCD ₁ from the CCD drive circuit 4 in accordance with the CCD drive timing signal φ ₁ , and the above-mentioned photoelectric conversion, frame transfer, and horizontal transfer are performed. The CCD drive timing signal φ ₁ is a CCD
It is supplied from the drive circuit 4 to the phase shift circuit 6, and the phase shift circuit 6 changes the phase of the CCD drive timing signal φ ₁ at time t.
_The shutter synchronization signal φ ₂ advanced by ₁ is supplied to the shutter drive circuit 5. The shutter drive circuit 5 has a period t _off corresponding to the frame transfer period t _s of the CCD drive timing signal φ _1.
An OFF signal (f _H ) for turning off the liquid crystal shutter 3 is supplied to the crystal shutter 3 during the operation, and the liquid crystal shutter 3 is turned _on during the t _on period corresponding to the photoelectric conversion period t _a of the CCD drive timing signal φ _1. ON signal for operating ON (f _L )
Is supplied to the liquid crystal shutter 3. The liquid crystal shutter 3 to which the OFF signal is given during the period t _off of the shutter synchronizing signal φ ₂ is changed from the ON state to the CC operation as shown in the shutter operation state diagram of FIG.
The CCD frame transfer is started from the time when the liquid crystal shutter 3 is closed by about 50% by starting t ₁ earlier than the frame transfer period t _{s of} D, and when the liquid crystal shutter 3 is closed by about 50%, it is almost completely turned off after the time t _3. Become. Then, the opening operation of the shutter is started earlier by the time t ₂ than the time when the frame transfer of the CCD ends.
At about 50% open, the frame transfer of the CCD is completed, and after a lapse of time t ₄ , the shutter is turned on almost completely, and at this time, photoelectric conversion of the CCD is performed. By repeating the above-described operation, the CCD sequentially outputs the images formed on the image pickup unit 1a as electrical video signals. In this case, the liquid crystal shutter is turned off during the frame transfer period, and the light radiated to the image pickup unit is blocked, so that the electric charge generated by the light irradiation during the frame transfer period is not added to the image signal as noise.

A specific applied electric field for driving the above-mentioned liquid crystal shutter and its operation characteristics are shown in FIGS. 8 and 9.
FIG. 8 shows the liquid crystal 20 of the liquid crystal shutter 3 for continuously performing ON, OFF, and ON operations in correspondence with the CCD photoelectric conversion period, the frame transfer period, and the subsequent photoelectric conversion period. The waveform of the electric field applied to the is shown. Here, the frequency of the high frequency electric field F _H is 200 KHz, and the frequency of the low frequency electric field F _L is 2 KHz. To obtain the ON state of the liquid crystal shutter 3, an on electric field is continuously low frequency electric field F _L to the liquid crystal 20 is applied, about 0.25msec applying a high frequency electric field F _H in accordance with the OFF operation timing of the shutter synchronization signal phi ₂ And then continue
An off electric field in which no electric field is applied for 0.25 msec is applied to the liquid crystal. By applying such a high frequency electric field F _H of the off electric field, the liquid crystal molecules are obliquely aligned and become a light blocking state, and the light blocking state is maintained in the subsequent non-electric field state. By on-field of a low frequency electric field F _L again, the liquid crystal shutter 3 is turned on.

FIG. 9 shows the operating characteristics of the liquid crystal shutter 3 when the electric field shown in FIG. 8 is applied to the liquid crystal 20. As shown in the figure, the liquid crystal shutter 3 of the present embodiment operates at an extremely high speed with a response speed from the ON state to the OFF state of about 0.25 msec and a response speed from the OFF state to the ON state of about 0.15 msec. To do. Therefore, it is possible to block the incident light on the imaging unit 1a during an extremely short time during the CCD frame transfer period.

As described above, the liquid crystal shutter 3 used in this embodiment is turned on and off at an extremely high speed.
In this embodiment as well, there is a delay in the on / off operation. Therefore, in this embodiment, the off operation of the liquid crystal shutter 3 is started earlier by the time t ₁ than the start time of the frame transfer of the CCD, and the time t is longer than the end time of the frame transfer. The ON operation of the liquid crystal shutter 3 is started earlier by ₂ . It is preferable that the times t ₁ and t _{2 be} the times until the transmittance reaches about 50% of the transmittance in the ON state in the ON / OFF operation of the liquid crystal shutter 3. For example, in the above-described embodiment, the time is 0.05 msec. It is desirable to be in the range of ~ 0.3msec.

FIG. 10 shows the spectral transmittance characteristics of the liquid crystal shutter 3 driven in this manner in the ON state. As is clear from the figure, the liquid crystal shutter 3 has a substantially flat spectral characteristic over a wavelength range of 400 nm to 700 nm and is optimal as an optical shutter for CCD. As described above, according to this embodiment, since the contrast of the liquid crystal shutter is 20, the vertical smear noise can be reduced to 1/20.

Further, the liquid crystal shutter 3 used in the present invention is not limited to the above-mentioned embodiment, and may be any liquid crystal shutter 3 utilizing the birefringence of liquid crystal. For example, the liquid crystal 20 exhibits a smectic C ^* phase and has a ferroelectric property. A smectic liquid crystal having properties can be used. This ferroelectric liquid crystal shutter operates at an extremely high speed and has a high contrast between the on state and the off state, so it is possible to almost completely block the light incident on the CCD during the charge transfer period of the CCD, and to prevent vertical smear noise. Can be prevented.

[Advantageous Effects of the Invention] The present invention provides the following:
By using a birefringence effect, a liquid crystal shutter that can respond at high speed and has a flat spectral characteristic is arranged, and the liquid crystal shutter is turned on and off to block light transmission. Turn on at extremely high speed,
Since the off operation is performed, incident light can be transmitted and blocked in synchronization with the time when light irradiation to the CCD is unnecessary, and thus it is possible to effectively prevent generation of vertical sminoeyes.

[Brief description of drawings]

1 is a schematic configuration diagram showing an embodiment of the present invention, FIG. 2 is a plan view of a liquid crystal shutter used in the present invention, FIG. 3 is a sectional view of the liquid crystal shutter shown in FIG. 2, and FIG. FIG. 6 is a graph showing the frequency dependence of dielectric anisotropy for the liquid crystal used in this example, FIG. 5 is a conceptual diagram showing the schematic configuration of an FT CCD, and FIG. 6 is the operation of this example. FIG. 7 is a timing chart showing the timing, FIG. 7 is an operation state diagram of the liquid crystal shutter in this embodiment, FIG. 8 is a waveform diagram of a specific applied electric field for operating the liquid crystal shutter in this embodiment, and FIG. FIG. 10 is an operation characteristic diagram of the liquid crystal shutter in the present embodiment, and FIG. 10 is a spectral characteristic diagram of the liquid crystal shutter in the on state in the present embodiment. 1 ... Solid-state image sensor, 2 ... Optical system, 3 ... Liquid crystal shutter, 4 ... CCD drive circuit, 5 ... Shutter drive circuit, 6
...... Phase shift circuit, 11, 12 …… Transparent substrate, 13, 14 ……
Transparent electrodes, 17, 18 …… Alignment film, 20 …… Liquid crystal, 21, 22 ……
Polarizer.

Claims (5)

(57) [Claims] 1. A solid-state image sensor having an image pickup section in which a plurality of light receiving elements for accumulating charges according to incident light are arranged, and a transfer means for transferring the charges accumulated in the image pickup section, and light incident on the image pickup element. A liquid crystal shutter disposed in the optical path of
The liquid crystal shutter has a drive means for controlling the opening and closing of the liquid crystal shutter, and the liquid crystal shutter includes a pair of substrates arranged to face each other with an electrode forming surface that has been subjected to an alignment treatment in a predetermined direction inside, and a pair of these substrates. A liquid crystal sealed between the substrates, and a pair of polarizing plates arranged outside the pair of substrates, at least one of which has a polarization axis in a direction inclined with respect to the alignment treatment direction. Image pickup device. 2. A liquid crystal for two-frequency driving having a dielectric dispersion characteristic in which positive and negative of dielectric anisotropy are inverted between a high frequency electric field and a low frequency electric field. The imaging device according to the item. 3. A liquid crystal shutter drive circuit, wherein the drive means applies an off electric field to the liquid crystal for controlling the liquid crystal shutter in a light blocking state during a transfer period for transferring the charges accumulated in the image pickup section of the solid-state image pickup device. The imaging device according to claim 1, further comprising: 4. The image pickup device according to claim 3, wherein the off electric field is a high frequency electric field. 5. The off electric field is applied to the liquid crystal at a time earlier than a time when the charge transfer is started by a predetermined time according to a response speed of the liquid crystal shutter. The image pickup device according to item 3 or 4.