JPH0546931B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an image forming apparatus equipped with a liquid crystal shutter array in which n (rows) by m (columns) microshutter groups are arranged in a matrix and a printer head having a light source.

Until now, liquid crystal display elements and liquid crystal shutter arrays are well known, in which n scanning electrodes and m signal electrodes are arranged in a matrix, and many pixels and shutter openings are formed of liquid crystal, which is a capacitive load element. ing. The driving method for this liquid crystal element is time-division driving, in which address signals are selectively and periodically applied to the scanning electrode group, and predetermined information signals are selectively applied in parallel to the signal electrode group in synchronization with the address signal. It has been adopted. In this driving method, as the number of time divisions increases, as shown in equation (1) below, V _ON (on signal)/V _OFF (off signal) approaches 1, and the opening/closing efficiency of the liquid crystal elements that make up the pixels deteriorates. Become. For this reason, especially in the case of a liquid crystal shutter array, it is not possible to provide an optical signal with a sufficient S/N ratio, and when this is used in the image exposure section (printer head) of an electrophotographic camera, it is not possible to form a good image. It has its drawbacks.

(In the formula, 1/N: duty ratio, 1/a: bias ratio, V _O applied voltage) In other words, the best driving condition is 1/1 duty,
In other words, this is stake driving, but this driving method requires that each pixel be controlled by a driver circuit. For example, A-4 (Japanese Industrial Standards)
In the case of a liquid crystal shutter array that can generate light spots with a pixel density of 16 dots/mm in the short width of of
This will require an IC. For this reason, the static driving method has the disadvantage that it is not suitable for driving a liquid crystal shutter array having a high density of pixels or shutter openings.

That is, an object of the present invention is to achieve a sufficiently high S/N
An object of the present invention is to provide an image forming apparatus that is equipped with a printer head that generates optical signals having a high ratio under multi-dimensional time-divisional driving, and that can create high-contrast copy images.

The present invention provides an exposure light source, a microshutter composed of a pair of opposing electrodes, and a nematic liquid crystal disposed between the pair of electrodes, which are arranged along a plurality of rows and columns in the exposure optical path of the exposure light source. A group of microshutters is formed, and the microshutter group generates an optical signal by selectively controlling the light beam in the exposure optical path of the exposure light source to either a light blocking state or a light transmitting state. In an image forming apparatus, a thin film transistor is connected to each microshutter, and a gate of the thin film transistor is gated. It has a wiring structure in which the sources of the thin film transistors are connected by gate lines for each column, and the sources of the thin film transistors are connected by data lines for each column, and the electric field strength applied to the gate insulating film in the channel part of the thin film transistor is 5×10 ⁵ V/ The gate insulating film is formed of a silicon dioxide film with a film thickness of 6000 Å or more so that the film thickness is less than 6000 Å, and a scanning signal is applied to the gate line, and image information is transmitted to the data line in synchronization with the scanning signal. The image forming apparatus is characterized in that it applies electrical signals corresponding to the image forming apparatus.

The present invention will be explained below with reference to the drawings.

According to the experimental studies conducted by the present inventors, when the variation of the threshold voltage Vth when a DC gate voltage Vg _DC is applied per unit time (hr) is ΔVth, the gate voltage changes as shown in Fig. 1. When the electric field strength Eg of the insulating film exceeds the low level gate voltage Vgl (approximately 40 V to 60 V), ΔVth increases exponentially. This indicates that the lifetime is rapidly shortened when the gate voltage Vg exceeds Vgl.
However, when the electric field strength that ignites the gate insulating film is set to 5×10 ⁵ V/cm or less, the gate voltage Vg can be increased without shortening the TFT lifetime, and therefore a sufficient S/N can be achieved. Another feature is that it can generate an optical signal with a signal-to-noise ratio, preferably a signal-to-noise ratio of 5 or more, preferably a signal-to-noise ratio of 5 or more.

TFT is Vg−Vth>Vs(Vg; gate voltage,
Vth: threshold voltage, Vs: data voltage) is used in the non-saturation region, and in this region Vx(t)/Vs (Vx(t);
The output voltage) is expressed by the following equation (2) before the start of charging (t=0).

Vx(t)/Vs=1−Lexp[−(1−m)t/τ]/1−Lme
xp[-(1-m)t/τ]...(2) In the formula, m, τ (TFT charging time constant) and L (liquid crystal residual voltage potential) are expressed by the following formulas (3), (4) and (Five)
It is expressed as

m=Vs/2(Vgh-Vth)-Vs...(3) τ=m/KVsC...(4) L=1-Vx(0)/Vs/1-mVx(0)/Vs...(
5) In the formula, C is the load capacitance, Vgh is the positive gate voltage, t is the gate on time, and K is expressed by the following formula (4)'.

K=W/2L・1/dinsεo・εs・μ……(4) εo: Vacuum conductivity [F/cm] εs: Insulating film specific conductivity μ: Mobility [cm ² /Vsec] dins: Insulating layer Film thickness [cm] L: Channel length [cm] W: Channel width [cm] According to formula (2), to increase the output power Vx(t),
This can be achieved by increasing the data voltage Vs, but it is necessary to tighten the gate voltage Vg from the relational expression Vg-Vth>Vs.

However, increasing the gate voltage Vg will shorten the lifetime of the TFT, and for this reason, a practical liquid crystal shutter array, especially an output voltage Vx(t) of 20 volts or more can be obtained without shortening the lifetime of the TFT. There is a need for a time-division driving method for TFT matrices that can achieve this goal.

Therefore, the inventors of the present invention have conducted extensive studies on the above-mentioned points, and have found that it is possible to apply a high voltage (e.g., 30 volts or more, especially 40 to 60 volts) gate voltage Vg without shortening the TFT lifetime. We were able to find a time-division driving method for TFT matrices that allows for

That is, the present invention makes it possible to maintain the lifetime of the TFT by setting the thickness of the gate insulating film so that the electric field strength in the gate insulating film of the TFT is 5×10 ⁵ V/cm or less when the gate is on. can. In a preferred embodiment of the present invention, the gate insulating film is formed of 6000 Å silicon nitride (specific conductivity 6.6) doped with hydrogen atoms, and the semiconductor film is formed of 2000 Å
When the TFT was formed using amorphous silicon (specific dielectric constant of 12), no shortening of the TFT lifetime was observed even when a gate voltage Vg of 40 to 60 volts was applied.

FIG. 2 is a cross-sectional view schematically showing a liquid crystal mode that can be used in the present invention. In the figure, polarizing plates 26 and 27 are arranged in a crossed nicol state, and two polarizing plates 21 and 22 are also arranged. The liquid crystal 25 is aligned by a method such as a rubbing process so that the initial alignment direction of the liquid crystal 25 is at 45 degrees with respect to the polarization directions of the liquid crystals 26 and 27. At this time, a nematic liquid crystal (NP type liquid crystal) having positive induced anisotropy is used as the liquid crystal 25. When a voltage is applied to the common electrodes 23 and 24a, the molecular axis of the liquid crystal 25 between these electrodes is oriented in the direction of the electric field, and a dark state (light-blocking state) with respect to the incident light I is formed. On the other hand, electrodes 23 and 24b
When the voltage is lower than the threshold voltage of the liquid crystal 25, the molecular axis of the liquid crystal 25 between the electrodes is aligned in the initial alignment direction, that is, in the rubbing direction. At this time, the incident light I
becomes transmitted light T, and a bright state (light transmission state) is formed.

FIG. 3A is a cross-sectional view of a liquid crystal element used in the present invention, showing an embodiment in which a TFT is formed on a substrate 301 (glass, plastic, etc.).
The TFT has a gate electrode 302 connected to a gate line for applying a scanning signal, a source electrode 303 connected to a data line for applying an information signal, and a drain electrode 30 for extracting this data signal as an output signal.
It has three terminals of 4. Further, the drain electrode 304 is connected to a segment electrode 307 forming a microshutter section. Gate electrode 30
When a scanning signal is applied to the source electrode 30, the resistance of the amorphous silicon film 305 decreases.
3 and the drain electrode 304 are in a connected state.

In the TFT used in the present invention, 6000 Å silicon dioxide (specific dielectric constant 6.6) doped with hydrogen atoms is used as the gate insulating layer 306 sandwiched between the gate electrode 302 and the amorphous silicon film 305. This silicon dioxide film is composed of a chromium/aluminum laminated vapor-deposited film that will become the gate electrode 302 and an ITO film that will become the segment electrode 307.
A vapor-deposited film of (Indinm Tin Oxide) is formed over the entire surface of the substrate 301 patterned in a predetermined shape under glow discharge. Further, the drain electrode 304 and the segment electrode 307 are connected through a through hole 308 provided in the silicon dioxide film.

Substrate 3 with such TFT and segment electrodes
Further, an insulating film 309 and an orientation control film 310 made of a silicon oxide film doped with hydrogen atoms are formed on the film 01. As this orientation control film 310, for example, a 1000 Å polyimide film is used.

The liquid crystal element used in the present invention has a nematic liquid crystal 313 (NP
(type liquid crystal) are sandwiched in the alignment state shown in FIG. A common electrode 31 is provided on the counter substrate 311.
Further, in the case of the liquid crystal shutter array described above, a light shielding film 314 made of a chromium/aluminum laminated vapor deposited film is formed on the counter electrode 312 to shield areas other than the openings to form a microshutter section. is layered on top. These common electrodes 312 and light shielding film 31
4, an alignment control film 315 is formed of polyimide or the like.

FIG. 3B is a sectional view schematically showing a liquid crystal shutter array used in the present invention. In the liquid crystal shutter array of the present invention, the TFT section 316 is on the same substrate 301' as the substrate 301 of the liquid crystal element 317,
Moreover, it is formed outside the liquid crystal element 317. In particular, the TFT 316 is connected to the substrate 301 of the liquid crystal element 317.
The common electrode 312 is preferably disposed outside a sealing member 318 made of an epoxy adhesive or the like formed to seal the liquid crystal 313 between the opposing substrate 311 and the common electrode 312. Further, the TFT 316 can be provided on an external circuit board (not shown) such as an IC circuit, separately from the substrate 301 of the liquid crystal element 317. Components having the same reference numerals as those in FIG. 3A in the drawings represent the same members. In the figure, 319 and 320 are crossed Nicol polarizers, and 312 is the semiconductor film 30 of the TFT 316 made of chromium, aluminum, etc.
5 represents a light-shielding film.

FIG. 4A shows a circuit of a TFT matrix substrate used in the liquid crystal shutter array of the present invention, and FIG. 4B shows a plan view thereof. TFT matrix is
TFT4011, 4012, 4013, in an array
4014, 4015, 4016, 4017, 40
It has a structure in which 18,... (TFT; 401) are arranged. The TFT 401 has gate lines 4021, 4022, which apply scanning signals to the gate electrodes,
4023, 4024 group 402, data lines 4031, 4 that apply information (data) signals to the source electrodes
032, . . . group 403 and the segment electrode 404 of the microshutter connected to the drain electrodes 4051, 4052, 4053, 4054 to which the data signal from the data line 403 is applied as an output signal.
1,4042,4043,4044,4045,
4046, 4047, 4048, . . . groups 404 are connected to each other.

In this embodiment, a TFT 40 is connected to the data line 4031.
11, 4012, 4013 and 4014 are commonly connected, and TFTs 4015 and 40 are connected to the data line 4032.
16, 4017 and 4018 are commonly connected. On the other hand, TFT4011 is connected to the gate line 4021,
4015 are commonly connected. Similarly, other gate lines are commonly connected to the TFT as shown. Although this embodiment clarifies the 4-order time-division driving method, the present invention
It is possible to adopt a multi-order time-division driving method such as second order, third order, fifth order or higher order.

In such a TFT matrix structure, there is no overlap between the gate electrode (and the lead-out gate electrode drawn out from the gate electrode to the gate line) and the segment connected to the drain electrode, and therefore the overlap Unnecessary capacity generated
C ₀ will not occur.

Furthermore, in this embodiment, the segment electrode groups 404 of the microshutter are arranged in a staggered manner, but this is because the microshutter section sequentially writes information in a time-division manner.
This is because information is written on a photosensitive drum (not shown), which is an image holding member that is constantly moving to 05, in a straight line within one frame.

FIG. 4C shows a sectional view taken along line A-A' in FIG. 4B. In the figure, an insulating film 407 is entirely covered on a gate line 4021 formed on a substrate 409, and intersecting gate lines 4022, 4
023 and 4024, and are connected to each other by a conductive film 410 through a contact hole 406.

An insulating film 408 is provided on these gate lines arranged to intersect with each other, and a data line 403 is formed on the insulating film 408.
1 is placed.

FIG. 5 shows a schematic configuration for applying optical signals to a photosensitive drum using a liquid crystal shutter array. However, the charger, developer, cleaning, etc. are omitted. 53 is a liquid crystal shutter array as described above, 51 is a photosensitive drum (alloy silicon photoreceptor, organic photoconductive photoreceptor), 54 is a light source such as a fluorescent lamp, 52 is a lens array such as a self-occurring lens, and 55 is a focusing lens. It is a light cover. The photosensitive drum 51 rotates in a sub-traverse direction 56 and responds to information signals by irradiating the surface of the photosensitive drum 51 with optical signals generated from a printer head section 57 consisting of a light source 54 and a liquid crystal shutter array 53. It is possible to form an electrostatic charge image. For this reason,
Compared to electrophotographic copying machines that emit optical signals generated by laser beams, the device can be made more compact, and there is no mechanical drive unit like the polygon scanner used in laser beam emitting methods. It has the advantage of being noiseless and reducing the requirements for strict mechanical accuracy.

Next, the shutter openings in the arrangement shown in Figure 4
An example of forming a dot pattern when performing quaternary time division driving using W ₁ , W ₂ , . . . will be described.

FIG. 6 shows a specific example of a time chart of the drive signal applied to the liquid crystal shutter array.
Here, G ₁ to _{G 4} are gate lines 4021, 4022,
The voltage waveform applied to 4023 and 4024, the potential
When V ₂ is applied, the TFT turns on and becomes conductive between the source electrode and the drain electrode. On the other hand, when a potential of _-V1 is applied, the TFT is in an off state, and the source electrode and drain electrode are in a cut-off state and electrically disconnected. Therefore, when the voltage applied to the gate electrode is _V2 , the potential of the segment electrode connected to the drain electrode of the TFT is
When the potential applied to the data line connected to the source electrode of the TFT changes, and then the voltage applied to the gate electrode is set to _-V1 , the potential applied to the data line immediately before is held at the segment electrode.

C is a voltage waveform applied to the common electrode, which is always held at potential O in this embodiment. S ₁ is a voltage waveform applied to the source electrode (data electrode), and as the openings W ₁ , W ₂ , ... are set to either on or off, a voltage is applied that changes the potential to 0 or V. Ru.

Next, the operation control of opening and closing the shutter will be explained, focusing on the opening _W1 .

At time T ₁₁ , the TFT 4 connected to the segment electrode 4041 of the microshutter section W ₁
The potential of the gate electrode connected to the gate line 4021 (G ₁ ) of 011 becomes V ₂ and the TFT 4011 is turned on. Time τ ₁₁ and τ ₁₂ (τ ₁₁ + τ ₁₂ = T ₁₁ )
Since the potential of the data electrode 4031 (S ₁ ) is V, the potential of the segment electrode 4041 of the microshutter section W ₁ is also approximately V. Therefore, at this time, the microshutter section _W1 is in an off state.
At the subsequent time τ ₁₃ , the potential of the gate electrode connected to the gate line 4021 (G ₁ ) becomes −V ₁ , so even if a voltage is applied to the data electrode 4031 (S ₁ ), the microshutter section W ₁ The segment electrodes can hold a potential V. τ ₁₃ =T ₁₂ +
T ₁₃ +T ₁₄ , T ₁₂ is connected to gate line 4022 (G ₂ ),
T ₁₃ is a period in which the voltage of V 2 is applied to the gate line 4023 (G ₃ ), and T ₁₄ is a period in which the voltage of V ₂ is applied to the gate line 4024 (G ₄ ). Therefore, T ₁₁ +T ₁₂ +T ₁₃ +T ₁₄ is 1
This is the frame period. the duration of the following frame period
At T ₂₁ , the potential of the gate electrode (G ₁ ) becomes V ₂ again.
TFT4011 is turned on. At time τ ₂₁ in the first half of this time T ₂₁ , the potential of the data electrode (S ₁ ) becomes V, voltage V is applied to the segment electrode of the microshutter section W ₁ , and in the subsequent second half time τ ₂₂ (TFT
), the potential of the data electrode (S ₁ ) becomes 0, so the potential of the segment electrode of the micro-shutter W ₁ changes to 0, and the continuing time τ ₂₃ (=T ₂₂ +T ₂₃ +T ₂₄ ), a potential of 0 is maintained. Therefore, since the voltage applied to the liquid crystal corresponding to the micro-shutter section _W1 is 0, the on-state (light-transmitting state) of the shutter is formed in one frame period as explained in FIG. .

|W ₁ -C| in FIG. 6 shows the voltage waveform applied between the segment electrode and the common electrode of the micro-shutter section W ₁ , that is, to the liquid crystal, in chronological order. According to this, the time τ ₁₂ + τ ₁₃ + τ ₂₁
Then, |W ₁ -C| becomes a potential difference V, and at time τ ₂₂ +τ ₂₃ of the next frame period, |W ₁ -C| becomes a potential difference 0. Micro shutter at this time
The change in transmittance in the time series of W ₁ is shown in Figure 6.
This is made clear in Tr ₁ . According to this diagram, during the period of time τ ₁₂ +τ ₁₃ +τ ₂₁ , the transmittance of the microshutter portion W ₁ is Trd (dark level), and during the period of time τ ₂₂ +τ ₂₃ +τ ₃₁ , the transmittance of the microshutter portion W 1 is Trd (dark level). The transmittance of part W ₁ gradually increases to Trl (bright level), and at τ ₃₁ of the next frame period |W ₁ −C| becomes V
In this case, it returns to Trd as shown in the figure.

Also, |W ₁ -C| in the figure is the micro shutter W ₂
It shows the potential difference in time series between the electrode and the common electrode, and Tr ₂ shows the change in transmittance at that time.

FIG. 7 shows the sequence for producing dots d ¹ ₁ and d ² ₁ of the light spot image. Among the dots, the dots in the first row (d ¹ ₁ , d ² ₁ , d ³ ₁ , d ⁴ ₁ , ...) correspond to the on and off states of the microshutter section W ₁ , and the dots in the second row ( d ¹ ₂ , d ² ₂ , d ³ ₂ , d ⁴ ₂ , . . . ) correspond to the on and off states of the micro-shutter section W ₂ . Also, each dot in each row is a micro shutter section.
It corresponds to W ₁ , W ₂ , W ₃ , W ₄ , ... Here, it is assumed that the dots d ¹ ₁ , d ¹ ₄ , d ² ₃ , d ³ ₁ , d ³ ₂ , d ⁴ ₂ and d ⁴ ₄ are at the dark level, and the other dots are at the bright level.
In the figure, 71 represents the main scanning direction, and 72 represents the sub-scanning direction.

In the time-division driving method of the present invention, when the micro-shutter section is operated by the fourth-order time-division drive as described above, the micro-shutter section is either in the on state (light transmitting state) or the off state (light blocking state) during one frame period. state) can be maintained.

That is, when forming dots at a dark level, the transmittance is set to the dark level (Trd) over the dot generation time for one line (τ ₁₂ +τ ₁₃ +τ ₂₁ ), and when forming dots at a bright level, the transmittance is set at the dark level (Trd) for one line. The transmittance can be kept at the bright level (Trl) over the dot generation time (τ ₂₂ +τ ₂₃ +τ ₃₁ ). The brightness ratio at this time,
In other words, the S/N ratio corresponds to the ratio of areas A and B in Figure 6, and the S/N ratio is significantly higher than that of the simple matrix method used in conventional liquid crystal shutter arrays. can be improved.

Further, in this embodiment, as shown in FIG. 6, during the initial period when the gate line is scanned, a voltage V is added to the information signal inputted in synchronization with this scanning signal. This is because when the voltage applied to the liquid crystal of the type shown in FIG. 2 is set to 0, the transmittance changes in a waveform with time as shown in FIG. 8. This phenomenon is generally called a light bouncing phenomenon. Therefore, according to FIG. 8, if one micro-shutter part continues to be in the on state for 3τ, the transmittance decreases after time t, so the transmittance after each write is different. There was a problem in which the brightness ratio (contrast of the printed image) of each dot varied. Therefore, in this embodiment, in order to make the transmittance of the shutter part in the ON state uniform during each write, the information signal input in synchronization with the scanning signal during the initial period during write is used as described above. When a voltage V is applied and a dark state is formed by forcibly applying the voltage V to the liquid crystal, even if the micro-shutter section continues to be in the on-state, the state will be changed again at the one-dot writing time τ shown in FIG. The transmittance of each dot in the on state can be made uniform.

Therefore, by setting this forced shutter closing time immediately before or after data writing,
A stable amount of transmitted light is always obtained both when the shutter is open and when the shutter is closed when writing data, making it possible to always obtain a printed image with stable contrast. Before applying the data signal for forming each dot to the data electrode, a signal can be applied so that a voltage is applied to the liquid crystal layer, regardless of whether the previous dot formation was at a bright level or a dark level. . At this time, the time when voltage is applied to the liquid crystal layer and the transmittance becomes sufficiently low is defined as τ ₁₁ , and at time τ ₁₂ the potential of the segment electrode 404 changes to the potential of the data electrode 403 via the TFT 401. need to make time.

According to experiments by the inventors, 20V is applied to an 8mm liquid crystal.
When applying a voltage of , the time τ ₁₁ is approximately 0.24 msec,
Therefore, it is sufficient that the time is 0.2 msec or more, and it is sufficient that the time τ ₁₂ is several tens of μsec, and the time τ ₁₁ +τ ₁₂ is approximately 0.3 m.
It turned out that it is sufficient if it is sec or more. Also, if the aperture (shutter part) density is 16 dots/mm and the image forming speed (process speed) is 50 mm/sec,
The dot formation time in the first row (τ ₁₁ +τ ₁₂ +τ ₁₃ ) is
Since the time is 1.25 msec, it has been found that the fourth-order time division drive can be realized by setting the time τ ₁₁ +τ ₁₂ to 1/4 of 1.25 msec, that is, 0.3125 msec. Furthermore, it was also found that the contrast ratio was improved to 6.5, and the amount of light at the bright level was more than double that of the conventional simple matrix drive method.

FIG. 9 shows another specific example of the time division driving method of the present invention. The specific example shown in FIG. 9 is a driving method suitable for further increasing the number of time divisions.

That is, in the driving example shown in FIG. ₆ , the gate minimum on time (data electrode voltage is Since the gate electrode is kept in the ON state during the time (τ ₁₁ +τ ₁₂ ) which is the sum of τ ₁₂ (minimum time required to obtain the desired voltage), there is a limit to increasing the number of time divisions. That is, when the opening density is 16 dots/mm and the process speed is 50 mm/sec, the number of time divisions is expressed by the following equation (6).

n = 1.25msec/τ ₁₁ (msec) + τ ₁₁ (msec) = 1.25
msec/0.2 msec+τ ₁₂ (6) At this time, several μsec to several tens of μsec is sufficient for the time τ ₁₂ , and the number of time divisions n is controlled by the time τ ₁₁ .

Therefore, as shown in Fig. 9, the time required to forcibly close the shutter opening (in our experiments, τ ₁₁ = 0.24 msec, liquid crystal thickness 8 μm, drive
40V) and gate lines G ₁ , G ₂ , G ₃ ,...Gn
A gate-on pulse is applied sequentially to (n: the number of time divisions), and a voltage V is applied to the data electrode in synchronization with this. Therefore, the microshutter section is sequentially turned off, and a selection signal (voltage 0 or V) is applied to the data electrode in the following frame period. That is, the first
During the scanning period _t1 , a voltage V is applied to the liquid crystal corresponding to the microshutter section (assuming the potential of the common electrode to be 0).
By applying this, all of the microshutter sections are turned off, and this first scanning period _t1 corresponds to a refresh period. The following second scanning period
At t ₂ , a voltage according to the data signal is applied to the data electrode in synchronization with the scanning signal (τ ₁ ) of the gate line, thereby setting a predetermined microshutter section in the on state or off state. This second scanning period _t2 corresponds to a data writing period.

In the specific example shown in FIG. 9, the refresh period and the data write period described above are provided alternately for driving, and furthermore, in this example, an arbitrary voltage application period is set between the data write period and the refresh period.
Although t ₃ is added, this arbitrary period t ₃ can also be omitted. The number of time divisions n when using such a driving method is expressed by equation (7).

n = frame period t ₁ (msec) / gate line scanning signal application period τ ₁ (msec) ...(7) Now, frame period t ₁ is 0.24 msec, gate minimum on time (scanning signal application period τ ₁ is If 5μsec, the number of time divisions can be up to 48.Also, the time series transmission of the shutter section switched by a TFT in which the terminals of the gate line _G1 and the data electrode _S1 are connected to the Tr in Fig. 9 is possible. (Trl: light level, Trd: dark level).

FIG. 10 shows another specific example. In the driving example shown in FIG. 9 described above, a gate-on pulse is always applied to one TFT during time _t1 , and the TFT is turned on, but in the present invention, the number of time divisions n is relatively small. (In the example shown in FIG. 10, the time division number n is 4). This allows the data pulse application time to be shortened.
That is, the micro shutter section shown in FIG.
As shown in the transmittance waveforms at W ₁ , W ₂ , W ₃ and W ₄ (corresponding to the apertures in Figure 4), the microshutter part W ₁ is driven first and the aperture part is driven last.
The driving time difference of _W4 can be shortened.

Now, if the lighting waveform of the light source is set as L in Figure 10, the energy of the transmitted light in the on state at each aperture at that time is W ₁ S, W ₂ N, W ₃ S, W ₄ S
The area is shown as . In this case, the openings W ₁ , W ₃ ,
W ₄ is in the shutter-on state, and the energy of the transmitted light from there is W ₁ S>W ₃ S>W ₄ S.As for this drive, as mentioned above, since the data pulse application time τ ₂ is short, This difference is almost negligible.
For example, the opening (shutter) density is 16 dots/
mm, the process speed is 50 mm/sec, and the minimum gate on time τ ₁ is 5 μsec to perform quaternary time-division driving, the data pulse application time τ ₂ is 20 μsec. On the other hand, the time τ ₃ is about 1 msec.
Therefore, the light energy due to this degree of time difference
The difference between W ₁ S, W ₃ S and W ₄ S can be almost ignored.

In this way, just before applying a refresh pulse (refresh pulse application period τ ₄ ) to the light source at the first gate, turn it on (or increase the light intensity only during this time)
As a result, the S/N ratio becomes W ₄ S/W ₂ N even in the worst case. We are currently using a light source with an emission center wavelength of approximately 650 nm, an aperture density of 16 dots/mm, a process speed of 50 mm/sec,
If the light source lighting time τL is 200 μsec, W ₁ S/W ₂ N
is more than 10 times as large. Also, data pulse application time τ ₂
By setting the S/N ratio (W ₄ S/W ₂ N)
However, as mentioned above, if the time τ ₂ is set so that the energy difference between W ₁ S and W ₄ S is small, the S/N ratio will be greatly improved, and the electrophotographic print image will always be stable. can be obtained. Note that time τ ₅ in the figure is an arbitrary period.

FIGS. 11 and 12 show driving examples representing another aspect of the present invention.

FIG. 11 shows the voltage waveforms applied to the gate line and the voltage waveforms applied to the data electrodes when the number of time divisions n is 8. The time τ corresponds to one pixel formation time and is calculated using equation (8).

τ=1/Vp・N...(8) (In the formula, Vp: Process speed mm/sec N: Number of pels) In Fig. 11, when the minimum gate on time of TFT is τ ₁ , τa≧τ ₁ , Time is divided by alternating τa and τb with τb > τ _1. During time τa, an electric field is always applied to the liquid crystal, the aperture is turned off, and a potential is refreshed to the aperture electrode to prevent transmittance fluctuation ( In the erase period, time τb is a data write period in which a potential for controlling the on/off state of the aperture is applied to the segment electrode of the microshutter section.

FIG. 12 shows a TFT matrix 1202 used in eight-order time-division driving, a gate line 1201 connected to the TFT, a data line 1202, and an opening W ₁ ,
Segment electrodes 1204 connected to W ₂ , . . . are shown. A voltage V ₂ is applied to a selected gate line among the gate lines 1201 only during a refresh period τa and a write period τb within one pixel formation time τ,
A voltage -V ₁ is applied to other unselected gate lines. That is, as shown in FIG. 11, a voltage V 2 is applied to the gate line G ₁ during the first refresh period τa, and the voltage V ₂ is applied to the gate line G ₁ during the following two sets of write period τb and refresh period τa. -V ₁ is applied, voltage V ₂ is applied to the gate G ₁ in the next write period τb, and the potential of the gate line G ₁ is set to -V ₁ during the remaining one pixel write time. The next gate line _G2 has a waveform that is temporally shifted from the gate signal applied to the gate line _G1 by the refresh period τa and the write period. Similarly, other gate lines G ₃ ,
If G ₄ , G ₅ , G ₆ , G ₇ and G ₈ are sequentially shifted in time, voltage will be applied to two or more gate lines at the same time.
A gate signal waveform is formed such that V ₂ is not applied. Next, the operation at the opening will be explained.

At time τ ₁ , the potential of the gate line G ₁ reaches +V ₂ at the segment electrode connected to the microshutter section W ₁ .
Therefore, a voltage V is applied to the data electrode connected to the data line _S1 . During the subsequent write period τb, the potential of the gate line _G1 becomes _-V1 , and the TFT12021
is in the cut-off state, the segment electrode of the microshutter section _W1 is held at the potential V regardless of the potential of the data line _S1 . This means that at time τ ₁ , an electric field is always applied to the liquid crystal corresponding to the microshutter section _W1 , and the off state of the microshutter section _W1 is formed. During the subsequent write period τb, the potential of the gate line _G1 becomes + _V2 , so in order to bring the dot ^d11 to the dark level (off state), the potential of the data line _S1 must be set to V, and the potential of the data line _S1 must be set to V to bring the dot d11 to the bright level (on state). ), set the potential of data S ₁ to 0
As shown in _FIG .
Gate line G ₁ until pixel write period τ ends
The potential becomes _-V1 , and the TFT 12021 enters the cut-off state, so that the potential corresponding to the brightness level is held.

FIG. 13 shows an example in which the number of time divisions n is set to the maximum value when this driving method is used. For example, the aperture density is 16 dots/mm (16 pel), the process speed is 50 mm/sec, the time τc≒240 μsec, and the time τ ₁ ≒
When the time is 78 μsec, the number of time divisions n is 240/78=3 in the driving example shown in FIG. 9, but eight-order time division is possible in the driving example shown in FIG.

In the driving method shown in FIGS. 11 and 12, on-pulses are applied continuously to adjacent gate lines during one row writing time without any time difference as in the driving example shown in FIGS. 9 and 10. Instead, the time difference occurs only at the gate minimum on-time τ ₁ . Therefore, in the external circuit (interface circuit that distributes input data for time-division driving), the data transfer speed to the data electrodes can be reduced to half of the driving method shown in FIGS. 9 and 10. The circuit configuration in that part can be simplified, and it is also effective in reducing costs.

In addition, the drive example shown in Fig. 11 is a case where optimal time division is not performed, and the gate line is always connected at time τd.
A gate-on pulse is not applied to any one of the gate lines among G ₁ to _{G 8} , that is, there is a time period during which an off-voltage is applied to any gate line within the time τd. Therefore, the data transfer speed to the buffer circuit connected to each gate line can be reduced, which facilitates circuit setting and is effective in reducing costs.

Furthermore, in the present invention, when a liquid crystal shutter array is attached to the optical signal generating section shown in FIG. 5 and a toner image is formed on a cut print (transfer paper) using an electrophotographic copying machine as shown in FIG. It is necessary to provide a transfer paper feeding interval, and by setting the potential of the data electrode to 0 and the potential of the common electrode to +Vd at time Ty of this transfer paper feeding interval, as shown in Fig. 14, A voltage having a polarity opposite to that when forming one screen (for example, Tx when forming one copy) can be applied to the liquid crystal. (Tz: next screen formation period). For example, the transfer paper feed interval in an electrophotographic copying machine is generally 30 mm to 50 mm when viewed in terms of the paper distance. Therefore, for example, if the paper distance is 50 mm and the process speed is 50 mm,
mm/sec, the paper passes through the liquid crystal shutter array for one second, so by setting the voltage polarity during this period as shown in Figure 14, it is virtually no longer a direct current drive. Not yet,
Therefore, the life of the liquid crystal shutter array can be improved.

FIG. 15 shows an example of an electrophotographic copying machine using the above-mentioned liquid crystal shutter array. First, a photosensitive drum 1501 is driven to rotate in the direction of an arrow 1502, and a charger 1503 is used to drive the photosensitive drum 1501.
1 to uniformly charge the liquid crystal shutter array 150.
4 to selectively control the opening and closing of the light beam from the light source 1505 placed behind to generate an optical signal, and this optical signal is transmitted to the charged photosensitive drum 1501.
is irradiated to form an electrostatic latent image.

This electrostatic latent image is developed with toner in a developing device 1506, and this toner development is applied to a transfer guide 150.
A transfer charger 1508 transfers the image onto the copy paper P (transfer paper) that has passed through the transfer paper 7 . The copy paper P on which the image has been transferred is sequentially separated from the photosensitive drum 1501 by a separation belt device 1509, and then the image is fixed by a fixing device 1510. Further, the toner remaining on the surface of the photosensitive drum 1501 after transfer is removed by a cleaning device 1511.
, the photosensitive drum 1501 is neutralized by the pre-exposure device 1512, and the next copying cycle becomes possible again. By the way, the first
The liquid crystal shutter array 1504 in FIG. 5 employs the liquid crystal cell shown in FIG. 2 described above. In other words,
The light beam from the exposure light source 1505 is transmitted to the photoreceptor 15 through a liquid crystal shutter array 1504 equipped with a liquid crystal cell and a lens array 1513 such as a self-occurrence lens.
When forming an image on 01, the liquid crystal drive circuit 1514 is operated by a digital signal containing image information obtained by a document information reading device (not shown) to turn on the liquid crystal shutter array 1504.
By turning it off, a light signal having a pattern of image information is exposed onto the photoreceptor 1501. In this embodiment, the exposure light source 1
505 also has the function of heating the liquid crystal cell, and the liquid crystal temperature control circuit 1 is connected to the heat sensitive element 1520.
By operating the liquid crystal cooling fan 1517 at 516, overheating of the liquid crystal cell can be prevented and the liquid crystal cell can be maintained at a constant temperature.
In the figure, 1518 is a reflective shade, and 1519 is a member for attaching the lens array 1513 to the liquid crystal shutter device.

By the way, a conventional liquid crystal shutter array that does not use TFT has an electrode structure shown in FIG. 16, and in this electrode structure, a driving waveform shown in FIG. 17 is applied.

For example, the thickness of the liquid crystal is set to about 8 μm, a square wave of ±10 V is applied to the common electrode, and the signal electrode is
To turn on the shutter section of the selected row, apply the same voltage waveform as that of the common electrode, and to turn off the shutter section, apply Ov.
In Fig. 17, at time T ₁ , only A ₁ is on, and time
T ₂ is when only A ₁ ′ is turned on, and A ₁ and
The absolute value of the voltage applied to each liquid crystal layer of _A2 is expressed as |V _A 1| and |V _A 2|. In this way, when one of the shutter parts connected to the signal electrode is turned on, 2V is applied to the liquid crystal layer.
In other words, a voltage of 20V is applied, but when all the shutter sections connected to the signal electrodes are turned off, only V, that is, 10V is applied. Therefore, the transmittance of A ₁ is Td 1 at time T ₂ , while the transmittance of A ₂ is Td ₂ over times T ₁ and T ₂ , and _{Td 2 >} Td ₁ .

In this case, one dot corresponds to the times T ₁ and T ₂
formed over a period of time. Therefore, the amount of light received by the photosensitive drum is the area 1201 (1210a+1210b) when the shutter section is on, and the area 1202 (1202a+1202) when the opening section is off.
b) is proportional to

Therefore, if the voltage V is lowered, the contrast ratio cannot be maintained.

Also, if the number of time divisions is increased, the area will be 1701
Since 1702b is larger than a, the brightness ratio cannot be maintained.

On the other hand, when 16 dots per 1 mm are formed, the image forming speed is 50 mm/s (speed of forming 6 to 8 A4 size images per minute in vertical feed), and 2 time division driving is performed, T ₁ becomes 0.625ms. The shape of the change in transmittance of the liquid crystal shutter at this level is approximately shown in FIG. _12A1 . That is, T ₁
If the length of the shutter is further increased, the transmittance Te becomes larger, but since the length of the shutter is short, a very high transmittance cannot be obtained.

In fact, even when T ₁ is set to 1.25 ms, two-time division driving is performed, and the voltage is set to ±20 V, which is twice the voltage explained above, the brightness ratio becomes about three times as large as the irradiation light wavelength of 550 nm.

However, in the photoreceptor, for brightness and darkness,
When matching white and black, the contrast ratio is at least 5
More than twice as much is required. Therefore, in order to increase the contrast ratio by 5 times or more, the applied voltage must be further increased. Currently, the voltage resistance of CHOS and ICs that can be manufactured at low cost is around 30V, even for high voltage types. Therefore, even if the number of time divisions is increased and the number of ICs is decreased, in order to increase the contrast ratio, it is necessary to use a special high-voltage IC with a breakdown voltage of 60V to 80V.
Or, they had to choose whether to apply it only to those with slow image formation speed or low pixel density.

FIG. 18 shows the relationship between the voltage magnitude and the transmittance when an alternating voltage is applied to a conventional liquid crystal shutter array (8 μm liquid crystal layer), which was obtained through experiments.
In the figure, 181 indicates the transmittance at a wavelength of 489 nm.
2 shows the change in transmittance at 655 nm.

As can be seen from this experimental result, the voltage V _LC applied to the liquid crystal layer is 20 V or less when long wavelength (red side) illumination is used, and 30 V or less when short wavelength (blue side) illumination is used. If this happens, the transmittance will increase rapidly.

According to the present invention, the contrast ratio (bright area light energy [area A in Figure 6]/dark area light energy [area B in Figure 6]) could be made to be 6.5 or more under multi-dimensional time division driving. .

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing the relationship between the gate insulating film and ΔVth. FIG. 2 is a sectional view of a liquid crystal element used in the present invention. Figure 3A shows the TFT of the present invention.
FIG. 3B is a sectional view of another liquid crystal element of the present invention. Figure 4A is
FIG. 2 is an explanatory diagram showing an equivalent circuit of the liquid crystal shutter array of the present invention. FIG. 4B is a plan view of the liquid crystal shutter array of the present invention, and FIG. 4C is a sectional view taken along line A-A'. FIG. 5 is a perspective view of the printer head used in the present invention. FIG. 6 is an explanatory diagram showing a time chart of a drive signal applied to the liquid crystal shutter array of the present invention. FIG. 7 is an explanatory diagram showing the sheet tense when creating dots using the liquid crystal shutter array of the present invention. FIG. 8 is an explanatory diagram showing changes in light transmittance in time series during the shutter-on state. FIG. 9, FIG. 10, and FIG. 11 are explanatory diagrams showing other specific examples of time charts of drive signals. FIG. 12 is an explanatory diagram showing another specific example of the equivalent circuit of the liquid crystal shutter array of the present invention. FIGS. 13 and 14 are explanatory diagrams showing other specific examples of time charts of drive signals applied to the liquid crystal shutter array of the present invention. FIG. 15 is an explanatory diagram schematically showing the image forming apparatus of the present invention. FIG. 16 is a plan view showing the electrode structure of a conventional liquid crystal shutter array. FIG. 17 is an explanatory diagram of a time chart showing a drive waveform applied to a conventional liquid phase shutter array. FIG. 18 is an explanatory diagram showing the relationship between voltage and light transmittance in a conventional liquid crystal shutter array. 302; gate electrode, 306; gate insulating film,
305; semiconductor film, 303; source (data) electrode, 304; drain electrode, 307; segment electrode, 312; common electrode, 314; light shielding film, 3
10,315; alignment control film, 313; liquid crystal, 40
1,4011,4012,...;TFT, 402,
4021, 4022,...; Gate line, 403, 4
031, 4032,...: Data line, 404, 40
41, 4042,...; segment electrode, W ₁ ,
W ₂ , W ₃ , ...; Micro shutter section, 405; Sub-scanning line, 406; Contact hole, 57; Printer head section, 53, 1504; Liquid crystal shutter array, 51, 1501; Photosensitive drum, 54, 15
05; Light source, 52,1513; Lens array.

Claims (1)

[Scope of Claims] 1. A microshutter consisting of an exposure light source, a pair of opposing electrodes, and a nematic liquid crystal disposed between the pair of electrodes is arranged in a plurality of rows and columns in the exposure optical path of the exposure light source. A group of micro-shutters are arranged along the line, and the micro-shutter group selectively controls the light beam in the exposure optical path of the exposure light source to either a light-blocking state or a light-transmitting state. In an image forming apparatus including a printer head configured to generate a signal and configured to irradiate an image holding member with an optical signal from the printer head, a thin film transistor is connected to each of the microshutters, and a thin film transistor is connected to each microshutter. It has a wiring structure in which the gates are connected by gate lines for each row, and the sources of the thin film transistors are connected by data lines for each column, and the electric field strength applied to the gate insulating film in the channel part of the thin film transistor is 5 × 10. ⁵ V/cm or less, the gate insulating film is formed of a silicon oxide film with a film thickness of 6000 Å or more, a scanning signal is applied to the gate line, and a scanning signal is applied to the data line in synchronization with the scanning signal. An image forming apparatus characterized in that an electric signal is applied according to image information.