JPH06222334A - Driving method for liquid crystal shutter - Google Patents

Abstract

PURPOSE:To provide the driving method which makes it possible to obtain a >=100 contrast ratio and does not entail coloration even if a positive supertwisted nematic type liquid crystal shutter is driven by using a low-voltage CMOS-LSI. CONSTITUTION:The crest value of the AC voltage to be applied to a counter side electrode is set at (VL+VM)/2 and the crest value of the AC voltage to be applied to a signal side electrode at (VL-VM)/2 when the inter-electrode voltage at which the coloration of the liquid crystal pixels constituting the liquid crystal shutter begins to disappear is defined as VL and the inter-electrode voltage to minimize the transmittance of the liquid crystal pixels as VM. The counter side AC voltage waveform and signal side AC voltage waveform are maintained at the same phase at the time of selecting white and the phases are inverted at the time of selecting black. Then, full-color printing having bright color tones is executed in a short period of time. The color liquid crystal printing head which is low in cost and compact is provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal optical printer, and more particularly to a method for driving a liquid crystal shutter for optically writing an image on a photosensitive paper or a photosensitive member.

[0002]

2. Description of the Related Art Liquid crystal optical printers that have been developed or put into practical use are hard-copy printers that are monochrome and have no gradation, and their applications are mainly laser printers. It is considered as a counter model such as an LED printer and an LED printer, and the main focus of development has been on high speed.

By the way, in order to carry out a hard copy of an image having a gradation with a color such as a photographic image or a television image by the liquid crystal optical printer, the contrast of the liquid crystal shutter which is one of the main constituent elements. Is required to have a high contrast of 100 or more and gradation is required. Therefore, the amount of light transmitted through each pixel of the liquid crystal shutter per unit time must be controlled according to an image input signal.

One method for solving the above problems is disclosed in Japanese Patent Laid-Open No. 62-150330. Figure 5
Is an explanatory diagram of this method. The driven liquid crystal shutter is a positive type twisted nematic in which a pair of glass plates with transparent electrodes sandwiching a twisted nematic liquid crystal sandwiches a liquid crystal cell between a pair of polarizing plates arranged so that their absorption axes are orthogonal to each other. Type liquid crystal shutter. From each transparent electrode of such a liquid crystal shutter, a liquid crystal drive waveform of an AC rectangular wave as shown in (b) based on the image signal of (c) is applied. As a result, the shutter transmission intensity characteristic as shown in (a) is obtained. This method is characterized in that the amount of transmitted light is controlled by always applying a voltage to keep the light cutoff state and stopping the applied voltage to make the light transmission state when an image signal is input.

[0005]

However, it has become clear from experiments by the author that this driving method cannot actually obtain a sufficient contrast ratio. Normally, the number of signal electrodes of a liquid crystal shutter array for image printing is close to 1000, and therefore it is general to drive by using a CMOS-LSI in consideration of downsizing of mounting module and cost. However, the driving voltage value of the CMOS-LSI is about ± 15 V at most. When a 240 ° twist positive type liquid crystal shutter was driven with this voltage value, the obtained contrast ratio was about 60, although it depended on the liquid crystal material and the like. Writing a color image on Polaroid 669 instant film at the above contrast ratio did not yield a satisfactory high quality printed image.

Further, a usual super twisted nematic type liquid crystal element is colored with yellow or the like because it displays by utilizing a birefringence effect. Therefore, it is very difficult to achieve a vivid full color image print by using the liquid crystal element in this state.

In view of the above, the present invention is a compact, low-cost color printer that performs vivid full color printing on photosensitive paper such as instant film in a short time.
Low voltage CM to realize liquid crystal print head
It is an object of the present invention to provide a driving method of a positive super twisted nematic liquid crystal shutter which can obtain a contrast ratio of 100 or more even when driven by an OS-LSI and is free from coloring.

[0008]

In order to achieve the above object, the present invention provides a liquid crystal by applying an AC voltage to the counter electrode and the signal electrode of a positive super twisted nematic liquid crystal cell. In the driving method of the liquid crystal shutter for controlling the amount of light passing through the pixel, when the inter-electrode voltage at which the coloring of the liquid crystal pixel begins to disappear is VL and the inter-electrode voltage that minimizes the transmittance of the liquid crystal pixel is VM, The peak value of the AC voltage applied to one of the counter electrode and the signal electrode is (VL
+ VM) / 2, the peak value of the AC voltage applied to the other electrode is (VL −VM) / 2, and when white is selected, the AC voltage of the opposite electrode and the AC voltage of the signal electrode are in phase with each other, When black is selected, it is characterized in that the driving is performed by inverting the phases of each other.

[0009]

The operation of the present invention will be described below with reference to the drawings. A positive type in which a liquid crystal cell composed of two glass substrates with transparent electrodes and a nematic liquid crystal to which a chiral dopant is added is sandwiched by a pair of polarizing plates arranged so that their absorption axes are orthogonal to each other. In the super twisted nematic liquid crystal shutter, as shown in FIG. 6, the white level due to the birefringence due to the twisted state of the liquid crystal molecules 62 is selected in the voltage non-applied state (b), and in the voltage applied state (a). Most liquid crystal molecules are glass substrates with transparent electrodes 6
It should be arranged perpendicular to 1 and the black level of the polarizing plate cross should be obtained.

However, if the applied voltage is not sufficient, the liquid crystal molecules in the vicinity of the glass substrate are not completely stood up as shown in FIG. 6A, and the transmittance of the selected black level is increased. Particularly, since the contrast ratio is drastically reduced by a slight black level transmitted light, it is necessary to apply a sufficient voltage to the liquid crystal element. According to the author's experiment, the liquid crystal layer thickness is about 5 microns and the twist angle is 180 degrees to 27 degrees.
Most of 0 degree positive type super twisted nematic type liquid crystal elements reach a saturation value of a contrast ratio of 100 or more at a driving voltage value of 30 V or more, and a normal CMOS-LSI.
It was found that a voltage value more than twice the driving voltage of is required.

On the other hand, an ordinary super twisted nematic liquid crystal element displays various images by utilizing the birefringence effect, and therefore various coloring is caused in the state where no voltage is applied. This coloring is generally expressed by the following relational expression. T = sin ² (πΔnd / 2λ) T: Light transmittance Δn: Refractive index anisotropy of liquid crystal molecules d: Liquid crystal layer thickness λ: Wavelength

That is, a liquid crystal element having a Δnd value of 0.9 produces yellow, 0.8 produces green, and 0.6 produces blue. Then, from 0.35 or less, coloring is reduced, and the state is optimal for use as a liquid crystal shutter. By the way, the above-mentioned colored state can be controlled not only by the Δnd value peculiar to the liquid crystal element but also by applying a voltage to the same liquid crystal element and changing the arrangement of the liquid crystal molecules.

For example, FIG. 7 is a voltage-transmittance curve showing a coloring state of a typical positive type super twisted nematic type liquid crystal element. As can be seen from the figure, the hue changes drastically from green-pink-yellow-pale-yellow to the voltage VL from no voltage applied, but at higher voltages, the hue remains colorless and only the transmittance decreases. . The transmittance reaches almost saturation at the voltage VM. According to an experiment under a polarization microscope, the value of VL is 3 to 5 when driven by an alternating current of 10 KHz.
The values of Volt and VM were 30 to 40 V, and the transmittance at that time was 0.5% or less.

Therefore, if the white is selected with the voltage VL and the black is selected with the voltage VM, the liquid crystal shutter can be driven with a high contrast ratio of 100 or more and in a color-free state.
It is possible to achieve a full-color, high-quality print with a bright color tone on a photosensitive paper such as an instant film.

[0015]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is an explanatory view showing a driving method of a positive type super twisted nematic type liquid crystal shutter of the present invention,
In white selection, the phases of the signal-side electrode AC voltage waveform and the opposite-side electrode AC voltage waveform match, and in black selection, the phases of the signal-side electrode AC voltage waveform and the opposite-side electrode AC voltage waveform are inverted, and the opposite-side electrode AC voltage waveform is reversed. The value is larger than the signal side AC voltage value by the voltage value VL that eliminates coloring of the liquid crystal shutter, and the sum of the counter side electrode AC voltage value and the signal side AC voltage value is the liquid crystal shutter.
Is set so as to substantially match the saturation voltage value VM that minimizes the transmittance of the.

As a result, the combined voltage for white selection applied to the pixels of the liquid crystal shutter becomes the voltage VL at which the transmitted light becomes the strongest within the range where there is no color shift, and the combined voltage for black selection lowers the transmitted light most. Since the applied voltage is VM, the driving method according to the present invention makes it possible to perform switching with a high contrast ratio and a substantially flat transmission spectrum in the visible light region. For example, the twist angle 225 used in this embodiment.
The positive super-twisted nematic liquid crystal shutter with .DELTA.nd = 1.02 in degrees has a VL value of 4 volts and a VM value of about 32 volts. Therefore, the signal voltage value is set to (32-4) / 2 = 14 volts, the opposite side voltage value is set to (32 + 4) / 2 = 18 volts, and 10 KH is set.
When driven with an AC pulse of z, the contrast ratio is 120
Was obtained and the voltage value on the signal side was also within the drive voltage range of the CMOS-LSI.

Next, a color liquid crystal shutter array having a resolution of 200 dpi was prepared in which R (red), G (green) and B (blue) gelatin filters were provided on the inner surface of the glass substrate of the liquid crystal shutter. . As shown in FIG. 2, the color pixels 21 in the color liquid crystal shutter array form a staggered arrangement, and the extraction electrodes 22 are individually formed from each pixel.
Since it is arranged, static driving is possible, and the driving method of the present invention can be applied.

FIG. 3 is a schematic sectional view of a color liquid crystal print head 31 for forming a full color image. The color liquid crystal shutter array 32 and a white light source 33 such as a fluorescent lamp or a halogen lamp, and A selfoc lens array 34 for focusing the modulated light on an instant film 35 such as photosensitive paper is incorporated in the color liquid crystal printhead. Here, in actuality, the color liquid crystal print head was moved over Polaroid 669 instant film at a speed of 32 mm / sec for 2.4 seconds to try a full color image print.

FIG. 4 shows an example of driving with gradation for obtaining a full color image. In the transmittance curve of (a),
, The level of transmittance of is shown. (B) shows the voltage waveforms applied between the electrodes of the liquid crystal pixels in order to obtain such transmittance, corresponding to the numbers in (a).
Each color pixel of 200 dpi (128 μm size) is 4
The open / close time of the liquid crystal shutter is continuously modulated within the selected time and the white level within the selected time of m seconds, and the black level of the half tone level thereof. 64 by the method described above
Continuous gradation driving of 3 gradations is performed on an instant film.
It was possible to obtain vivid and high-quality prints in a short time of less than a second.

[0020]

As described in the above embodiments, according to the present invention, even if a low-voltage CMOS-LSI is used, a contrast ratio of 100 or more can be easily provided, and a positive type superimposing without coloring is provided. -Twisted nematic liquid crystal shutter-
Therefore, it is possible to provide a low-cost and compact color liquid crystal print head that performs full color printing having a vivid color tone on a photosensitive paper such as instant film in a short time.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a driving method of a liquid crystal shutter of the present invention.

FIG. 2 is a schematic plan view showing a color pixel arrangement of a color liquid crystal shutter array using the driving method of the present invention.

FIG. 3 is a schematic cross-sectional view of a color liquid crystal print head including a liquid crystal shutter using the driving method of the present invention.

FIG. 4 is an explanatory diagram of gradation control according to the driving method of the present invention.

FIG. 5 is an explanatory diagram showing a driving method of a conventional liquid crystal shutter.

FIG. 6 is a schematic view showing an arrangement of liquid crystal molecules in a liquid crystal element.

FIG. 7 is a voltage-transmittance curve of a liquid crystal element.

[Explanation of symbols]

 21 Color Pixel 22 Extraction Electrode 31 Color Liquid Crystal Print Head 32 Color Liquid Crystal Shutter Array 33 Light Source 34 Selfoc Lens Array 35 Instant Film 61 Glass Substrate 62 Liquid Crystal Molecule

Claims (2)

[Claims] 1. A driving method of a liquid crystal shutter for controlling an amount of light passing through a liquid crystal pixel by applying an AC voltage to a counter electrode and a signal electrode of a positive super twisted nematic liquid crystal cell. In Fig. 3, when the voltage between the electrodes at which the liquid crystal pixel starts to lose its color is VL and the voltage between the electrodes which minimizes the transmittance of the liquid crystal pixel is VM, an AC voltage applied to one of the counter electrode and the signal electrode. The peak value of (VL
+ VM) / 2, the peak value of the AC voltage applied to the other electrode is (VL-VM) / 2, and when white is selected, the AC voltage of the opposite side electrode and the AC voltage of the signal side electrode are in phase with each other, A method of driving a liquid crystal shutter, which is characterized in that when black is selected, the phases are inverted and driven. 2. A white selection period within the selection period of each pixel,
The method of driving a liquid crystal shutter according to claim 1, wherein gradation control is performed by changing a ratio of black selection periods.