Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2002-08-20
2004-05-25
Ben, Loha (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C345S031000, C345S055000, C345S107000
Reexamination Certificate
active
06741387
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a display element. More specifically, it relates to a display element for repeatedly displaying an image by driving colored particles with an electric field.
2. Description of the Related Art
Examples of display element having excellent holding property and capable of being rewritten repeatedly include twisting ball displays (two-color painted particle rotating display), electrophoresis display mediums, magnetophoresis display mediums, thermal rewritable display mediums, liquid crystals having a memory property, or the like. However, since these elements cannot display a white color like paper, they have a problem of low image contrast.
In order to solve the low contrast problem, display techniques using toners have been proposed (Japan Hardcopy, '99 Article Collection, pp. 249-252, and Japan Hardcopy, Fall Preliminary Report Collection, pp. 10-13). According to the display technique, a conductive colored toner and a white particle are sealed in a space between a display substrate and a non-display substrate, which face each other. The display substrate and the non-display substrate each comprise an electrode. A charge is injected to the conductive colored toner via a charge conveying layer provided in the electrode of the non-display substrate. The conductive colored toner into which the charge was injected is moved to a side of the display substrate disposed facing the non-display substrate by the electric field therebetween. By adhesion of the conductive colored toner to the display substrate, the color of the conductive colored toner is displayed on the display substrate. Accordingly, a high contrast image is displayed with the color of the conductive colored toner and the white of the white particle.
According to the display technique, since the solid colored toners and white particle are used, unlike the case of using a liquid instead of either one of the toner and the particle, deterioration of the display density due to intrusion of the liquid into the toner or the particle does not occur. Therefore, theoretically the display can be switched completely between the white and the toner color (for example, black). However, according to this display technique, a charge is not injected to a toner, which does not contact the charge conveying layer provided in the electrode of the non-display substrate and is isolated from the other conductive colored toners. The toner without the charge injection exists in a space between the substrates randomly without being moved by the electric field so that the toner and the white particles exist in a mixed manner, and thus a problem of the image display with a low contrast is involved in practice.
A method for forming an image by moving or rotating a particle that was charged by the effect of the electric field, is proposed in the official gazette of the Japanese Patent Application Laid-Open (JP-A) No. 2001-33833. According to the method, substrates provided with a conductive layer or a rectifying layer are disposed facing each other, and two kinds of colored particles having colors different from each other and without a charge of the same polarity are sealed between the substrates. By transferring a charge to each particle from the conductive layer or the rectifying layer and moving or rotating the particles by the effect of the electric field, the image is displayed. However, the method has a problem of image density irregularities and contrast deterioration being generated due to local charge leakage.
An image display medium comprising a pair of substrates and a group of different kinds of particles having different colors and charge characteristics, sealed between the substrates, movably between the substrates by an applied electric field is proposed in the official gazette of the Japanese Patent Application No. 2000-165138. According to this image display medium, high whiteness degree and contrast can be obtained. However, the driving voltage is increased, because the image display medium requires an applied voltage of several hundred volts for displaying an image, therefore, this image display medium has a problem that the degree of freedom in designing the driving circuit is lowered.
Moreover, with repetition of contact electrification of the particles and the substrate, and frictional electrification among the particles, the amount of the charge of the particles becomes too large relative to an initial charge amount, and the particles coagulate or adhere to the substrate. As a result, there are also problems of the contrast being reduced, the unevenness in image density, and the driving voltage increasing.
SUMMARY OF THE INVENTION
Accordingly, in order to solve the above-mentioned problems, the present invention has been achieved, and an object thereof is to provide a display element capable of displaying an image with a high contrast and few density irregularities, which is driven at a low voltage, and has an improved image holding property.
In order to solve the above-mentioned problems, a first aspect of the present invention provides a display element comprising a first substrate and a second substrate, which are disposed facing each other, and multiple kinds of particles having different colors and charge polarities, which particles are sealed in a space between the first substrate and the second substrate and can be moved between the first substrate and the second substrate by an electric field applied thereto, wherein the surface coarseness of at least one of the first substrate and the second substrate, on the surface thereof that faces the other substrate, is more than 1 nm and no greater than 10 &mgr;m, and the coefficient of a shape of the particles is more than
100
and no greater than
140
when FE represents the coefficient of shape of the particles, L represents the circumference of the particles, S represents the area of the particles and FE=((L
2
/S)/4&pgr;)×100.
According to the first aspect of the present invention, when the surface coarseness of at least the first substrate and the second substrate on the surface of the side facing the other substrate is more than 1 nm and 10 &mgr;m or less, and the coefficient of shape of the particles is more than
100
and
140
or less (wherein FE is the coefficient of shape of the particles, L is the circumference of the particles, S is the area of the particles and FE=((L
2
/S)/4&pgr;)×100), the distance between the substrates and the particles becomes longer and the contact area of the substrates and the particles becomes smaller, and thus the particles adhered to the substrate can be separated easily. Thus, the driving voltage of the display element can be reduced. In this case, the coefficient of shape FE represents the roundness of the particles are projected onto a plane, and the circumference L and the area S each represent a value measured by projecting the particles onto a plane.
A second aspect of the present invention provides a display element comprising a first substrate and a second substrate disposed facing each other, and multiple kinds of substantially spherical particles having different colors and charge polarities, which particles are sealed in a space between the first substrate and the second substrate, and can be moved between the first substrate and the second substrate by an electric field, applied thereto, wherein the surface coarseness (Rz) of at least one of the first substrate and the second substrate is smaller than the average diameters of the multiple kinds of particles.
According to the second aspect of the present invention, since the surface coarseness (Rz) of at least one of the first substrate and the second substrate is smaller than the average diameters of the multiple kinds of particles, the particles cannot be fitted into the concave surface of the substrate and the particles adhered to the substrate can be separated easily. Accordingly, the driving voltage of the display element can be reduced.
A third aspect o
Machida Yoshinori
Mashimo Kiyokazu
Matsunaga Takeshi
Miyamoto Hiroshi
Nishikawa Kyoko
Fuji 'Xerox Co., Ltd.
Oliff & Berridg,e PLC
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