Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2001-09-05
2004-09-14
Pham, Long (Department: 2814)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S130000, C349S155000
Reexamination Certificate
active
06791657
ABSTRACT:
RELATED APPLICATION DATA
The present application claims priority to Japanese Application(s) No(s). P2000-269446 filed Sep. 6, 2000, which application(s) is/are incorporated herein by reference to the extent permitted by law.
BACKGROUND OF THE INVENTION
This invention relates to a light control device wherein incident light enters thereinto after control in quantity of the light and also to a pickup device using the same.
In general, a polarizer is used in a light control device using a liquid crystal cell. For the liquid crystal cell, there is used, for example, a TN (twisted nematic) liquid crystal cell or a guest-host (GH (guest-host)) liquid crystal cell.
FIGS. 14A and 14B
are, respectively, a schematic view showing the working principle of a known light control device. This light control device is constituted mainly of a polarizer
1
and a GH cell
2
. The GH cell
2
is sealed between two glass substrates and has working electrodes and liquid crystal alignment films although not particularly shown in the figures. The GH cell
2
has liquid crystal molecules
3
and dichromatic dye molecules
4
sealed therein.
The dichromatic dye molecules
4
have anisotropy with respect to the absorption of light and are made, for example, of positive-type (p-type) dye molecules that absorb light along the major axis of the molecules. The liquid crystal molecules
3
have dielectric anisotropy of the positive type (p-type).
FIG. 14A
shows a state of the GH cell
2
in case where no voltage is applied thereto (or under conditions of applying no voltage). Incident light
5
is linearly polarized after transmission through the polarizer
1
. In
FIG. 14A
, the polarization direction and the direction of the major axis of the dichromatic dye molecules
4
are coincident with each other, so that the light is absorbed with the dichromatic dye molecules
4
, thereby causing the transmittance of the GH cell
2
to be lowered.
When a voltage is applied to the GH cell
2
as shown in
FIG. 14B
, the liquid crystal molecules are turned toward a direction of an electric field, under which the direction of the major axis of the dichromatic dye molecules
4
becomes perpendicular to the direction of polarization of the linearly polarized light. Thus, the incident light
5
is transmitted by the GH cell without being absorbed.
It will be noted that where negative type (n-type) dichromatic dye molecules which are capable of absorbing light along the direction of the minor axis of the molecules are used, the light is not absorbed under conditions where no voltage is applied, but is absorbed when a voltage is applied, unlike the case using the positive type dichromatic dye molecules
4
.
With the light control device shown in
FIGS. 14A and 14B
, a ratio between the absorbances under voltage-applying conditions and no voltage-applying conditions, i.e. an optical density ratio, is at about 10. This optical density ratio is two times higher than that of a light control device constituted of the GH cell alone without use of any polarizer
1
.
The optical density ratio is influenced by the gap or distance between the two glass substrates constituting the GH cell (hereinafter referred to as cell gap).
More particularly, the greater the cell gap is or the thicker the liquid crystal layer is, the greater the difference becomes between light transmittances under transparent or transmitting conditions and light-shielding conditions. Thus, although the optical density ratio (contrast ratio) can be made great, the light transmittance under transmitting conditions become lower.
When the cell gap is changed, the response speed of a light control device using the GH cell also changes. More particularly, when the cell gap is made greater, the response speed certainly tends to become slower.
Under these circumstances, there is a strong demand for a light control device using a liquid crystal cell, which ensures a great contrast ratio and, at the same time, can realize a rapid response speed.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a light control device wherein while ensuring a satisfactory optical density ratio (contrast ratio) of a liquid crystal optical element, a liquid crystal optical element that can be driven at such a high response speed as required for a light control device is employed. It is another object of the invention to provide by pickup device having a light control device of the type mentioned above arranged in a light path, thereby improving properties, image quality and reliability thereof.
The invention contemplates to provide a light control device comprising a liquid crystal optical element having a liquid crystal sealed inbetween substrates (e.g. a pair of glass substrates) provided by face-to-face relation with each other wherein the liquid crystal which is made of a guest-host liquid crystal which is made of a negative type liquid crystal as a host material, and a cell gap between the substrates is at least controlled to be in the range of 2 &mgr;m to 4 &mgr;m in an effective light path. The invention also contemplates to provide a pickup device having the light control device arranged in a light path of a pickup system.
The light control device and the pickup device of the invention make use of a guest-host type liquid crystal element arranged in a light path thereof wherein a negative type (wherein a dielectric anisotropy (&Dgr;∈ is negative) liquid crystal is used as a host material. Accordingly, the light transmittance under light transmitting conditions (especially, under transparent conditions) is significantly improved by the case using a positive type (i.e. a value of &Dgr;∈ is positive) liquid crystal. In addition, the cell gap is at least defined within a range of more than 2 &mgr;m to less than 4 &mgr;m at least in an effective light path, so that a response speed can be made greater while keeping a high ratio between the optical densities (contrast ratio) under light transmitting conditions (transparent conditions) and the light intercepting condition (light-shielding conditions).
The invention provides a liquid crystal optical element having an improved optical function for use as a light control device and relies on an invention of earlier Japanese Patent Application No. Hei 11-322186, assigned to the same assignee. According to this invention, a light control device is constituted of a liquid crystal element and a polarizer arranged in a light path incident on the liquid crystal element, and a guest-host type liquid crystal using a negative type liquid crystal as a host material. Eventually, a ratio between absorbances under no voltage applying conditions and voltage applying conditions (i.e. a ratio between optical densities) is improved by a great contrast ratio of the light control device. Thus, one enables to normally conduct light control operations in bright to dark places.
In the guest-host type liquid cell (GH cell)
2
shown in
FIGS. 14A and 14B
, a positive type liquid crystal where a dielectric anisotropy (&Dgr;∈ is positive is used as a host material, a positive type dye
4
which has dichromaticity and in which a light absorption anisotropy &Dgr;A is positive, is used as a guest material
4
, and the polarizer
1
is provided at an incident side of the GH cell
2
. When a working voltage using a rectangular wave as a drive wave is applied to so as to measure a change of light transmittance, it has been found that, as shown in
FIG. 15
, as the working voltage increases, an average light transmittance of visible light in the air (wherein a transmittance through the liquid crystal cell along with the polarizer is taken as a reference (100%) herein and whenever it appears hereinafter) increases. Nevertheless, when the voltage increases up to 10V, a maximum transmittance arrives only at about 60%, with the gentle change of the light transmittance.
This is considered for the reason that where a positive type host material is used, liquid crystal molecules whose director does not change (or undergoes little chan
Kawabata Masaru
Udaka Toru
Yanagida Toshiharu
Depke Robert J.
Holland & Knight LLP
Pham Long
Weiss Howard
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