Liquid crystal microlens

Details Liquid crystal microlens Liquid crystal microlens

2002-11-25

2004-07-27

Dudek, James A. (Department: 2871)

Liquid crystal cells, elements and systems

Liquid crystal optical element

Liquid crystal lenses other than for eyewear

Reexamination Certificate

active

06768536

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a liquid crystal microlens used as a means for forming an image in a lens array.
There is commonly known a contact-type sensor having a construction such as that of
FIGS. 14 and 15
.
Referring to
FIG. 14
, a sensor
110
has a frame
108
in which are mounted a linear light-emitting element (LED) array
105
, a rod lens array
106
, and light-receiving element array
104
. The light-receiving element array
104
comprises a substrate
103
formed at the bottom of the frame
108
, a protection film
102
mounted on the substrate
103
, and a sensor IC
101
comprising a plurality of photoelectric converters. A transparent plate
107
on which a text sheet
109
is set is mounted on the upper portion of the frame
108
.
In operation, a light beam from the LED array
105
irradiates the text sheet
109
. The light beams diffused and reflected at a particular reading line of the sheet
109
passes through the rod lens array
106
so as to form an image on the text upon the sensor IC of the light-receiving element array
104
. Information regarding the shades of the text sheet conveyed by the reflected light, taking the form of the intensity of light, is converted into an electric signal by the sensor IC
101
and serially outputted in accordance with the scanning direction. After scanning one line in the scanning direction, the next line in the direction perpendicular to the scanning direction is scanned. By repeating the scanning operation, two-dimensional information on the text sheet
109
is converted into an electric signal in time sequence.
FIG. 15
shows the arrangement of the rod lens array
106
of the contact-type sensor
110
shown in FIG.
14
and the operation thereof.
The principle and the construction of the rod lens array
106
are described hereinafter with reference to
FIGS. 16
a
to
16
c
. Each rod lens of the rod lens array
106
is a graded index lens, each having a refractive index distribution shown in
FIG. 16
a
.
FIG. 16
b
shows the transmission of a light beam through the rod lens.
In
FIG. 16
a
, the distribution of the refractive index n can be approximately expressed as
n=n
0
(1−(
A
/2)
r
2
)
where n
0
is the refractive index on the optical axis, r is the distance from the optical axis in a radial direction, and A is the constant of the refractive index. The light beams tend to travel slower in a range where the refractive index is large and faster where the refractive index is small.
Due to such a characteristic, the light beam entered in the rod lens follows a path according to the winding interval P, which depends on the distribution of the refractive index, and is emitted out from the opposite end of the lens as shown in
FIGS. 16
a
and
16
b.
As shown in
FIG. 16
c
, by setting an appropriate rod lens length Z
0
in relation to the winding interval, an erecting image Q″ of an image Q equal in size thereto can be formed at the opposite side of the rod lens at a distance TC. The image forming operation is also described in FIG.
15
.
The reference L
0
in
FIG. 16
c
is a working distance between the rod lens and the object Q (Q″).
The rod lens is provided with the following characteristics.
(1) The rod lens has end faces which are flat, and is light in weight.
(2) The condition of the formed image can be arbitrarily changed dependent on the length of the rod lens.
(3) The image can be formed on the end surface of the lens, and furthermore, a lens with a short focal length can be provided.
(4) The optical axis of the lens coincides with the geometric center so that the lens can be easily adjusted.
Methods for imparting the refractive index distribution to a glass rod include ion implantation, molecular stuffing, and ion exchange method. In the case of rod lens, the ion exchange method is used so that the distribution becomes smooth and symmetrical.
Referring to
FIG. 17
, the ion exchange method employs a kiln
112
containing a fused salt
113
of high temperature. A glass rod
116
is immersed in the salt
113
so that an alkali ion A in the glass rod and an alkali ion B in the salt
113
are exchanged with each other. As a result, there is formed in the glass rod
116
an ion concentration distribution which is in proportion to the refractive index distribution described above.
However, the rod lens thus formed has the following problems.
(1) In order to manufacture the rod lens, there is a need to provide a device for the ion conversion treatment so that the manufacturing cost increases.
(2) The conjugation length TC, which is the distance between the original object and the image formed, can only be selected from the lineup of the rod lens products. Thus the distance TC cannot be shortened for manufacturing a thin contact-type sensor.
In order to solve the problem, there has been proposed a lens where a known liquid crystal lens shown in
FIGS. 18
a
and
18
b
is used instead of the rod lens array. The construction and the features of the liquid crystal lens are described in a known publication OplusE., October, 1998, Vol. 20, No. 10, Kabushiki Kaisha Shingijutsu Communication, featuring liquid crystal optical elements and their applications: liquid crystal microlens.
In order to form an optical element which serves as a lens with a liquid crystal, a liquid crystal layer, which becomes a medium, may be shaped into lens as in glass lenses. Alternatively, the optical element maybe constructed so that a spatial refractive index may be imparted. In a nematic liquid crystal cell, liquid crystal molecules are aligned in the direction of an electric field. Thus, due to the distribution effect of the liquid crystal molecules in the electric field which is symmetric with respect to the axis and inhomogeneous, a lens having a spatial refractive index distribution can be provided. When such a liquid crystal lens is employed, a microlens array where a plurality of miniaturized lens are arranged in two dimensions in a flat plate is easily provided.
Referring to
FIGS. 18
a
and
18
b
, the nematic liquid crystal cell
121
comprises a lower transparent glass substrate
123
, an upper transparent glass substrate
122
, a pattern electrode
124
a
on the lower transparent glass substrate
123
, a pattern electrode
124
c
formed on the underside of the upper transparent glass substrate
122
, a transparent alignment layer
125
a
on the electrode
124
a
, a transparent alignment layer
125
b
on the electrode
124
c
and an enclosing member
127
provided between alignment layers
125
a
and
125
b
. The pattern electrode
124
a
is formed by a conductive electrode film and has a plurality of circular holes
124
b
, and the pattern electrode
124
c
is also formed by a conductive electrode film and has a plurality of circular holes
124
d
. Each of the circular holes
124
d
is concentrically formed with an opposite hole
124
b
. A liquid crystal material
128
is injected into a space defined by the enclosing member
127
and the alignment layers
125
a
and
125
b
. The alignment layers
125
a
and
125
b
are rubbed so that the alignment of each layer is antiparallel and homogenous to one another. The pattern electrodes
124
a
and
124
c
are so disposed that the holes
124
b
and the holes
124
d
coincide.
When the liquid crystal cell
121
is applied with a voltage higher than a threshold, electric potentials are distributed as shown by contour lines in
FIG. 19
a
. As shown in the figure, the electric field intensity has such a spatial distribution as to be increased as the distance from a center of the hole
124
b
(
124
d
) of the pattern electrode
124
a
(
124
c
) increases in the radial direction, that is, a distribution is symmetrical about the axis of the cell.
In
FIG. 19
b
, a section of the liquid crystal material
28
is divided into a plurality of regions by the contour lines and the vertical division lines, and a typical director is shown for each region.
Namely, when a voltage larger than a threshold voltage is applied, liquid crystal molec

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