Note: Descriptions are shown in the official language in which they were submitted.
2093281
LENS FOR EYEGLASSES
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to lenses for
eyeglasses or spectacles comprised of a pair of plastic
lenses.
2. Description of Related Art
Expensive lenses for eyeglasses (i.e. eyeglass or
spectacle lenses), such as multi-focus or progressive
multi-focus lenses, have been made by adhering a pair of
plastic lenses into a plastic composite lens. The
provision of such eyeglass or spectacle lenses is made
possible by the use of plastic lenses which can be
independently designed with a great degree of freedom.
Such an eyeglass lens consists of a front lens, which is
located on an object side of the eyeglass lens, and a rear
lens, which is located on the user's eye side of the
eyeglass lens. In known plastic composite lenses, the
front lens and the rear lens are usually made of
diethyleneglycolbisallylcarbonate polymer (CR-39plastic).
Known plastic composite lenses, however, have drawbacks as
discussed below.
Firstly, the impact resistance of such lenses is
low. In particular, some plastic composite lenses having
a small center thickness do not meet the requirements
prescribed in the drop-ball test (i.e., crushing ball test)
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standardized by the U.S. FDA (U.S. Food & Drug
Administration). In the drop-ball test, used in a sampling
inspection, a steel ball having a weight of 16g is dropped
from a height of 127 cm onto the center of a lens. Only
the lenses corresponding to the test samples that are not
destroyed or crushed by the steel ball aEe authorized by
the FDA. In the case of lenses having a thickness at the
center of around 1.5 mm, the rear lens tends to shatter
before the front lens. From a safety standpoint, this is
an unacceptable result.
Secondly, there is a large difference in
thickness between the center portion and the edge portion
of either the front or rear lens. This results in a
significant difference in polymerization shrinkage between
the center portion and the edge portion of the front or
rear lens particularly in high negative power lenses.
Consequently, during the molding operation, the lens can
easily become separated from the molding die, resulting in
a decreased yield.
A feature of one embodiment of the present
invention is to provide a strong composite eyeglass or
spectacle lens consisting of a front and rear lens.
Another feature of an embodiment of the present
invention is to provide a thin and light composite eyeglass
or spectacle lens.
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SUMMARY OF THE INVENTION
The inventors of the present invention have
focused on the plastic material of which the front and rear
lenses of a composite eyeglass or spectacle lens are made.
They have found that by making the rear lens on the user's
eye side out of polycarbonate resin or polyurethane resin,
rather than the conventional diethyleneglycol
bisallylcarbonate polymer, a strong, thin, and light
plastic composite eyeglass lens can be realized.
In accordance with an embodiment of the present
invention there is provided a composite plastic eyeglass
lens comprised of a front lens located on an object side of
the eyeglass lens, and a rear lens located on a user's eye
side of the eyeglass lens, wherein the rear lens is made of
polycarbonate resin or polyurethane resin.
Polycarbonate resin or polyurethane resin has a
greater mechanical strength and a higher refractive index
than that of diethyleneglycol bisallylcarbonate polymer.
~- Accordingly, a composite plastic eyeglass lens made of
polycarbonate resin or polyurethane resin will have a
` greater mechanical strength than the conventional composite
lens. Consequently, a lens that is relatively thin at the
center can satisfy the requirements prescribed in the drop-
ball test mentioned above.
Furthermore, a thinner lens having the same power
as that of the conventional composite plastic lens can be
realized, owing to the high refractive index.
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A thinner lens is particularly advantageous in
the case of a rear lens on the user's eye side which has a
negative (i.e. minus) power. The thickness of a negative
power lens at the edge thereof increases as the power
increases, resulting in an increase in thickness of the
spectacle or eyeglass lens as a whole. The thickness of
the eyeglass lens can be reduced if the rear lens is made
of a material having a high refractive index, since the
edge thickness of the rear lens is reduced. Namely, the
thickness of the composite plastic lens can be reduced on
the whole without decreasing the mechanical strength
thereof.
The front lens on the object side is preferably
made of diethyleneglycol bisallylcarbonate polymer, similar
to the conventional product. This is in view of the
molding process. Namely, diethyleneglycol
bisallylcarbonate polymer can be precisely molded by a
glass molding die due to thermal polymerization of the
plastic. In general, the surface of the front lens has a
curvature which is designed to give the composite lens
multi-focus or progressive multi-focus characteristics and
is sometimes defined by an aspheric surface. Consequently,
; it is necessary to precisely mold the front lens.
Polycarbonate resin or polyurethane resin can be
molded in a predetermined shape by an injection molding
process. Injection molding of such resins does not yield
a lens having a shape as precise as the lens made by the
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2093281
molding of diethyleneglycol bisallylcarbonate polymer in
the glass molding die. However, the shape of the rear lens
is simpler than that of the front lens. Accordingly, in
the preparation of the rear lens, the simpler injection
molding process will suffice.
Moreover, the percentage of shrinkage of
polycarbonate resin or polyurethane resin is usually about
8%. This is considerably less than that of
diethyleneglycol bisallylcarbonate polymer which has a
percentage of shrinkage of about 12%. Consequently, the
percentage of defective molded lenses that are produced is
decreased, resulting in an increased yield of rear lenses,
and accordingly, an increased yield of the composite
plastic eyeglass lenses as a whole.
As can be seen from the above discussion,
according to the present invention, the front lens, which
usually has a complex shape that must be precisely molded
in accordance with the desired multi-focus, progressive
multi-focus, or aspheric surface characteristics, is made
of diethyleneglycol bisallylcarbonate polymer. Conjointly,
the rear lens, which is usually simpler in shape than the
front lens, is made of polyurethane resin or polycarbonate
resin having the necessary degree of mechanical strength.
The separate functions of the front lens and rear lens also
contribute to an increase in the freedom of design thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be described below in detail with
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reference to the accompanying drawings, in which:
Fig. 1 is a sectional view of a composite
eyeglass lens according to the present invention;
Fig. 2 is a schematic sectional view of a molding
die which is used to mold a front plastic lens of a
composite eyeglass lens located on an object side thereof,
according to the present invention; and,
Fig. 3 is a schematic sectional view of a molding
die which is used to mold a rear plastic lens of a
composite eyeglass lens located on a user's eye side
thereof, according to the present invention.
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DESCRIPTION OF THE PREFERRED EMBODIMENT
In Fig. 1, a composite eyeglass lens 11 according
to the present invention is comprised of a pair of plastic
lenses, i.e., a front lens 12 on an object side and a rear
lens 13 on a user's eye side thereof. In the illustrated
embodiment, the composite eyeglass lens 11 has a negative
power as a whole. The front lens 12 is made of
diethyleneglycol bisallylcarbonate polymer (CR-39, n=1.5,
; ~ =55) and has a positive power. The rear lens 13 is made
of polycarbonate resin (n=1.58, ~ =29) and has a negative
power.
To mold the front lens 12, a molding cavity 24
de~ined by upper and lower lens molding dies 21 and 22, and
gaskets 23 is filled with diethyleneglycol
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bisallylcarbonate polymer, as shown in Fig. 2. The front
lens 12 can be molded by thermal polymerization of
diethyleneglycol bisallylcarbonate polymer per se known.
On the other hand, the rear lens 13 is molded by
injection molding polycarbonate resin in an injection
molding cavity 27 defined by upper and lower molding dies
25 and 26, as shown in Fig. 3.
The mating surfaces 12a and 13a (defined by the
molding surfaces of the lower molding die 22 and the upper
molding die 25) of the front lens 12 and the rear lens 13
are identical in shape. The molded front lens 12 and the
injection-molded rear lens 13 are adhered to each other at
the mating surfaces 12a and 13a by, for example, a W
setting adhesive 14 ~Fig. 1) to form a composite plastic
eyeglass lens 11 comprised of the front and rear lenses.
Since the rear lens 13 is made of polycarbonate
resin having a large refractive index n, the thickness,
particularly the thickness at the edge thereof, can be
reduced in comparison with the conventional rear lens which
is made of diethyleneglycol bisallylcarbonate polymer. The
conventional rear lens made of diethyleneglycol
bisallylcarbonate polymer is designated at a phantom line
in Fig. 1. Consequently, the overall thickness and the
edge thickness of the composite eyeglass lens 11, according
to the present invention, is less than that of the
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conventional composite eyeglass lens, resulting in a good
appearance thereof.
Furthermore, the rear lens 13 provides sufficient
mechanical strength for the eyeglass lens. In spite of the
thin center portion thereof, the lens will not be destroyed
or crushed by a steel ball in the drop-ball test.
Two examples of products (i.e., composite plastic
eyeglass lenses) are shown below.
Example 1
Four rear lenses made of CR-39, polycarbonate
resin, polyurethane resin 1, and polyurethane resin 2 were
prepared. Numerical data of the four rear lenses is shown
in Table 1 below
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Table
CR-39 PC PU 1 PU 2
lens (dioptric) power(D) 5-3.40 5-3.40 5-3.40 5-3.40
refractive index 1.50 1.58 1.60 1.66
..
first surface R (mm) 93.62 93.62 93.62 93.62
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second surface R (mm~ 57.10460.32461.041 63.014
_
center thickness (mm) 0.8 0.8 0.8 0.8
edge thickness (mm) 7 0 6.0 S 54 5 33
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wherein,
S represents a spherical lens surface;
PC represents polycarbonate;
PU 1 represents polyurethane 1; and,
PU 2 represents polyurethane 2
In Table 1, the first surface R designates the radius
(mm) of the mating lens surface 13a of the rear lens 13
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which is connected to the mating surface 12a of the front
lens 12. The second surface R designates the radius (mm) of
the lens surface of the rear lens 13 opposite the mating
surface 13a.
As can be seen from Table 1, since the rear lens made
of polycarbonate resin or polyurethane resin has a
refractive index larger than that of the rear lens made of
CR-39, it is possible to increase the radius R of the
second surface to reduce the edge thickness thereof.
Example 2
Four rear lenses made of CR-39, polycarbonate resin,
polyurethane resin 1, and polyurethane resin 2 were
prepared, similar to Example 1 above. In Example 2,
however, the four lenses are identical in shape. Namely,
the radii R of the first and second surfaces are 93.62 mm
and 57.104 mm, respectively, the center thickness 0.8 mm,
and the edge thickness 7.0 mm. The lens degrees of the
four lenses are shown in Table 2 below.
Table 2
~IPU 1
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lens power (D) 5-3.40 5-3.94 S-4.078 S-4.484
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wherein,
S represents a spherical power
PC represents polycarbonate;
s PU 1 represents polyurethane 1; and,
PU 2 represents polyurethane 2
As can be seen from Table 2, as long as the lenses
have the same shape, the lens made of polycarbonate or
polyurethane has a lens degree higher than that of the lens
made of CR-39.
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