U.S. patent application number 10/675510 was filed with the patent office on 2005-03-31 for contact lens with transition.
Invention is credited to Campbell, Charles E., Mandell, Robert B..
Application Number | 20050068490 10/675510 |
Document ID | / |
Family ID | 34313994 |
Filed Date | 2005-03-31 |
United States Patent
Application |
20050068490 |
Kind Code |
A1 |
Mandell, Robert B. ; et
al. |
March 31, 2005 |
CONTACT LENS WITH TRANSITION
Abstract
A monocentric bifocal contact lens with upper and lower optical
power zones is connected by a transition comprising a family of
sigmoidal curves. The sigmoidal curve begins with a common tangent
at the boundary of the near zone and, with a reversal of sign from
the near zone curve, climbs with increasing positive slope to an
inflection point, whereupon it continues to climb with decreasing
positive slope until reaching the distance zone curve, with which
it has a common tangent. There is no discontinuity in the first
derivative of the curve throughout its length. A sigmoidal curve
can be constructed from numerous mathematical functions, examples
of which include polynomial, conic, transendental, or cumulative
distribution curves.
Inventors: |
Mandell, Robert B.; (Moraga,
CA) ; Campbell, Charles E.; (Berkeley, CA) |
Correspondence
Address: |
Robert B. Mandell
69 Sullivan Drive
Moraga
CA
94556
US
|
Family ID: |
34313994 |
Appl. No.: |
10/675510 |
Filed: |
September 29, 2003 |
Current U.S.
Class: |
351/159.41 |
Current CPC
Class: |
G02C 7/042 20130101;
G02C 7/045 20130101; G02C 7/048 20130101; G02C 7/043 20130101 |
Class at
Publication: |
351/161 |
International
Class: |
G02C 007/04 |
Claims
We claim:
1. A bifocal contact lens formed of at least one optical material
and comprising, a back surface of generally concave shape, and a
front surface of generally convex shape, said front surface joining
said back surface at an edge perimeter, at least one of said
surfaces comprising an upper zone of optical power and a lower zone
of different optical power, said upper and lower zones connected by
a transition, a region between said bifocal area and said edge
perimeter comprising a peripheral zone, said upper zone and said
lower zone together with said transition comprising a bifocal area
that is monocentric, said transition comprising a family of curves
with an upper and a lower boundary and having a plurality of
sigmoidal shapes, whereby said lens provides a wearer with vision
that is free of image jump and allows minimal resistance to lid
movement across said lens.
2. The lens of claim 1 in which at least one of said boundaries of
said transition forms a straight line in plan view.
3. The lens of claim 1 in which at least one of said boundaries of
said transition forms a curved line in plan view.
4. The lens of claim 1 in which a midpoint of said transition
extends from a geometric center of said lens to said peripheral
zone of said lens.
5. The lens of claim 1 in which said midpoint of said transition
extends from a position decentered from said geometric center of
said lens to said peripheral zone of said lens.
6. The lens of claim 1 in which at least one surface of said upper
and lower power zones is selected from the group consisting of
spherical, aspherical and toric surfaces.
7. The lens of claim 1 in which at least one of said front and back
surfaces has a set of curvatures that correct for aberrations of
the eye.
8. The lens of claim 1 comprising prism power in at least a portion
of said lens, in addition to optical power.
9. The lens of claim 1 in which at least one of said optical power
zones is comprised of at least two optical power zones to form a
multifocal contact lens.
10. The lens of claim 1 whereby said sigmoidal curves are based on
conic functions.
11. The lens of claim 1 whereby said sigmoidal curves are based on
polynomial functions.
12. The lens of claim 1 whereby said sigmoidal curves are based on
transendental functions.
13. The lens of claim 1 whereby said sigmoidal curves are based on
cumulative distribution functions.
14. A bifocal contact lens formed of at least one optical material
and comprising, a back surface of generally concave shape, a front
surface of generally convex shape, said front surface joining said
back surface at an edge perimeter, at least one of said surfaces
comprising an upper zone of optical power and a lower zone of
different optical power, said upper and lower zones connected by a
transition, a region between said bifocal area and said edge
perimeter comprising a peripheral zone, said upper zone and said
lower zone together with said transition comprising a bifocal area
that is monocentric, said transition comprising a family of curves
with an upper and a lower boundary and slopes that are equal to the
slopes of said power zones at said boundary, each curve of said
family of curves comprising a portion that rises from its lowest
point with increasing positive slope to an inflection point,
whereupon said curve continues to rise with decreasing positive
slope until reaching its highest point, whereby said lens provides
a wearer with vision that is free of image jump and said lens
allows minimal resistance to lid movement across said lens.
15. A bifocal contact lens formed of at least one optical material
and comprising, a back surface of generally concave shape, a front
surface of generally convex shape, said front surface joining said
back surface at an edge perimeter, at least one of said surfaces
comprising an upper zone of optical power and a lower zone of
different optical power, said upper and lower zones connected by a
transition, a region between said bifocal area and said edge
perimeter comprising a peripheral zone, said upper zone and said
lower zone together with said transition comprising a bifocal area
that is monocentric, said transition comprising a family of curves
which begin at a connection to a positive radius of said lower zone
with an inflection, that is followed by a negative radius that
increases continuously until reaching a radius of infinity at an
inflection of positive slope, which is followed by a positive
radius that decreases continuously until reaching said upper zone,
where it changes to the radius of the upper zone, whereby said lens
provides a wearer with vision that is free of image jump and said
lens allows minimal resistance to lid movement across said
lens.
16. A bifocal contact lens formed of at least one optical material
and comprising, a back surface of generally concave shape, a front
surface of generally convex shape, said front surface joining said
back surface at an edge perimeter, at least one of said surfaces
comprising an upper zone of optical power and a lower zone of
different optical power, said upper and lower zones connected by a
transition, a region between said bifocal area and said edge
perimeter comprising a peripheral zone, said upper zone and said
lower zone together with said transition comprising a bifocal area
that is monocentric, said transition comprising a plurality of
curves of a length that is greater than the chord width of a
cutting tool surface at the depth used in cutting said transition,
whereby said lens provides a wearer with vision that is free of
image jump and said lens allows minimal resistance to lid movement
across said lens.
17. A bifocal intraocular lens formed of at least one optical
material and comprising, a back surface of optical power, and a
front surface of optical power, said front surface joining said
back surface at an edge perimeter, at least one of said surfaces
comprising an upper zone of optical power and a lower zone of
different optical power, said upper and lower zones connected by a
transition, said upper zone and said lower zone together with said
transition comprising a bifocal area that is monocentric, said
region surrounding said bifocal area comprising structures to
support said lens in the eye, said transition comprising a family
of curves having a plurality of sigmoidal shapes, whereby said lens
provides a wearer with vision that is free of image jump and
provides a lens with smooth surfaces.
18. A method of manufacturing a monocentric bifocal contact lens in
which a computer controlled lathe capable of an oscillating motion
of the cutting tool uses a points file to generate the following, a
first surface area about a center of curvature that lies on a
predetermined position from the axis of a second surface area about
a center of curvature which is a greater distance from the spindle
axis than is said center of curvature of said first optical power,
to form a second optical power zone in the lower portion of said
lens, a transition between said zone of said first optical power
and said zone of said second optical power which is defined by a
sigmoidal function that is selected so as to produce no change in
slope where joining surfaces of said zones of optical power,
whereby said lens provides a wearer with vision that is free of
image jump and said lens allows minimal resistance to lid movement
across said lens.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field
[0002] This invention relates to bifocal contact lenses and bifocal
intraocular lenses with upper and lower optical power zones.
[0003] 2. Prior Art
[0004] Bifocal contact lenses are lenses with at least two regions
of different optical powers, known as zones or segments. Usually,
one power is chosen to provide the wearer with clear distance
vision and the second power to provide clear near vision, but any
two powers may be selected. Bifocal contact lenses also may be
called multifocal contact lenses, although the latter term is
sometimes reserved for lenses comprised of at least three regions
with different optical powers or regions of variable power, as in
U.S. Pat. No. 5,517,260 (Glady) and U.S. Pat. No. 5,754,270
(Rehse.) Bifocal contact lenses have some features in common with
bifocal intraocular lenses and some differences.
[0005] Bifocal contact lenses generally are classified into two
types, concentric and vertically segmented. Both types can be
produced as rigid or soft contact lenses.
[0006] Concentric bifocal contact lenses have a central power zone
surrounded by one or more annular zones of different powers or a
sequence of alternating powers. Generally, the lens is designed so
as to have little motion on the eye and the wearer views through
portions of more than one zone at the same time, a process called
simultaneous vision, as described in U.S. Pat. No. 4,636,049
(Blacker); U.S. Pat. No. 4,752,123 (Blacker); U.S. Pat. No.
4,869,587 (Breger); and U.S. Pat. No. 5,864,379 (Dunn). The
distance and near zones, together with optional transition curves,
comprise the bifocal area. The peripheral portion of the lens is
comprised of one or more curves that are used to connect the
bifocal area to the edge perimeter, including options currently in
use such as prism ballast, slab-off, tapers, peripheral curves,
lenticular curves, truncations and Vertically segmented bifocal
contact lenses have vertically separated power zones, an upper zone
that usually provides the appropriate correction for viewing far
distances and a lower zone, which usually provides the appropriate
correction for viewing near distances. The lenses are designed to
alternate their position in front of the pupil when the lens moves
up and down on the eye as the result of lid forces, which occur
when the wearer changes gaze between different distances, a process
called alternating vision, as described in U.S. Pat. No. 3,597,055
(Neefe) and U.S. Pat. No. 3,684,357 (Tsuetaki). If there is little
vertical movement then vertically segmented bifocal contact lenses
may also function as a simultaneous vision lens.
[0007] The two vertically separated power zones maintain their
relative positions by various features that can be added to control
the lens position and stabilize the meridional rotation as
described in U.S. Pat. No. 4,095,878 (Fanti); U.S. Pat. No.
4,268,133 (Fischer); U.S. Pat. No. 5,760,870 (Payor); U.S. Pat. No.
5,296,880 (Webb); and U.S. Pat. No. 4,573,775 (Bayshore). This is
commonly accomplished in rigid bifocal contact lenses by
incorporating a prism into the lens, which provides a progressively
greater thickness from the top to the bottom of the lens. The prism
serves to maintain the desired lens orientation and keep the lower
zone of the lens downward on the eye as described in U.S. Pat. No.
5,430,504 (Muckenhirn) and U.S. Pat. No. 4,854,089 (morales) and in
Burris, 1993; Bierly, 1995, and Conklin Jr. et al, 1992. The lower
edge of the lens is designed to rest upon the lower lid margin of
the wearer and the lens shifts up and down relative to the eye as
the result of lid forces. There are several subtypes of vertically
segmented bifocal contact lenses, based on the shape of the near
zone, including round, D-shaped, flat, crescent, and others as
described by Conklin Jr. et al, 1992 and in U.S. Pat. No. 4,618,229
(Jacobstein) and U.S. Pat. No. 5,074,082 (Cappelli).
[0008] There have been attempts to incorporate prism into soft
bifocal contact lenses for the same functional purpose as prism
provides for rigid lenses. U.S. Pat. No. 4,549,794 (Loshaek); U.S.
Pat. No. 5,635,998 (Baugh); U.S. Pat. No. 4,618,229 (Jacobstein)
Ezekiel, 2002, but generally these lenses have inadequate lens
movement or produce discomfort to the wearer. There also have been
attempts to induce a vertical shift of a soft bifocal contact lens
by adding features to the lower periphery of the lens, as described
in U.S. Pat. No. 4,614,413 (Obssuth); U.S. Pat. No. 5,635,998
(Baugh); U.S. Pat. No. 6,109,749 (Bernstein): U.S. Pat. No.
5,912,719; and European Pat. EP0042023 (Muller).
[0009] A more successful soft bifocal contact lens design (U.S.
patent application Ser. No. 09/908,296 (Mandell)) contains at least
two prisms. A primary prism controls lens positioning and
meridional orientation, while a secondary prism controls lens
movement.
[0010] Transition
[0011] The power zones of a bifocal contact lens contain different
surface curvatures, which are linked together by a transition. The
transition may have zero or finite width and may vary in design,
depending upon how the adjacent power zones are oriented with
respect to each other.
[0012] If a bifocal contact lens has a transition of zero width and
there is a change in slope of the adjacent zones at their junction,
the transition will appear as a line when viewing the front surface
of the lens as shown in U.S. Pat. No. 4,752,123 (Blacker). If there
is no change in slope, the transition will appear smooth and will
not be detectable by surface inspection.
[0013] Concentric bifocal contact lenses are available with a
greater variety of transition designs than are found in vertically
segmented lenses. An example is shown in FIG. 1, which illustrates
the midline cross-section construction of a prior-art concentric
bifocal contact lens 14. On a front surface 15 a distance power
zone 16 is located in the center of the lens and a near power zone
17 surrounds the distance zone in a concentric arrangement. A
center of curvature for the distance zone 18 and another for near
zone 19 lie on a common axis of symmetry, so that at a transition
20 between the zones there is not only a change in curvature, but
also a change in slope that is equal to an angle 21 between a
radius for the distance power zone 22 and a radius for the near
power zone 23. The front surface 15 of lens 14 has a visible
transition line in the shape of an arc of a circle. Distance zone
16 and near zone 17 comprise a bifocal area 24, limited by a
bifocal perimeter 25, which is surrounded by a peripheral zone 26
that extends to an edge perimeter 27.
[0014] FIG. 2 shows another example of a prior-art concentric
bifocal contact lens front surface, which has center of curvature
19 for near zone 17 that occurs on lines connecting center of
curvature 18 for distance zone 16 and transition 20. There is no
slope change at transition 20 and there is no visible transition
line when viewing the front of the lens. However, in three
dimensions the center of curvature for the near segment is a locus
of points that form a circle and the near zone is part of a torus,
rather than a sphere. If the radius of the torus increases towards
the periphery, near zone 17 is an aspheric curve. This arrangement
can be used to connect a spherical distance zone to an aspherical
near zone with no slope change at the transition. In a similar
manner various combinations of spherical and aspherical curves can
be combined to produce a variety of concentric bifocal designs
having no slope change at the transition. A front view of the
surface any of these of lenses does not show a visible
transition.
[0015] There are several design options currently available for the
transition of bifocal contact lenses with vertically segmented
zones. One design is a front surface bifocal in which the center of
curvature of the near zone is displaced upward with respect to the
distance zone. This creates a transition in which the two adjacent
zones join together at the same height, relative to the back
surface, but with an instantaneous change in slope as revealed in
U.S. Pat. No. 4,854,089 ((Morales). A front view of such a lens
appears as a line that is arc shaped, concave upward.
[0016] Unfortunately, an abrupt change in slope at the transition
between two adjacent power zones is accompanied by a prismatic
optical difference, which causes the wearer of the contact lens to
observe a change in the image position when his gaze is shifted
between the distance and near zones, a phenomenon known as image
jump. Most contact lens wearers find that Image jump is disturbing
and generally poorly tolerated. Furthermore, if the lens assumes a
position on the eye such that the transition lies in front of the
pupil, the prismatic difference of light passing into the eye
simultaneously from both the distance and near zones will cause the
wearer to experience image doubling, which is intolerable to the
lens wearer.
[0017] In another prior-art bifocal contact lens design, image jump
and doubling were avoided by making the bifocal contact lens
monocentric. (R Mandell, 1967, 1974, 1988) A monocentric bifocal
lens is a lens that has no prismatic difference between the power
zones at the transition (FIG. 3). Monocentricity can be produced on
front surface 15 of bifocal contact lens 14 by locating center of
curvature 18 for distance zone 16 and center of curvature 19 for
near zone 17 on a common line that also passes through transition
20. Unfortunately, the optical advantage of monocentricity is
accompanied by a physical limitation.
[0018] FIG. 4 shows how the two power zones of FIG. 3 can be made
to coincide at a midpoint 28 of transition 20 but they do not
coincide at peripheral points 29 on transition 20 due to the
difference in curvature of the zones. The height of distance zone
16 along transition 20 increases towards the periphery relative to
the height of near zone 17. Consequently there is a step up in
height in passing from the near to the distance zone and the step
increases towards the lens periphery. When the lens is worn, the
transition step interrupts the smooth flow of the lid across the
lens during blinking.
[0019] FIG. 5 shows the magnitude of the step height for a range of
moncentric bifocal contact lens parameters. The step height
increases with the power of the bifocal add as well as from the
midpoint to the periphery of the transition. For example, given an
add power of 3.00 diopters and an optic zone diameter of 8 mm the
step height would range from zero at the midpoint to 0.052 mm in
the periphery. Typical maximum values for step heights of bifocal
adds between 1.00 and 4.00 diopters would range from about 0.02 to
0.07 mm.
[0020] The step height of the transition can be changed by a
modification of the monocentric bifocal design. For example, the
centers of curvature for the distance and near zones can fall on a
common line with the transition, but the distance zone is displaced
inwardly at the midpoint of the transition. A step down occurs from
the near to the distance zone at the midpoint of the transition.
The two zones have the same height at two points along the
transition in the periphery, as in U.S. Pat. No. 4,549,794
(Loshaek).
[0021] Another option for the design of a moncentric bifocal
contact lens is intermediate to the other two designs. The distance
and near zones are positioned such that there is a step inward near
the midpoint of the transition that is less than the step in Option
2. The step decreases towards the periphery until at some
intermediate position on the transition there is no step, followed
at more peripheral locations by a step outward that would increase
towards the periphery.
[0022] In theory the step that occurs at the transition of the
monocentric bifocal of FIG. 4 consists of an abrupt increase in
height, which can be represented by a square wave function. The
surface of the near zone reaches the transition and then an
instantaneous height increase occurs in passing to the distance
zone as shown in U.S. Pat. No. 5,245,366 (Svochak). In practice, a
square wave function is not produced on the lens because of the
constraints of manufacturing. For example, in using a standard
lathe to manufacture the monocentric bifocal contact of FIG. 4, the
entire surface of the lens is first generated using the radius of
the distance zone. Then, the cutting tool is adjusted for the near
radius and the center of rotation is offset to fall on the line
connecting the centers of curvature for the distance zone and the
transition point. Next the near zone is generated up to the
transition, at which position the curvature of the cutting tool
will be imparted into the portion of the near zone that is adjacent
to the distance zone. The impression made by the cutting tool
becomes the transition curve of the lens. The mathematical function
that represents the transition shape is determined by the shape of
the tool. Since the cutting edge of the tool is a convex circle or
asphere, the transition that is formed will be a concave negative
replica of the cutting tool.
[0023] Generally it is found that most cutting tools used in the
manufacture of contact lenses have a radius of curvature at the
cutting edge that is between 0.1 and 0.6 mm. Therefore, it is this
same radius that will be found in negative form on the transition
of the lens that is manufactured. The result is fortuitous because
the curvature of the transition is more gradual than would be the
case were an actual square wave function created. The more gradual
slope of the transition creates a smoother surface for the lid to
pass over and adds to the comfort of the lens when worm. However, a
transition created by the shape of the cutting tool is not ideal
since it presents abrupt slope changes in passing from the
transition to the power zones. In addition, the width of the
transition, and hence the rate of change of its slope, is limited
by the radius of the lathe cutting tool.
[0024] An alternate method of manufacturing a monocentric bifocal
contact lens can be achieved by using an oscillating lathe, such as
the Precitech Optomform 40 with Variform Generator by Sterling Co.
of Tampa, Fla. or DAC Series IV/2 Axis ALM by DAC International of
Carpinteria, Calif. This type of lathe can be used to generate two
or more zones on the front surface of a monocentric bifocal lens in
a single continuous motion by varying the distance of the cutting
tool relative to the lens back surface during each rotation of the
lathe spindle. The cutting tool moves in and out from the lens
surface with each cycle at the same time it traverses from lens
edge to center. A problem is presented when the lathe tool passes
between the distance and near zones, which are at different
heights. The cutting tool cannot make an instantaneous change in
height, which would require a square wave motion. Instead, the most
efficient transition curve is used that will allow the lathe to
change the height of the cutting tool as rapidly as possible. The
curve that is usually chosen to do this is based on a sinusoidal
function. This results in a transition curve that is smooth but
which has a change in slope at the connections to the adjacent
zones. In addition, the curve may not be smooth in a radial
direction.
[0025] Another attempt was made at smoothing the transition between
the distance and near zones of a bifocal contact lens is by using a
mechanical device that changes radius of curvature while cutting
the transition area, as in U.S. Pat. No. 5,430,504 (Muckenhim).
This device produces a curve for the transition that varies
continuously from the radius of the distance zone to the radius of
the near zone. This results in a curve instead of a step at the
transition, but also produces a line at the boundary between the
junction and the distance power zone, where a slope change is
evident.
[0026] Intraocular (implanted) bifocal lenses are manufactured by
essentially the same process as bifocal contact lenses, except for
their biconvex shape. In these lenses, if there is a rough or
abrupt transition it can cause the accumulation of debris in the
eye and produce adverse reactions. Although an intraocular bifocal
lens might have restricted movement the optical advantage of
monocentricity would still provide optimal simultaneous vision. The
principles of the present invention for bifocal contact lenses
would also apply to intraocular lenses.
OBJECTS AND ADVANTAGES
[0027] It is accordingly one object of the present invention to
provide contact and intraocular lenses that will provide viewing
both distance and near objects without image jump and doubling.
[0028] A further object of the present invention is to provide a
monocentric lens that has a step in height between the power zones
of the lens that is not apparent.
[0029] Further objects of the present invention are to provide a
bifocal contact lens that will give maximum comfort to the wearer,
to provide a bifocal contact lens that can be manufactured using
standard lathing, oscillating tool lathing or molding techniques,
to expand the number of patients who are able to wear bifocal
contact lenses, and to provide a bifocal lens that gives optimal
vision for both distance and near vision without image jump or
doubling.
[0030] Another advantage is to provide a contact lens that has a
bifocal surface that is smooth and free of sudden height steps that
may interrupt the smooth flow of the lid across the lens during
blinking.
[0031] Further objects and advantages of the present invention will
become apparent from a consideration of the drawings and ensuing
description.
SUMMARY
[0032] According to the present invention, monocentric bifocal
contact lenses and intraocular lenses with upper and lower optical
power zones are connected by a transition comprising a family of
sigmoidal curves. The ends of the sigmoidal curves have common
tangents with the curves of the adjacent power zones at their
boundaries. Each sigmoidal curve begins with a common tangent at
the boundary of the lower zone and climbs with increasing positive
slope to an inflection point, whereupon it continues to climb with
decreasing positive slope until reaching the upper zone curve, with
which it has a common tangent. There is no discontinuity in the
first derivative of the curve throughout its length. A sigmoidal
curve can be constructed from numerous mathematical functions,
examples of which include polynomial, conic, transendental, or
cumulative distribution curves. The height of the sigmoidal curve
is determined by the step height between the two power zones to be
connected. The length of the family of sigmoidal curves is chosen
independently and is longer than the narrowest curve that can be
produced by the manufacturing process. The sigmoidal curves provide
a smooth transition and make the transition less visible.
[0033] As the step height of a monocentric bifocal contact lens
increases from the midpoint to the periphery of the transition, the
sigmoidal curves that form the transition will increase in length
and height towards the lens periphery. As a result, the family of
sigmoidal curves has smoothness in a radial direction as well as in
the direction of the sigmoidal curves.
[0034] The general shape of the transition area, as it appears from
the front of the lens in isometric projection, can vary widely. In
general the transition forms two sectors of the bifocal area, which
are usually symmetrical with respect to a midpoint but can be
oriented at different angles. The boundaries between the transition
and the power zones may be straight or curved and the transition
can be decentered along any meridian.
[0035] Various combinations of boundary shapes and positions can be
created. The parameters that can be used to control the
characteristics of the transition are as follows:
[0036] 1. Midpoint position. Centered or displaced up or down, left
or right, or at an angle from the lens geometric center.
[0037] 2. Radius of the lower boundary of the transition as
constructed by orthogonal projection from the front of the
lens.
[0038] 3. Radius of the upper boundary of the transition as
constructed by orthogonal projection from the front of the
lens.
[0039] 4. Angle of the transition as measured by its angular
subtense from the midpoint of the transition to the edge of the
bifocal area.
[0040] 5. Type of sigmoidal function used to connect the two
adjacent power zones.
DRAWINGS--FIGURES
[0041] FIG. 1 is a midline cross-section of a prior-art concentric
bifocal contact lens in which a prismatic change occurs at the
transition.
[0042] FIG. 2 is a midline cross-section of a prior-art concentric
bifocal contact lens front surface in which no prismatic change
occurs at the transition.
[0043] FIG. 3 is a midline cross-section of a prior-art vertically
segmented bifocal contact lens front surface in which no prismatic
change occurs at the transition, making it monocentric.
[0044] FIG. 4 is a perspective view of a prior-art monocentric
bifocal contact lens showing the increase in transition step height
towards the periphery.
[0045] FIG. 5 is a graph of the transition step height as a
function of the distance from the transition midpoint, for
representative bifocal powers.
[0046] FIG. 6 is a plan view of a front surface of one embodiment
of a bifocal contact lens, in accordance with the present
invention.
[0047] FIG. 7 is a representation of a sigmoidal curve used to form
the bifocal transition of FIG. 6.
[0048] FIG. 8 illustrates the relative relationship of a single
sigmoidal curve to the family of sigmoidal curves that form the
transition of FIG. 6.
[0049] FIG. 9 is a plan view of the bifocal area of FIG. 6,
modified so that the lower boundary is straight and the upper
boundary is angled upward on each side.
[0050] FIG. 10 is a plan view of the bifocal area of FIG. 6,
modified so that the lower boundary is straight and the upper
boundary is curved upward on each side.
[0051] FIG. 11 is a plan view of the bifocal area of FIG. 6,
modified so that the lower boundary is curved upward and the upper
boundary is curved upward a greater amount than the lower
boundary.
[0052] FIG. 12 is a plan view of the bifocal area of FIG. 6,
modified so that the lower boundary is curved downward and the
upper boundary is curved upward.
[0053] FIG. 13 is a plan view of the bifocal area of FIG. 6,
modified so that the lower boundary is curved downward and the
upper boundary is curved less than the lower boundary. The
transition midpoint is decentered.
DETAILED DESCRIPTION
[0054] FIG. 6 is a plan view of a front surface 15 of a bifocal
contact lens 14, in accordance with the invention. Front surface 15
is comprised of bifocal area 24 that is limited by bifocal
perimeter 25, and surrounded by peripheral zone 26 that extends to
edge perimeter 27. Bifocal area 24 is comprised of an upper zone 30
of an optical power for distance vision and a lower zone 31 of an
optical power for near vision, which are connected by transition
20. Transition 20 is comprised of a midpoint 28, a periphery 32, a
lower boundary 33 with lower zone 31 and an upper boundary 34 with
upper zone 30. Lower boundary 33 follows a straight line in a
horizontal orientation and upper boundary 34 follows two straight
lines that are angled upward on each side from transition midpoint
28 to transition periphery 32.
[0055] A sigmoidal-curve path 35 is represented by an arc that is
concentric with midpoint 28 of transition 20. Midpoint 28 of
transition 20 coincides with a geometric center 36 of lens 14.
[0056] FIG. 7 is a representation of a sigmoidal curve 37, which
follows path 35 of transition 20 of FIG. 6. Sigmoidal curve 37
begins at its connection with a positive radius 23 of lower zone 31
with a common tangent at a first inflection point 38, where the
radius of curvature is infinity. Sigmoidal curve 37 then becomes a
negative radius 39, which decreases radius value as the curve
climbs with increasing slope to a second inflection point 40, at
which the radius is again infinity. From second inflection point 40
the curve climbs further with decreasing slope and a positive
radius 41 which decreases from infinity until it reaches positive
radius 22 of upper zone 30 with a common tangent at their
connection 42.
[0057] FIG. 8 shows the relative relationship of sigmoidal curve 37
of FIG. 7 to the family of sigmoidal curves 43 used to form the
increasing width of transition 20 of FIG. 6, from zero at midpoint
29 to a maximum at bifocal perimeter 25.
[0058] FIG. 9 is a plan view of bifocal area 24 of FIG. 6 showing
transition 20a with lower boundary 33a that is a line in a
horizontal orientation and an upper boundary 34a that is a line
angled upward on each side from transition midpoint 28a to
transition periphery 32a. Transition midpoint 28a is displaced
upward from geometric center 36 of bifocal area 24.
[0059] FIG. 10 is a plan view of bifocal area 24 of FIG. 6 showing
transition 20b with lower boundary 33b that is a straight line in a
horizontal orientation and an upper boundary 34b that is concave
upward on each side from transition midpoint 29b to transition
periphery 32b. Transition midpoint 29b is displaced downward from
geometric center 36 of bifocal area 24.
[0060] FIG. 11 is a plan view of bifocal area 24 of FIG. 6 showing
transition 20c with lower boundary 33c that is a concave upward and
an upper boundary 34c that is more concave upward on each side from
transition midpoint 28c to transition periphery 32c. Transition
midpoint 28c is displaced downward from geometric center 36 of
bifocal area 24.
[0061] FIG. 12 is a plan view of bifocal area 24 of FIG. 6 showing
transition 20d with lower boundary 33d that is concave downward and
an upper boundary 34d that is concave upward on each side from
transition midpoint 28d to transition periphery 32d.
[0062] FIG. 13 is a plan view of bifocal area 24 of FIG. 6 showing
transition 20e with lower boundary 33e that is concave downward and
an upper boundary 34e that is concave upward on each side from
transition midpoint 28e to transition periphery 32e. Transition
midpoint 28e is displaced laterally from geometric center 36 of
bifocal area 24.
Manufacturing the Lens
[0063] The bifocal contact lens can be manufactured using an
oscillating lathe with computer controller such as the Precitech
Optomform 40 with Variform Generator from Sterling of Tampa, Fla.
or DAC Series IV/2 Axis ALM from DAC International of Carpinteria,
Calif. This type of lathe is programmed to generate a surface for
the lens from a three-dimensional points file or family of curve
segments. The file can be based on any mathematical function that
fulfills the requirements of a sigmoidal curve.
[0064] In order to create the points file, the data specifying the
front surface of the sigmoid lens are formed as an array of surface
elevation values above a fixed plane transverse to the axial of
rotation of the lathe that will form the surface. Instructions to
the lathe are best given in its natural coordinate system, a
cylindrical coordinate system whose axis of rotation is that of the
lathe. In a plane perpendicular to this axis of rotation the
discrete data locations form a polar coordinate grid whose radial
values are evenly spaced from the center to a value equal to the
radius distance from the center to the edge of the lens on
meridians that are evenly spaced around the lens. It is convenient
to specify 256 radial positions, so that the radial distance to the
edge of the lens is divided by 256 to create a radial increment
value and this is the distance between data locations from the
center to the edge on each meridian. Sufficient data density is
created by spacing the meridians 2 degrees apart. This completely
specifies the data locations. For ease of calculation, it is better
to store these locations in Cartesian form rather than in polar
form so each data location pair, (r,.theta.) is converted to (x,y)
values in standard fashion, i.e.
x=r cos(.theta.)
y=r sin(.theta.)
[0065] The data array itself should be thought of as an array of
locations, in a memory file, into which the surface elevation
values will be placed, once they are found. Associated with this
array are two other arrays called the data position arrays, one
labeled the x array and the other labeled the y array. They are the
same size as the data array and in each location of the position
arrays is placed the position value, either x or y, of the
associated elevation value.
[0066] The two surfaces may now be combined with a sigmoid
transition zone. This is done using a transition mask that takes
the value 1 in areas where the full add is wanted and the value 0
were the full distance power is wanted. In between, in a crescent
shaped area the mask smoothly changes value following a sigmoid
curve from 1 to 0. The mask is multiplied times the value of the
distance zone curve minus the near zone curve at each point and
then this array of values is subtracted from the distance curve
values. This creates a combined power surface with a proper sigmoid
transition zone.
[0067] The principle of the mask used for this purpose is to create
a function that when multiplied times a second function causes that
value of the combined function to smoothly change from the value of
the first function to zero over some distance at which the mask
operates. The logical transition mask has the value 1 at all data
locations where it is decided that there is to be no change in the
value of the second function. Therefore after the application of
the mask via a point by point multiplication of the mask-to the
second function, the combined function still equals the first
function in this area. Outside the zone in which there is to be no
change, the value of the mask changes smoothly in some specified
manner until the value of zero is reached at the edge of the
transition zone. This causes the value of the combined function,
created by multiplying the mask times the second function, to vary
in smooth fashion from the value of the second function at the edge
of the transition zone to zero at the other edge.
[0068] Naturally there are cases in which it is not desired that
the combined function goes to zero at the edge of the transition
zone and this is accomplished by subtracting from the second
function the value desired at the edge of the of the transition
zone before applying the mask. Then after applying the mask, the
value subtracted is added back to all points. This technique can be
used in applying the sigmoid crescent mask.
[0069] An alternative method for the manufacture of the lens is to
first prepare a mold ether by direct lathing using the method
followed for constructing the lens or by molding lens surfaces of
the desired shapes. The molds may then be used to form a contact
lens of any suitable optical material that can be molded.
[0070] Conclusions, Ramifications, and Scope
[0071] There are a number of general considerations that apply to
the present invention.
[0072] A sigmoidal curve is ideal for connecting the height
difference between the two adjacent power zones of a monocentric
bifocal contact lens. It presents the smoothest transition for the
lid to pass over the junction. By choosing a sigmoidal curve
function for the transition of a vertically segmented bifocal
contact lens that is monocentric, and spreading the transition over
a greater area than that created by the cutting tool shape, it is
possible to produce a more comfortable bifocal contact lens than
previous designs.
[0073] We have found that if the transition zone has a sigmoidal
shape, there is no appreciable decrement to the image seen by the
contact lens wearer. This occurs because light passing through the
transition is dispersed over a wide area and is not perceived as
spurious images by the wearer.
[0074] The exact shape of the sigmoidal curve can vary over a large
range of parameters. Its length will exceed the chord width of the
cutting tool surface at the depth used in its construction. The
sigmoidal curve need not be symmetric about the inflection point
and the inflection point need not be in the center of the curve.
The ends of the sigmoidal curve will meet each adjacent zone curve
with a common tangency so that there is a perfectly smooth
transition. If the sigmoid curve has a slope that is equal to the
slope of the curve in the connecting power zone, the connection
will be smooth and no line will be apparent when observing from in
front of the lens.
[0075] The principles of the invention may be applied to a variety
of bifocal contact lenses, including those designed for
simultaneous or alternating vision.
[0076] A curve that is similar to a sigmoid curve can be produced
on a vertically segmented bifocal contact of monocentric design by
beginning with a lens of the prior art design of FIG. 4 and then
polishing the abrupt connection between the transition and each
adjacent zone until there appears to be a smooth connecting curve.
The procedure would not produce a sigmoid curve and would not have
a common tangent with each adjacent zone at its connection.
Further, it would not be based on a known mathematical function and
would not be reproducible.
[0077] In the preceding examples of the present lens, the bifocal
portion of the contact lens was placed on the front surface. The
back surface of the lens can be designed in a manner so as to fit
the cornea of the wearer using methods known to those familiar with
the state of the art. Generally, the curvature of the back surface
of the contact lens is made very similar to the curvature of the
cornea. However, there are purposeful differences made in the lens
curvatures from the corneal curvatures, which are governed by the
shape of the cornea, the shape of the contact lens and the
interrelationship that is desired by the fitter in order to control
the lens riding position on the cornea and the lens movement.
[0078] The surface of the power zones may be comprised of
spherical, toric, or aspherical curvatures.
[0079] The transition may extend to the bifocal perimeter or may be
connected to the bifocal perimeter by a blending curve or other
connecting curve.
[0080] The bifocal lens may have one or more non-optical features
such as prism ballast, slab-off, tapers, peripheral curves,
lenticular curves, truncations and edge contours, as are found in
present contact lenses as well as oval or other commonly known
perimeter shapes. The edge is the most peripheral contour on the
lens and ends at the edge perimeter, which is the most peripheral
limit formed by the maximum diameters in all meridians. The
peripheral zone may vary in width at different meridians and may
not extend around the entire bifocal area.
[0081] The principles that are described may be applied to lenses
made of any optical material, hard, flexible, soft, hydrophobic or
hydrophilic, that is suitable for a lens.
[0082] The principles of this invention may also be applied to one
surface of an intraocular lens in order to eliminate image doubling
and to avoid step boundaries, which would otherwise occur and tend
to collect intraocular debris and deposits.
[0083] Since the bifocal has a smooth surface it is possible to
make a contact lens in which the bifocal surface is either on the
front surface, back surface, or both surfaces. The lens may be
designed for simultaneous or alternating vision.
* * * * *