Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
s
B~ckground of the Invention
Field of the Invention:
This invention relates to improvements in ophthalmic
lenses and more particularly to the correction of off-axis
errors by means of a radial gradation of refractive index
between the optical center and edges of the lenses together
with proper control of surface curvatures and center thicknesses.
Discussion of the Prior Art:
In the designing of corrected spectacle lenses, it is
common practice for the designer to make extensive off-axis
computation and select the base curve which achieves the best
performance, i.e. the "tool" used by the designer is the
overall amount of bending employed in a given lens.
Off-axis errors of prime concern are astigmatism and
curvature of field (power error) with a third aberration,
distortion, being of lesser but important consideration.
Relatively recent approaches to improving off-axis
correction in ophthalmic lenses have included the use of
aspherics wherein non-spherical surface curvatures are used in
conjunctin with proper base curve selection to further
reduce off-axis astigmatism and curvature of field with
improvement in distortion as well.
While aspheric surface curvatures in off-axis correction
situations are most easily and economically applied to lenses
which can be cast, e.g. resin lenses, this approach to oblique
correction can also be used on glass.
., .
jl/ -2-
,.
. .
L5
Thc art, beinL pres~!rlcly Llmil:ecl to ooe or combinations
of the ~Eoresaid tecllni(lues for redllclng o~-axLs aberratlons
in ophthalmic lens desigrl~ Ls need~ll of improved designs and
methods o~ their applicatlon which can be i~nplemented wlth
greater ease and economy, especlally on glass, and which can
offer greater variety and versatiliî:y to designers in their
selections oE aberrations to be corrected and the order of
priority or emphasis applLed thereto ln particular oblique
correction situations.
Accordingly, lt is an object of the present invention to
make possible off-axis correction in ophthalmic lens design
without aspheric surface tre~tmel-t, if desired, but which is
comparable to and/or improved over prior art accomplishments
with aspherics.
Another object is to apply a novel conjunct to aspheric
correction in lens design wherewith a higher degree of oblique
correction can be achieved and further wherewith more than the
, usual prime aberrations of astigmatism and curvature of field
may be successfully dealt with.
~20 ~ Still another object is to render possible a designer's
selection of aberrations to be attended to with emphasis
thereon in an order of his choice and with greater versatility
than the hèretofore limited "tool" of controlled surface
bending but not without surface bending; and
A more general object is to provide for greater ease and
versatility in the designing of corrected ophthalmic lenses,
improvements of substantial significance and importance in `
resulting lenses, greater economy in the implementation of
corrections of prime concern aberrations and others of
importance and greater freedom of manner of applying variables
of base curve selection and asRhericity in the correction of
oblique aberrations.
jl/ -3-
Other ob~ects and advantages of the inventlon
will become apparent fron~ the ~ollowlng summary of the
invention and descript:ion of preferred embodiments.
SU~MARY OF T~IE INVENTION
The present invention relates to the method of
~ correcting off-axis errors in the manufacture of an
: ophthalmic lens comprising in addition to affording the
lens with optically finished conve:~ and concave opposite
sides of preselected curvatures, a particular center
thickness and a pr~-established refractive index value
adjacent its axis, further effecting a gradation of
refractive index in the material of the lens radially from
its axis toward its edge.
In its article aspect, the invention relates to
a corrected ophthalmic spectacles lens of meniscus config-
uration having optically finished convex and concave opposite
- side surfaces of preselected curvatures and a particular
: center thickness and refractive index value adjacent its
axis, one of the curvatures being the base curve of the lens
and there being a gradation of refractive index in the
material of the lens extending in directions radially from ~ :
its axis toward its edge, the geometrical surface configura- :
tion of said base curve being selected and formed in conjunc~
tion with the axial refractive index value, center thickness
and opposite surface curvature according to off-axis correction ;~
desired for at least one of ophthalmic lens aberrations
including power error, astigmatism and distortion and the
gradation of refractive index affording off-axis correction
of at least one of the lens aberrations of power error and
astigmatism.
In another aspect, the invention relates to an
ophthalmic lens blank having at least one curved side surface,
bc/~.
1~'7'~
the curved surface constitutil-g tl~e basc curve of a lens to be
produced from the blank and t~lcre bcing a gradatLon of refractive
index in the material of the lens blank extending in directions
radially from its axis toward its eclge, the geometrical surface
configuration of the base curve being selected and formed in
conjunction with the axial refractive index value according
to a center thickness and opposi te surface curvature required
for off-axis correction of at least one of ophthalmic lens
aberrations including power error, astigmatism and distortion
and the gradation of refractive index affording off-axis correc-
tion of at least one of the aberrations of power error and
astigmatism in the finished lens to be formed from the blank.
Thus, according to the present invention, off-axis
correction in ophthalmic lens design is accomplished, at
least in part, by varying the index of refraction of a lens
by controlled amounts from its optical center radially to its
. j .
edge. Utilizing regular spherical and/or toric lens surfaces
of preselected dioptric values, off-axis corrections of lens
aberrations comparable to and improved over those which may be
accomplished with aspheric surface design are possible. Thus,
the relatively complex and expensive processes of applying
aspheric corrections to glass lenses can be avoided without
sacrifice of oblique correction quality. In this connection,
a lens base curve may be chosen so as to minim;ze astigmatism
`~ and a refractive index gradient used to control curvature of
`~ field.
Still higher degrees of correction are contemplated
by using an index gradient along with an aspheric surface, and,
of course, with careful selection of base curve. Base curve
selection may be made with reduction of distortion in mind,
asphericity chosen to minimize astigmatism and a refractive
- 4a -
,,~ .
bc/
:,
~3'7'7;~
index gracl:ient util:iæed to reducc curvatllre of field tpower
error).
Details of the invent:Lon will become more readily
apparent Erom the following description when taken in
conjunction with the accompanying drawings.
''
:i
~, '
:i 20
~'
.'
.
.:
; 30 ~ ,
.~ .
~ - 4b -
:. h r / -~ r~
7'~ 5
I~i IIIE _~WLNG.~
Fig. I is ~ scllemltlc illustration of traditionnl
geometry and Issumptions basic to spectacle lens desig-l;
Fig. 2 is a chart indicating the optical perforlnance of a
conventional +3.00 diopter spherical lens prescription in terms
of its tangential and sagittal power errors;
Fig. 3 is a chart similar to Fi~. 2 but illustrating the
optical performance of a +3.00 diopter lens having off-axis
corrections applied according to the invention;
Fig. 4 is a cross-sectional view of an ophthalmic lens
with strippling included for purposes of illustrating a
refractive index gradient;
Figs. 5 and 6 illustrate a technique useful in preparing
lens blanks having radially directed gradations of refractive
index; and
Figs 7 and 8 are illustrations of other techniques for
accomplishing gradations of refractive~index in lens blanks.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
For ease in understanding principles o~ the present
invention, Fig. 1 illustrates the traditional geometric
assumptions which are basic to spectacle lens design. In Fig. 1
point P is a point on the reference sphere C at which it would
be desirable to present the same optical corrections as are
present~at the vertex V of lens L. Problems associated with
this endeavor are classical and reported in the literature, e.g.
Bechtold, Edwin W. "The Aberrations of Ophthalmic Lenses",
Am. Jl. of Op. and Arch. Am. Acad. Optom., 35 (1) 10-24, 1958;
Davis, John K., Henry G. Fernald, and Arline W. Raynor, "An
Analysis of Ophthalmic Lens Design", Am. Jl. of Op. and
~Arch. Am. Acad. Optom. 41 (7) 400~421, 1964; Davis, John K.,
Henry G. Fernald, and Arline W. Raynor, "The Design of a
. .
5-
:
10'7'7~
~en~raL Purpos~ ~LIl~Le VisiOll l.el1.Y Scri~s" ~m. _1 o~ æ~ and
~rch. Am. Acad. Optom., ~prll 19G5; an(l DavLs, .lohn K. "Stock
Lenses and C~stom Design" m._Jl. of 0~ , December, 1967.
Fig. 2 disp]ays the result of the traditional calculations
and indicates in terms of tlle tangclltial (t) and sagittal (s)
meridional power errors the performance possible for a -~3.00
spherical prescription. Data is given for a commonly encountered
28.5 mm center-of-rotation (CR) distance and index of refraction
of 1.56. Curvatures of front surface, rear sur~ace and center
thickness are 6.72 diopters, -4 diopters and 3.76 mm respectively.
It is immediately obvious that for a concave base curve
of approximately -4.00 diopters, the average field curvature
(power error) is approximately "O", i.e. the sagittal error
is about -0.9 diopters and the tangential error is about +0.9
diopters. In order to reduce this astigmatism to "O", however,
` a concave base curve slightly steeper than -6.00 diopters would
be required and the field curvature (power error) would become
increased to about -.17 diopter. Thus, it can be seen that
; power error and astigmatism cannot both be reduced to zero by
- 20 conventional lens design techniques. Accordingly, the differences
in lenses produced by various manufacturers stem from differences
in the type of compromise favored by their designers.
Table I which follows sets forth sagittal and tangential
powers, sagittal and tangential errors and astigmatisn occurring
in the exemplary +3.00 diopter lens at various angles A (Fig. 1).
:-
jll -6-
,. :.
~LO'773~5
1 j~ TABLE I
I' ,j
2 Ij Index is 1.56
3 j Index Increment is 0.0000
4 1! Center of Rotation Distancs is 28.5 m~
. i . .Axial Pow~r is 3.00 diopters . ~ ¦
6 ' Angle Sagittal Tangential~ 5 t Astig-
7 A Power Power Error Error matism
8 5~ 3~00 3~00 ~0~00 0~00 0~01
9 10~ 2~99 3~01 -OoOl 0~02 0.02
15~ 2~98 3~03 -0~02 0~04 O~OS
11 20~ 2~97 3~06 ~0~03 0~06 0~09
12 j 25~ 2~95 3~08 -0.05 0~09 0~14
13 jj 30~ 2~92 3~11 ~0~09 0~09 0~19
14 j 35~ 2~88 3.12 -0~12 0~12 0~2~
,,~ I
'''~'' r
- 15 The optimum in o-axis ophthalmic len~ correction would,
- 16 of course, be to accomplish simultaneouR correction of oblique
.: 17 astigmatism and fiela curvature (power error). This can be
18 ¦accomplished according to the present invention through the use
19 If' a gradient index of refraction in the lens blank used to
~prepare the lens as showp in T~le I~ which follows:
:~
. . ` _7_ I
. , 1.
v
"~ '
, ~: , :
-: ~ . ~:
. .,
:. . :
~ 773~ ,
.; .
,, j,,
I' .
¦! X ooooooo
I' ~ U~ o ~ o ~ o U~
, ,i~ ,
oo-~ooo-
~
.~ I . I
In ~ ~ ~ ' -
, , ooooooo
~ l oooo'o'o l'
I O O O O O O O O
: ~ ~ Hl ~ h O O O O O O O
U~ ~ ~1 ~ ~ ~
000000~ 1 -
.. ~ .......................... ~
," , . ~ ~ ,
. O o)~ E~
.~ . O '~ o
. . ~ C t~ 1 ~ h¦ o ~ o ~ o O
, ~ U'~ 2~
n ~
. ~I H ~
~ o .......
I ~ ~ tnoU ou-o-n
C ~ rl N N 10 ~ 1
`~ ~ ~8~
: I . ~-.
!
.
... .
: - :
:~
~.43~ "7;~1~
Table Il rel)r~e~ C119 II~IViIIg n Lront surf.lce
curvature of ~8.64 diopters, n rcar surface curvatllre of -6.00
diopters, thicklless of 3.8~ mm alld index o~ re~raction o 1.56
at its axis affording an axlal power of 3.00 diopters. The lens
is provided ~ith a re~ractive index gradient increasing from
center to edge by increments of 0.0050 per each 5 increase in
angle ~ (Fig. 1). The center of rotation (CR) distance is 28.5
mm which is most common in ophthalmic lens precriptions. At a
point of 30 obliquity astigmatism is reduced to nearly "0" (i.e.
0.02) while field curvature ~power error) has been reduced to
essentially "0" (i.e. sagittal error is -0.00 and tangential
error is 0.02). At this 30 position, the refractive index has
been increased to 1.5900.
Referring more particularly to Fig. 3, the sagittal (s)
and tangential (t) errors have been plotted for the lens used in
the example of Table II. This illustrates that by the selection
of a concave base curve of -6.25 diopters or slightly less,
essentially complete correction of oblique astigmatism and
field curvature (power error) can be accomplished for a viewing
angle A of 30. For obliquities less or greater than 30, it
can be seen from Table II that only slightly less but sub-
stantially complete correction of oblique astigmatism and field
curvature has occurred.
It should be understood that all examples given hereinabove
have employed a stepped gradient of refractive index, i.e.
0.0050 per each 5 changes in angle A. By employing a continuous
and/or non-linear gradient of refractive index between the
center and edges of a lens, a still further improvement in
oblique astigmatism and field curvature correction can be
accomplished.
j 1/ _9_
, , - .
~: .. . .
,1~ .7 r~ 5
Using the fore~oing exilm~le o~ a lens llavlng axlal power
of 3.00 diopters and a center of rotutlon (CR) distance of
28.5 mm, Table III which follows illustrates a variation in
index of refraction between the lens center and peripheral
portions which may be incorporated to accomplish the effect
of off-axis correction over the entire lateral fleld of view,
e.g. from 0 to 35 in ophthalmic lens design.
:
'~' '.
~ .
,
j 1/ -10- ~ -
~'7~3~
,
X o o o o m U In
Ql O ~ ~D O CD CO N
. ~ U~ O ~ ,
b~ el O ~
. ~ . O O ~ O O O O
I . ~ e 0OOOOOO
!
o o ,, , o,l ,, ,, I
~, ooooooo
~ ooo,l,l,lo
. H O O O O O O O _ .
i
O ~ ~1 0 ~
X~ 0000000
I ~
' ~ I o o o ~
.......
u ~ ~`7 ~ ~ ~ ~ ~
P~
,,o l
.: ~ ~ u~ o In o u~ o u~
.:~ H 8 ~ ~ I ,, , ., ~ ~ ol ~
. .
,~' . ' ~ .
,., .
' '. -11- I,
. . I
: ......
.
...... .. . . .
:~., : . ,
-
. - .
,: . ,
. . .
Rcferring in sLill Illore dctail ~o l'ables tl and ~[r~ it
can be seen th~lt tllc graclation of re~rilctlve -Index has very
little effect on of~-axls astlgmatism. Astlgmatism remains at
zero or very close thereto throughout aLl lateral viewing angles
A, i.e. from 0 to 35. The refractive index ~radation, however,
has the effect of dran~atically reducing Eield curvature
(power error~. It can be seen, for example, that at a 35
obliquity (Table II) the error in the sagittal meridian has
been reduced to -0.04 diopters, and in the tangential meridian
to -0.03 diopters. Thus, one may strategically select base
curve to correct for off-axis astigmatism according to
conventional practice, and then be able to employ a gradient
of refractive index according to the present invention to
eliminate or reduce field curvature (power error) to an
insignificant value.
In the foregoing examples of the use of a refractive
index gradation according to the present invention, the index
of refraction has been lowest at the center or axis of the lens
and increased in directions outwardly toward the edge of the lens.
This, however, is not necessarily the direction of refractive
- index gradient which should be used for a]l lenses. The direction
of index gradation, i.e. whether decreasing or increasing in
; directions away from the center of a lens will depend upon the
power range in which one is working and just what is being
attempted to achieve in terms of off-axis correction. For
example, in working with lenses of high plus power such as
cataract lenses, a refractive index of highest value at the lens
center and dropping off from center to edge may produce the most
; desirable results.
The following tables IV, V and VI demonstrate that a ~-useful direction of index gradation for a high plus power lens
e.g. a cataract lens, is one which decreases from center of the
lens toward its periphery.
jl/ -12-
.:
'7~ LS
' X ~n g u~ o ~ g ",
0~ ~n m
m u~ u~
H rl ~^1 ~1 ~1~_1 ~1 ~1
' I E~ , .
~r~o~,~m~
i O ~I d' ~-- C~) CO , I E3
) ! U~ Id O O O O ~ d~ ~ ~ ~ O ~ 1~1
I l¢ ~ '~'~i o~
. ' ~ ~ O O O O
.~ ~ 0 ~
P h o
h O O O O ~1 ~J N ~ O ~1 ~ ~ a~
~1 ~ h ~1~ 0 ~J ~) ~ ~ I
.~ . ~ 000000~i
h ooo,lo--l~r
O oooOooo
~~ h h~ m al
.`~ . ~ 1~ ~ ~ hl . - ... , ¦
~ o hl O O O O O o O
~ . ~ o ~1 1 1 1 1 1 1 1 ~ .
.` ~ ~ ~d ~ ~ i
I ~q ~ ~ u) o~ O ~r N 1` 1` ~
:. .j ~S ~rl ~5~ ~ h O ~1~ ---1`1 co 1` ~ ~1
E~ C~ ah~ ~ ~r ~r ~ In ~ ~ ,1 ,l o ~ ~ o O ~-
U ~ ~ 0 ~ ~ ~n ~ ,l ~ co ~
I g c ~ r ~ O ci~ ~ c
. o~ o i
. ~o orl~ E~:: ~ o
.1 o~ o o o o o ~ ~ cq a .
~1 ~ h oOooOoo ~,1 Od~ ,1
~i ~ 0J~ Q~ - - . . - ~ r~ ~ co ~ G~ U~
~ a~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ 5~ ~ r~ ,l O
,., n E3 0 tJ' O ~ ..... - -
,, ~ P, ~ rl 3
U~ ~; O tJ O
h
~ oq a ci 3
X X 0 ~1 0 rl H ~4
O ~1 0~ Ln O U~ O U~ O Ul X X aJ ~1 ~
I ~ x ~ ~ ~ o
. . I H ~ ~ ~ ~i ~ u~ o o o In
-
; . 13-
..... i, . i
., . :
:. .. . .
' '~ ,
. .
. . . .
7~15
xl o c~ ~ ~o o
Il 0 U~OG~ :
1, H _I ~I r-l ~ ,1 _~ ~1
1 ~ r co a~ ~ O ~
o ~ o,~ou~O
I ~ ~ ooo'o ~
~1 C ~1~_lo~
~ H ~ Ul ol , ~D O ~r O
`. ~Cj ~ U') ~ I I I I I I I I
~, I ~ ~
I ~ 8 0 o -'~ o o o o
I ~ ~ ..... ~ . I
! Q~ ro o ~ 3
o h
I ~ O ~
` ~ ~ o
.. ~ ~
~ ~ ~o~ ~ ~
~5 ~ ' ~
I . . I'
.~ I.'
' I' ~
::. ~ . . ~ . .
:`. . . . ~ , ,
. . ...
~ .
.`
1~'7'~ 5
Tabl~ IV ilLustr~ltes wh.lt Call llat)pell with respect to
off-axis aberrations in a lens of constall~ index value from
center to edge wllen, for e~clrnple, axial power is 1~.00 diopters
with a concave curve of -3.50 tliopters ~nd center thickness of
11 mm. Sagittal power remains nearly constant Erom center to
edge of the lens, i.e. ~rom 0 to 35 of angle A while the
tangential power increases in value to a point where at 35, it
is nearly 3.00 diopters strong. The resulting astigmatism at
35 rotation in the eye is 2.72 diopters.
Table V, with the same prescription of 14.00 diopters
axial power illustrates the improvement that can be accomplished
with an incremented index of refraction according to the
invention. The index incrementation in this example is provided
by dropping .005 for each 5 of eye rotation away from the center
of the lens. By such means, it can be seen that while the
less important sagittal power errors have increased somewhat,
the more important tangential errors have dramatically decreased.
,
At 35 the tangential error has been reduced to approximately
2.00 diopters with astigmatism accordingly being reduced to
about 2.00 diopters.
By employing a non-linear refractive index gradient as
shown in Table VI, a substantially constant tangential power
(nearly 0 tangential error) can be accomplished. It should be
understood that although tangential error is generally considered
more serious than sagittal error, it is not necessary to reduce
it to zero at the expense of substantial amounts of sagittal
error. Accordingly, Table VI is intended mainly to show that
additional control in off-axis correction according to the
present invention can be achieved by ~arying the refractive -
index gradient not only in predetermined increments but rather
in a non-linear manner.
jl/ -15-
~3'~ 5
~ hile the exalllpLes o~ 'l`abLes I-VI h.lve Lllustruted the
invention as applled to conclitions using the most common 28.5
mm center of rotation distance (CR) for genelal purpose
ophthalmic lens prescriptions and 25 mm CR for high plus Si.e.
cataract lens) prescriptions, it should be understood that
astigmatism and curvature of field tpower error) can be
essentially eliminated, i.e. reduced to negligible amounts,
for other center of rotation (CR) distances as follows:
Tables VII and VIII illustrate a control of astigmatism
and power error available by gradation of refractive index in
a lens having a front surface curvature of +8.64 diopters, a
rear surface curvature of -6.00 diopters1, center thickness of
3 84 mm and index of refraction of 1.56 at its axis affording
aXial power of 3.00 diopters. The refractive index is increased
radially from center to edge of the lens.
~ ',
. .
.'' ~'" . ~"
' ~
"~ ,;~;
`' .
, ~ ' '
` .
' ' .
' ~
- jl/ -1 a-
.: , ' '' ~ ,
! ' '
i
', :
Il X oOOo~nln.r~ X OOoou~mu~
I' ~ o~Doalco~ c~ O~DOCOCOC`J
~, ~d ~ ~ o ~ ~o ~D ~ ~ r~ CO C~
I ~i ul ~ n u7 u u~ ~ C u~
' H .... H .......
I
i ~, e o ,~ o o o o o ,~
I ~ o O O O O O O "- I o o o o o o o
, ~ .................... ~ l .......
I ~ ooooooo ~ I ooooooo
~1 O ~I t~ ~ ~D a) O ~1 O O ~J ~ ~
I O 000000~ 0 00001::100
I ~ h O O O O O O O O O O O O O O
! ~i ~oooo~ oo~
. o ooooooo o o ooooooo
I H O ~ 1 0 0 0 0 0 0 0 H ~`1 h O O O O O O O ' .
~> ~ l l I ~ ~ I I I I I I I
' ! ~ 01 , rj ,1
m ~ _
m c) h r~
El ~ ~ ~ D CO O ~ ~ ~ X ~ o o
r ~r ~ o o o o o o ~ o o
r 3 ~ O ~ ~N
. r~ ~1 ~ ~ ~ h
h o o o o ,1 ,l r~ ~D ~ ,1 ~ ~ cn o cn ~ co ~ t~ i
~-,,~ G) ooooooou~ O ~--1 3 cno~cn~ns~ cn
. u~ Orl ~E ~ h tJ~ O i
~r~f~)~~1 C~ ~ ~
~ P. ~0~ ~q ~'
~ ~ ~ a) o l ~ o u~ o u~ o' Lr
~ N Ir~ O In
:~ ~:
~' ' ~ .'
.' . ~
Il -15b- 1
`.. i ,1 ' .. . i
..
~ . :
::
Refcrring now to 'I.lbl.~s L~C nnd X wh:i.cll follow, the
same lens design datl but w:i.thout refractivc index gradation
is presented to iL:Lustrate l:he respective correctlons in
curvature of field (power crror) wh:ich were accomplished wlth
tlle refractive index gradation of the Tables VII and VIII
situations.
Comparing Tables VII and IX it can be seen that curvature
of field (sagittal and tangential errors) was considerably
reduced by refractive index gradation (Table VII). For the
:~ 10 25 mm center of rotation distance situation, l.e. with constant
refractive index (Table IX) s error equals -0.20 and t error
equals -0.14 at 35 while with refractive index gradation
(Table VII) s error equals 0.02 and t error equals 0.10.
Similarly, a comparison of Tables VIII and X shows a
highly significant correction of curvature of field for the
32 mm center of rotation situation. There, for the 45 angle of
viewing, s error has been reduced from -0.25 to -0.03 and t
erro~ from -0.30 to -0.06.
TABLE IX
Index is 1.56
; ~ Center of Rotation Distance is 25.0 mm
Axial Power is 3.00 diopters
Angle Sagittal Tangential s t Astig- :
Power Power Error Error matism
5. 3.00 3.00 -0.00 -0.00 0.00
10. 2.99 3.00 -0.01 -0.00 0.01
: 15. 2.97 2.99 -0.03 -0.01 0.02
20. 2.94 2.98 -0.06 -0.02 0.04
25. 2.91 2~96 -0.09 -0.04 0.05
2.86 2.92 -0.14 -0.08 0.06
35. 2.80 2.85 -0.20 -0.14 0.05
jl/ - -15c-
. ~ ... . ~
7;:~..5
1 T.~BLE X
2 ~ Index i9 l 56
3 Center o~ Rotation Di~tance is 32.0 mm
4 Axial Power is 3.00 diopter~ I
lj Angle Sagittal Tangential 3 t A3tig-
6 , Pow~r PowPr Error Error mati?m
7 Ij 5. 2.9g 3.00 -0.00 -0.00 0.00
8 lO. 2.98 2.98 -0.02 -0.02 0 00
9 1. 15. ~.96 2.96 -0.0~ -0.04 0 00
1, 20. 2.92 2.92 -0.08 -0.08 0.00
~ 25. 2.88 2.87 -0.12 -0.13 0.01
12 1 30. 2.82 2.80 -0.18 -0.20 0 02
l3 35. 2.75 2.70 -0.25 -0.30 0 05
14 ! It should also be under~tood that whil^ all e~ample.~ I
~given hereinabove have been based upon the use of an ind~x of
refraotion of 1.56 at the enter of a lena, hl3her or low~r
'
' ` ' , ' '
.. . .
.
-1 5d-
. , .
:~ I
j
:
~, ~ . , `.
. ~ ,
- - . . : '
:, . . ~`:
,
10'7'7~ ~
~efractive indiccs ~Ily bc u~d for purpose~ oF nvol~llng undue
lowering or raising of re~ractlve iu(lex at e~ges of the lenses.
The fore~oLng illustrcltcs the advantages of deslgning
ophthalmic lenses according to the :Lnventlon with refractive
index gradients uniquely employed ac; "tools" in the correction
of of f-axis aberrations. Data emboclied in the various examples
of Tables I-VIII has been arrived at by conventional lens
designer's ray tracing calculations and the assistance of
; programmable electronic computer technology, the latter being
dispensable but extremely useful to the designer. While the
data of Tables I-VIII has been selected to illustrate
principles of the present invention, it is believed to have
been made apparent that similar information can be arrived at
or adjusted according to the requirements of any one of the
virtually unlimited number of ophthalmic lens prescriptions
encountered in the art whether these prescriptions are for
spherical lenses of the single vision or multifocal types or
whether, in either case, they contain a cylinder correction.
` Those interested in details of ray tracing as used in
ophthalmic lens design work may refer to U.S. Patents Nos.
` 3,434,781 and 3,169,247 and/or one or more of the above-
identified pieces of literature on the subject.
As mentioned earlier in this specification, the present
inventive concept of correcting off-axis aberrations in
ophthalmic lenses with a controlled gradation of refractive
index can be utilized to obviate a need for aspheric surface
` design or may be incorporated in and/or with aspheric design
for greater "fine tuning", i.e. correction, of oblique aberrations.
A~ in all cases of lens design including the present
concept of using refractive index gradation) base curve is
carefully chosen and in using a refractive index gradient to
replace or obviate a need for off-axis correction by aspheric
... .
~ jl/ -16-
.
. :,
.. . . . . .
'7;~5
urv.lture, base curve can he so choscn ~s to miniml~e a~stigmatism
and the rcfractlv~ lndex gradient so applied as to minimi~e
curvature of field (power) error.
In the case of 'If ine tuning" with aspheric surface design,
particular attelltion may be paid to correction of a third off-
axis aberration, distortion whether of the "barrel" or "pin
cushion" type. In this case, base curve selection may be
made more specifically with reduction of distortion in mind,
surface asphericity chosen to minimize astigmatism and a
refractive index gradient chosen and utilized ~o reduce curvature
of field (power error), the latter having been demonstrated
hereinabove. Those interested in details of manipulating base
curves and applying surface asphericity for reduction of oEf-
axis aberrations may refer ~o applicant's U.S. Patents Nos.
3,169,247 and 3,960,442, issued February 9, 1965 and June l,
1976, respectively.
While the present invention relates more particularly to
matters of the use of a refractive index gradient in ophthalmic
lenses as a "tool" for correcting off-axis aberrations and is
applicab~e to lens blanks having a gradation of refractive index
` from center to edge regardless of how the lens blanks may be
fabricated, treated or otherwise provided with the refractive
index gradation, examples of techniques useful in producing such
; lens blanks have been illustrated in Figs. 5-8. These exemplary
lens blank manufacturing techniques, however, are not intended
to be restrictive to matters of the present invention but merely
illustrative of some of the schemes available to the artisan for
providing lens blanks which are useful in practice of the above-
described invention.
Referring to Figs. 5 and 6, a billet 10 of lens material
e.g. optical glass, having a diametral dimension equal to or
greater than that of the maximum transverse dimension required
jl/ -17-
` : -. ~ . '. :
t 7 ~ - ~ 5
~E an opllthalrllic ~ s lo be produced ~ccordlng to the inventlon,
is providcd. ~illc~ 10 is -Immersed in a salt 12 contalning
alkali metals to cause ion-excilang~ between ions in the glass
and those ill the salt in amounts graclually penetrating radially
into billet 10.
Details of techniques having utility in the manufacture
of ion-exchanged billets may be had by reference to U.S. Patents
Nos. 3,650,598 and 3,827,785.
With a controlled passage of time in salt 12, ion exchange
can be caused to proceed a predetermined distance toward and/or
to the axis of billet 10 with refractive index varylng
according to the gradation of ion exchange having taken place.
- The resulting substitution of monovalent alkali metal ions of
one size in salt 12 for ions of another si~e in billet 10 pro-
gressively radially inwardly thereof produce a variable
denseness of the material of billet 10 which results in corres-
ponding refractive index changes. While not shown in Fig. 5,
opposite ends of billet lO are preferably covered with a
protective coating or the like which prevents ion exchange in
direction axially of billet 10.
~ aving so treated billet lO in salt 12, removal from salt
12 and cleaning to terminate the ion exchange process renders
` billet 10 adaptable to transverse cutting into sections 14
(Fig. 6) of thickness and diameter necessary for the formation
of lenses such as lens L in Fig. 3. The meniscus configuration
of lens L is accomplished with conventional grinding and
polishing operations.
; As illustrated with stippling and arrow 16 in Fig. 4, lens
L is provided with a gradation of refractive index from its
axis outwardly in the direction of arrow 16. The density or
refractive index may be caused ~o decrease in the direction of
arrow 16 or vice-versa.
jl/ -18-
.~ ........... . ...................................... .
: ~
~8'7 7~15
In Fi~. 7 ~.n ~llL~rnaL~ t~chni(lue ~or ~abricatLng lens
blanks h~vin~ a gr~dat~d refractlv~ index is illustrated. Thls
comprises Ll~e fabrication of a bil]et lO' of a central rod 18
having a preselec~ed refractive lndex and successively
surrounding closely inte~fitted sleeves 20 each of a preselected
different refractive index. The assembly may comprise more or
less than the five concentrically related components which have
shown, i.e. the incremental gradation of refractive index from
rod 18 radially outwardly may be of any desired step function
either increasing or decreasing in value.
Components 18 and 20 of billet 10' would normally be
heated and/or otherwise fused together as a unit and cut trans-
versely to the thickness desired of a lens blank 22, for
example. Fusion of components 18 and 20 together and transverse
cutting of billet 10' to form blank 22 can be followed by
irradiation and/or other treatment of blank 22 to produce a
blending or gradation of index of refraction of one of components
- 18 and 20 with an adjacent component. It is also contemplated
- that components 18 and 20 may be surface treated before
assembly by immersion in a diffusant such as heated silver
chloride to provide a graduated transition of refractive index
therebetween in the final assembly. Operations of producing
refractive index gradation by irradiation and/or diffusion are
well-known in the art. For those interested in details, however,
reference may be made to U.S. Patents Nos. 3,610,924 and 3,563,057.
Still another technique for producing a gradient refractive
index billet 10" from which lens blanks may be cut is
illustrated in Fig. 8. This includes the provision of a
multiple chamber g:Lass furnace 24 having a plurality of con- ~;
centric orifices through each of which a lens material of a
preselected refractive index may be directed and caused to form
the composite billet 10". Following the formation of billet 10"
j 1/ -19-
', ,
~L~l'î'7~
and coolinE~ ~hereo~ ~o a ~;ol:id state, transverse c~ltting a].ong
lines 28 w:ill pro~uce lens blanks 30. tt should be understood
that a multiple oriEicc plastic (i.e. ophthalmic resin)
dispenser may be substit-lted ~or furnace 24.
The artisan will readily appreciate that there are
various other modifications and adaptations of the precise forms
of the invention herein shown which may be made to suit
particular requirements. Accordingly, the precise forms of
the invention presently shown and described have been presented
for purposes of illustration only and are not to be interpreted
as restrictive of the invention beyond that necessitated by
the following claims.
jl/ -20-
