Note: Descriptions are shown in the official language in which they were submitted.
``" 12~95~L3
The present invention relates to a multi-focal
spectacle lens, wherein dioptric power varies progressively
between different zones of vision.
Such lenses are referred to generally as
progressive lenses, and that surface of the spectacle lens
which effects the progressively varying course of the
dioptric power of the lens is referred to as the progressive
surface. Canadian Patent 1,152,369 issued on August 23,
1983 having as inventors Gerhard FURTER and Hans LAHRES,
are generally descriptive of design principles for a progres-
sive lens.
Progressive lenses are used to compensate for
impaired vision of presbyopes (persons whose vision has been
impaired by age), in that the power of accommodation of the
eye has decreased. To date, progressive lenses for
presbyopes have had an upper distance-vision region and a
lower near-vision region, and these regions have been
connected by a progressive zone. Within said zone, the
dioptric powers of the upper and lower regions pass
-20 progressively and continuously into each other. Such a
progressive lens is made in a single piece; it does not
contain any disturbing separation line between the two
viewing
, ~ _
regions, and it is therefore esthetically very
satisfactory. It is also very comfortable for ~2~9513
the person wearing the spectacles, since the two
viewing regions pass continuously into each other,
without any jump in the viewed image, so that the
transition from far vision to near vision occurs
very naturally.
There are tasks which present difficulties
for a person wearing spectacle lenses having two
viewing-distance regions. Thus, for example, in
the case of a computer-screen work station, the
screen is at a viewing distance of 50 to 75 cm
while the keyboard is at a viewing distance of
40 to 50 cm, and documents may also have to be
viewed at positions alongside the screen and/or
alongside the keyboard. In principle, it would
be possible in such case to work with spectacle
lenses which have only the two regions which
correspond to said viewing distances. But
certainly, the user would then not clearly see
objects or persons away from his work station;
3 i.e., he would have to remove his glasses to look
a distance away or, if his vision is impaired, to
change them.
To avoid this serious disadvantage, a paper
in the journal "Optometrie" 5 (1984), 208/213,
proposes special multiple-power lenses having
three viewing-distance regions, in the manner of
the known trifocal lens. A separation line delimits
these regions from each other, and the separation
lines are clearly visi~le; thus, each change in
glance from distance vision to the region of the 127~S13
screen, or from the screen region to the region
of the keyboard, entails a jump in the image.
The condition is similar in the case of
spectacle lenses known from West German Patent
No. 3,127,1~8. Such lenses can have three spherical
regions of different dioptric value and all of
these regions have a common tangent in the plane
of the vertical meridian, i.e., they adjoin each
other without seam. However, at lateral offset
from this meridian, the viewing regions are connected
by a transition surface, wherein seam lines form
edges and thus points of optical jump due to the
local difference in the involved radii of curvature.
It can readily be seen that such a spectacle lens is
pervaded by edges and can thus in no way be satis-
factory. Furthermore, such a lens cannot be
considered a progressive lens since it has jumps
in curvature and thus in the action, even along
the meridian.
From U.S. Patent No. 2,878,721, it is known
to develop a progressive lens having an extremely
long progressive zone. In this way, regions lateral
of this zone can be shaped such that they are relatively
low in error, over a limited lateral extent. But the
vertical distance between distance effect and near
effect is too great in the case of such a lens. There
is the additional difficulty that such a lens has no
regions which are associated with specific fixed
lZ79S13
viewing distances, so that the user must in every viewing
situation incline his head to seek the suitable viewing
orientation.
BRIEF STATEMENT OF THE INVENTION
The object of the present invention is to provide
a progressive spectacle lens which makes it possible for the
user to work suitably and without fatigue in three different
viewing-distance regions and which is esthetically
satisfying in every respect.
To date, those skilled in the art have experienced
great difficulties in developing a progressive lens having
two viewing-distance regions and a progression zone between
them, wherein the user's vision is disturbed as little as
possible by aberration. This fact
is apparent from a large number of different proposals,
such as those to be found, for example, in West German
Patents 2,918,310 and 3,016,935.
These difficulties have caused those skilled in
the art to consider it impossible to provide, on the
relatively small surface of a variable-focus lensj
regions associated with different viewing distances
in such manner that they merge continuously into each
oth~r via two progressive zones which are vertically
spaced from each other, without entailing aberrations
of such magnitude as to result in highly questionable
utility for the intended progressive lens purpose.
In the present invention, the logic of the
progressive-lens design approach of West German Patent
15 No. 3,016,935 has been followed, namely to calculate
the progressive lens in accordance with the mathe-
matical method of spline analysis set forth in the
text, "Spline Analysis", by M. H. Schultz (Prentice
Ha]l Series in Automatic Computation, Englewood Cliffs
20 No. 9, 1973). This analysis provides the mathematician
with the basis for finding the area equation which
finally makes it possible, by means of an optimizing
method known, for example, from the journal Optik 18
(1961~, page 577, to satisfy meaningful optical require-
ments in the lens surface. In this case, the requirement
must be satisfied that the resulting surface is twice
continuously differentiable. The surface resulting
from this calculation is an aspherical surface in the
most general sense, i.e., its regions are not related
to each other by any formula. But such an aspherical
surrace can be produced with the new CNC-machines
795~3
available on the market.
By calculating a variable-focus lens of the
above nature, namely, with spaced progressive zones
in vertical interlace with and continuously merging
into spaced different viewing regions, wherein the
calculation is effected on basis of thorough optical
and ophthalmological knowledge, there surprisingly
results a progressive lens which achieves the pur-
pose of the present invention in an unpredictably
optimal manner. ~efining features which assist in
achieving this result will appear from detailed
description below, but will be briefly noted here
to include:
a. a middle viewing-distance region within
the useful field of view of which the dioptric
power varies by not more than a given low frac-
tional multiple of the difference in dioptric
power for the greatest and for the shortest of
the viewing distances;
b. a lower viewing-distance region in which
the dioptric power decreases laterally outward
) directions within the useful field of view by at
most a given fractional multiple of the difference
in dioptric power for the greatest and for the
shortest of the viewing distances;
c. a distant-vision region which comprises
at least one surface which is described by two
adjacent sides of a square or rhombus standing with
the included vertex at a point at a principal sight
line of the lens at which point the progression
commences;
12'YgS~3
d. a diopter-power relationship for assuring low values
of astigmatism throughout the progressive zone of transition
between the middle and the lower viewing-distance regions,
and for assuring tolerably low values of astigmatism
throughout the progressive zone of transition between the
upper and the middle distance-viewing regions; and
e. adaptation of distortion in lateral areas of the
upper viewing-distance region to distortion in the lower
viewing-distance region, such that the pattern of distortion
throughout the lens is akin to the distortion pattern to
which the wearer of spectacles is already accustomed, as in
use of his normal reading glasses.
According to the present invention, there is
therefore provided a multi-focal continuous-focus spectacle
lens with at least one progressive surface of progressively
varying power which is so shaped that spaced lens regions of
different but constant viewing distances pass progressively
and continuously into each other. The lens has two separate
progressive zones of continuously varying optical power,
which zones are spaced from each other and change the
dioptric powers of three viewing-distance regions,upper
middle and lower, continuously into each other, each of the
viewing-distance regions being of substantially constant
dioptric power. The progressive surface is twice
continuously diffexentiable nasally and temporally at least
up to a horizontal viewing angle of substantially 25C.
Preferably, between the two progressive zones
there is a middle viewing-distance region within which the
dioptric power varies along the principal-sight line by not
more than 0.2 times the difference in dioptric power of the
viewing-distance regions for the greatest and for the
shortest distances.
The invention will be described in detail in
conjunction with the accompanying drawings, illustratively
~279513
applicable to a progressive spectacle-lens design for use at
computer-screen work stations. However, it should be
expressly understood that the viewing distances of the
viewing regions of the spectacle lens of the invention can
also be selected differently, depending upon the particular
other task or tasks to be satisfied. For example, in
spectacles for use by pilots of transport planes, the middle
viewing-distance region serves for distance vision, while
the upper and lower viewing-distance regions serve for
observing instruments which are at different but closer
distances.
,~
- 7a -
~'
Speclfically, in said accompanying drawings: lZ79S~3
Fig. 1 is a view in front elevation to SilOW a
progressive lens of the invention;
Fig. 2a graphically displays an illustrative
distribution of dioptric powers along the meridian
of the lens of Fig. l;
Fig. 2b similarly displays a different distri-
bution of dioptric powers, for another embodiment;
Fig. 3 is another view in front elevation of
the lens of Fig. 1, showing the distribution of
viewing-axis intercepts over a progressive-lens
surface, at which optical demands forming the basis
of a presently disclosed calculation are estimated;
Fig. 4 is a plan view of the progressive-lens
embodiment of Fig. 2a, showing the distribution of
lines of equal astigmatism;
Fig. 5 is a view similar to Fig. 4, but showing
lines of constant average surface dioptric power, for
the embodiment of Fig. 2a;
Fig. 6 is a chart of sagittae at equidistant
points along various horizontal sections over the pro-
gressive-lens surface of the embodiment of Fig. 2a;
and
Fig. 7 shows the distortion of an equidistant-
ob~ect grid, by a spectacle lens according to the
embodiment of Fig. 2a.
Fig. 1 shows a progressive lens 1 developed in
accordance with the invention, as seen in plan view
of its progressively characterized surface. This lens
has an upper viewing-distance region 2, an intermediate
or middle viewing-distance region 3, and a lower 12795~3
viewing-distance region 4. An upper progressive
zone 5 is interposed between (interlaced with) the
viewing-distance regions 2 and 3, while a lower
S progressive zone 6 is interposed between (inter-
laced with) the viewing-distance regions 3 and 4.
The course of and distances between region/zone
separating lines are indicated merely by way of
example.
The lens 1 may be developed with a vertical
meridian 7, in which case it constitutes a symmetri-
cal lens. It is also possible to develop the lens 1
as an asymmetrical lens which is divided, by a
sloping or non-linear principal-sight line 8, into
a nasal region and a tempora] region, the principal
sight line 8 extending as a curve which deviates
toward the user's nose, for shorter-distance viewing.
Fig. 2a is an illustrative plot showing the
distribution of the dioptric power of the lens 1
along the meridian 7. This same distribution of
the dioptric power results along the principal-sight
line 8, in the case of an asymmetrical development
of lens 1.
Fig. 2b is a similar plot but for a different
illustrative example of dioptric-power distribution,
for a progressive lens suitable for use by aircraft
pilots.
From Fig. 2a, it can be noted that the lens 1
has a dioptric power of 0 dpt in the upper viewing-
distance region 2. In this development of lens 1,
this upper region 2 represents the distant-vision lZ7~513
region, and it is combined with the other regions
3, 4 to particular advantage when used at computer-
screen work stations.
Within the upper progressive zone 5, dioptric
power varies continuously, to a value of +1.25 dpt at
merger with and throughout the intermediate viewing-
distance region 3. This region 3 is used for observing
the computer screen and possibly a document positioned
alongside of it. Within the lower progressive zone 6,
dioptric power again varies continuously, to a value
of 1.75 diopters at merger with and throughout the
lower viewing-distance region 4 which is used for
observing the keyboard.
A progressive lens having the dioptric-power
distribution shown in Fig. 2a is suitable for a
presbyope with normal vision whose power of accommo-
dation, for instance, still covers 1.5 diopters. It
is convenient and easy for this presbyope to create
an accommodation of 0.75 diopters. Thus in the middle
region 3, the assumed presbyope obtains a maximum total
effect of 1.25 ~ 0.75 = 2.0 diopters, which corresponds
to a viewing distance of 50 cm; and in the near region
4, he obtains a maximum total effect of 1.75 + 0.75 =
2.5 diopters, which corresponds to a viewing distance
of 40 cm.
The greatest viewing distance results when the
user's eye is relaxed and does not accommodate. In
the example shown, the diopt~ic power of +1.25 diopters
is then available in the middle region 3, which, for
the relaxed eye, corresponds to a viewing distance
,_ --10--
o~ about 80 cm; thus t~e user, upon looking through
the middle region 3, can sharply see, without strai ~ 2 7 9 51 3
objects which are at viewing distances between 80 and
50 cm. In the near region 4, an effect of 1.75
diopters is available, which, for the relaxed eye,
corresponds to a viewing distance of about 60 cmi
thus the observer, upon looking through the near-
distance region 4, can sharplv see objects which are
at viewing distances between 60 and 40 cm.
10For the case of a presbyope whose power of
accommodation is only 1 diopter, the central region
3 is illustratively characterized by a dioptric
power of 1.5 diopters, with the near region 4 having
a dioptric power of 2.0 diopters. In this way, clear
15viewing is possible in region 3 between 70 and 50 cm
and in region 4 between 50 and 40 cm.
The foregoing examples show that when a pro-
gressive lens is developed in accordance with the
examples indicated, relaxed and clear vision of
screen, keyboard and copy is realizable for any
possible arrangement of their working distances.
Depending upon the user's viewing distance to
the computer screen, diopter addition in the middle
region 3 should be in the range 0.4 to 0.8 times the
maximum diopter addition, namely, the increase in
dioptric power between the distance-viewing and
near-viewing regions 2 and 4. Other viewing tasks
result in a correspondingly different distribution
29 of effect and region sizes (see Fig. 2b).
--11--
The calculation of a progressive lens in accord ~2 79 5~ 3
ance with the invention will now be discussed. First
of all, the eye-side (rear) surface of the lens is
selected on basis of required strength (dioptric power),
with due consideration of simplicity of manufacture,
good appearance and good compatibility. This rear
surface may, for example, be sphericali and if
astigmatism of the eye is to be corrected, this rear
surface may be toric, or atoric.
Second, the object-side (front) surface of the
progressive lens is calculated, bearing in mind that
the desired strength of the lens is to be obtained
cooperatively with the rear surface. This front
surface incorporates the progressive features and is
divided into the three viewing-distance regions 2,
3, 4; of these, the middle region 3 should have a
vertical extent of at least 7 mm, and may range up
to about 20 mm, depending upon particular requirements.
Next, the course of the meridian or principal-
sight line, i.e., curve 7 or 8, is established. Anddeterminations are made for the vertical extent of the
two progressive zones 5, 6 and for the course of
dioptric power along the meridian (principal-sight line)
in these zones. Finally, it is decided whether astig-
matism will be permitted along the principal-sight line,
curve 7 or 8. Such astigmatism should not exceed values
of 0.5 diopter.
The designer then plots on the progressive surface
a plurality of dots, so-called "peepholes", the distri-
bution of which is effected on basis of his experience.
-12-
r~
One such illustrative distribution of the peepholes lZ795~3
`" over a progressive surface for a lens according to
Fig. 1 is shown in Fig. 3. At each peephole, t~e
desired dioptric effec-t (power, astigmatism, prismatic
effect) is established.
In this connection, it is, in particular,
required that:
a. In the central region 3 and within the
total useful field of view, the average dioptric
effect may vary, to a maximum extent of 30 percent
of the total diopter addition. In the present
illustrative example, the field of view comprises
a horizontal viewing angle of about 25, nasally
and temporally, and the total dioptric addition
is the difference between dioptric effects at the
viewing-distance regions for the greatest and
smallest distances;
b. In the near-vision region 4 and within
the useful field of vision, the average dioptric
effect may decrease laterally of the principal-
sight line, to a maximum extent of one-half of
the total diopter addition, thus permitting
account to be taken of the fact that a document
located alongside the keyboard is at greater
distance from the eye than is the keyboard; and
c. The distant-vision region 2 comprises at
least one surface, which can be described by two
adjacent sides of a square or rhombus standing
with the included vertex at a point on the
principal-sight line and diverging upward and
outward of the principal-sight line, said point
being where progression commences.
-13-
The designer then inserts a network of points
l~9S13
which is independent from the distribution of the
peepholes, and which is preferab]y in the form of
a uniform grid, over the variable-focus surface,
and he starts to calculate the surface with a spline
function Sp (X, Y) which appears suitable to him.
This initially produces at the peepholes of Fig. 3
given dioptric effects which in general do not agree
with the values desired. But his initial calculations
are followed by a number of optimization steps which
are continued until the surface parameters -- preferably,
the height (sagittae) of the network points -- are so
established that the spline function Sp (X, Y) which
is defined thereby provides at the peepholes the desired
dioptric values with sufficient, attainable precision.
This spline function (X, Y) of the surface defines a
progressive surface which is twice continuously differ-
entiable, since this is a basic property of every spline
function.
If the calculation is performed in the described
manner and with the indicated parameters, there results
a lens which not only satisfies stated requirements
but is additionally characterized as follows:
-- Within the useful field of view and at lateral
transition regions between the middle region 3 and
the near region 4, i.e., in the lateral regions of
the progressive zone 6, the value of the astigmatism,
expressed in diopters, is contained in the range 1.0
to 1.5 (1.0 being preferable~ times the total
dioptric addition; and
-14-
ithin the useful field of view and at lateral
transition regions between the middle region 3 an~ 9 ~3
the distance-vision region 2, i.e., in the lateral
regions of the progressive zone 5, the value of the
astigmatism, eYpressed in diopters, does not exceed
3 times, and preferably not 2.5 times, the value of
the total dioptric addition.
In this way, the result is obtained, on the one hand,
that the useful field of view is completely usable in the
middle, near-vision and corresponding transition regions.
The glance of the user can therefore travel between
screen, copy and keyboard without being lmited by dis-
turbing imaging errors. For eY~ample, normal writing can
still be clearly read with a value of astigmatism of 1
5 diopter.
On the other hand, disturbances of the field of view
in the transition region between middle and distance-
vision regions are tolerably slight, in view of the fact
that values of the astigmatism are maintained within the
limits indicated. Larger values of astigmatism would
lead in dynamic viewing not only to migrating regions of
lack of sharpness but, as a result of the induced dis-
tortion, also to unnatural movements of objects.
Fig. 4 shows the distribution of astigmatism over
the surface of a lens according to Fig. 2a; and it can
be seen that in the middle and near regions of the lens,
the value of the astigmatism scarcely rises above 1.0
diopter. The power of the spectacle lens is shown in
Fig. 5; and it can be seen that in the middle region of
the lens, the power remains constant practically to the
-15-
edge, while in the near region it decreases somewhat
in the advantageous manner required. 12~9S~3
In Fig. 5, it can also be seen that, along the
principal-sight line, the power of the lens smoothly
declines in the upper portion of the middle region 3.
In this way, the usable distance range in the middle
region becomes greater and the upper progression zone
5 becomes somewhat wider. This variation should,
however, not exceed 0.2 times and, to particular
advantage, 0.15 times the total dioptric addition
since in such case the accommodation of the eye would
be noticeably affected. For this same reason, the
vertical extent of the middle region 3 should be at
least 7 mm.
The height and width of the usable far-distance
region 2 depends on the vertical extent selected for
the progressive zone 5. The greater the vertical
extent of zone 5, the more its useful width increases;
on the other hand, if one selects a progressive zone
that is too short, the useful far-distance region 2
becomes larger, but lateral portions of the progressive
zone 5 are degraded by astigmatic lack of sharpness and
by variations in power increase.
As shown in Figs. 4 and 5, large usable regions
result over the entire progressive lens 1. Furthermore,
distortion is user-tolerable for both static and dynamic
viewing. This can readily be noted from a glance at
Fig. 7 which shows the distortion of an equidistant-
object grid by a spectacle lens in accordance with the
embodiment shown. It can be seen that one has
-16-
,_
7~S13
intentionally dispensed with shaping the progressive surface
in such manner that vertical lines are imaged as vertical
lines. Xather, the distortion in the far-distance region is
adapted laterally to the distortion in the near-distance
region; and in the far-distance region 2, horizontal
magnification increases laterlly outward, with the result
that vertical object lines are imaged with less strong an
inclination. It can be seen that distortion in the far-
distance region (dioptric power: 0 diopter) is very slight
and that in the course of diopter addition, the permitted
distortion is of the type to which the wearer of spectacles
is already accustomed, as in use of his normal reading
glasses.
If importance is placed on a progressive lens
which assures the same viewing conditions for both eyes in
every direction of viewing and thus permits undisturbed
binocular vision, the foregoing calculation can be performed
in such manner that this result is essentially achieved by a
suitable predetermination of the dioptric effects in the
peepholes, and using principles of symmetry with respect to
the principal-sight line, as discussed in Canadian patent
1,152,369 above mentioned.
Fig. 6 shows the sagittae of the progressive
surface of the :Lens of Fig. 2a. In this figure, a vertical
plane is passed through a point (.00~ in the region of the
upper progressive zone 5, and the distances (in mm) from
this plane are indicated for the points shown.
If one cuts a progressive lens of the invention
along horizontal planes, the resultant horizontal sections
, --
r ~
of the variable-focus surface cannot be described by
a conic-section curve.
As pointed out, the progressive lens which has
been described and shown is calculated for a given
back surface. But it should be noted that the pro-
gressive surface corresponding to a preselected back
surface can also be used together with back surfaces
which deviate in the range of about + 0.5 diopter
from the original rear surface, all without imparting
recognizable defects in the finished product.
It will be readily understood that, within the
scope of the present invention, there can be deviations
from the parameters selected for present illustration.
Thus, it is possible, for example, to dispense with
having the entire progressive surface twice continuously
differentiable. For those surface portions (e.g., edge
regions) for which the horizontal viewing angle is
greater than 25, i.e., which lie outside the useful
fiela of view, one can dispense with this requirement
in order to reduce the distortion within this region
or to further reduce the astigmatic lack of sharpness.
It may also be advantageous for special uses to pro-
gresively increase the power of the lens in the lower
part of the near-vision region 4, with consequent need
to tolerate a narrowing of this region.
In connection with the drawings, the invention has
been described as applied to a progressive lens for use
at computer-screen work stations. Other developments
of the variable focus lens are also possible, for which
an initial example is ment:ioned in connection with Fig. 2b.
-18-
For users who must perform close work in the ~2795~3
region above their heads, the invention will be seen
to be further applicable to a progressive lens in
which the distance region is located at the bottom
and the near-vision region is at the top.
Still further, the described invention will be
seen to be advantageously applicable to a progressive
lens which differs from a traditional progressive lens
by the provision of an additional distance-vision
region at vertical offset below the near-vision region,
there being a progressive zone spacing this additional
distance-vision region from the bottom of the near-
vision region. Such a lens would, in particular,
make it easier for the user to handle stairs.
In the various cases of use, requirements differ
as to nature, size and mutual arrangement of the viewing-
distance regions as well as the development and vertical
extent of the progressive zones. But in all cases, care
must be taken that viewing can be effected undisturbed
in regions of good imaging which correspond to the parti-
cular viewing tasks, and that disturbances outside these
regions are small. This is possible, in use of the
indicated method of calculation, by suitable change in
requirements a, b and c in the example of the lens for
a computer-screen work station.
In all cases, it is advisable to keep the vertical
distance between the two outer viewing-distance regions
at less than 35 mm.
--19--
