Optics: eye examining – vision testing and correcting – Spectacles and eyeglasses – Ophthalmic lenses or blanks
Reexamination Certificate
2000-07-11
2003-03-25
Sugarman, Scott J. (Department: 2873)
Optics: eye examining, vision testing and correcting
Spectacles and eyeglasses
Ophthalmic lenses or blanks
C351S161000, C359S565000, C359S838000, C623S006300
Reexamination Certificate
active
06536899
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to multifocal lenses, and more particularly to multifocal lenses with powers which are intrinsically both diffractive and refractive powers. The invention relates even more particularly to multifocal lenses which provide simultaneous refractive and diffractive powers without exhibiting optical steps on a lens surface, common with diffractive lenses. The invention also relates to multifocal lenses in which at least two powers can be attributed to arbitrary relative intensities completely independent of one another.
2. Description of the Prior Art
A diffractive lens generally consists of any number of annular lens zones of equal area; such zones are usually called Fresnel zones. Between adjacent zones optical steps are provided with associated path length differences t which usually are absolutely smaller than a design wavelength &lgr;. The area or size of the zones determines the separation between the diffractive powers of the lens; this separation increases with decreasing zone area. The optical path length difference t determines the relative peak intensities of the various diffractive powers, e.g. for t=&lgr;/2 there are two principal diffractive powers, the 0-th and the 1-st order diffractive power, respectively, and both exhibit a peak intensity of (2/&pgr;)
2
=40.5%, where 100% is the peak intensity of a lens with identical Fresnel zones but with zero path length differences between any and all zones. The latter lens is a “normal” refractive lens. For absolute path length differences smaller than half the design wavelength, the zeroth order power is dominant, for &lgr;>abs(t)>&lgr;/2 the first diffractive order power carries the maximum relative intensity.
It is of paramount importance to note that with any single Fresnel lens zone of a diffractive lens, a refractive power is associated; this refractive power can be calculated by refracting an incoming light ray using Snell's refraction law. The Fresnel zone may exhibit a uniform refractive power, but it can also exhibit a certain blaze design in such a way that the refractive power of the zone varies across said zone; then the refractive power of this zone is an average power.
In conventional multifocal diffractive lenses with optical steps between adjacent zones, none of the various diffractive powers of the lens are equal to the refractive power of the zones. In particular, this is true also for the zeroth diffractive power of a diffractive lens, in apparent contradiction to the terminology used by some authors who call this zeroth diffractive power the “refractive” power of a diffractive lens (see e.g. Freeman, U.S. Pat. Nos. 4,537,697 and 4,642,112). But even if the average optical path lengths of light rays between an object point and its conjugated image point through any two zones are equal—as is the case in the zeroth order diffractive power—this power is not a refractive power, since it cannot be calculated or derived on the basis of the refraction law for light rays, i.e. without wave considerations.
There are two principal designs of diffractive lenses. In the first design, the optical path length difference t between the first and second zone is equal to the path length difference between the second and the third zone, etc. Embodiments of such diffractive lenses usually exhibit a saw tooth profile on one of the surfaces of a lens made from a material of some given refractive index. This saw tooth profile can be embedded in a material of different refractive index in order to obtain e.g. smooth outer surfaces of the bulk lens.
FIG. 1
is a schematic sketch of the center portion of a diffractive lens according to such a design of the prior art. When applied to a contact lens, the saw tooth profile is usually present on the back surface of the lens in order to control the phase relations of such lenses. The saw tooth profile
4
is completely embedded in the tear layer
1
between the cornea
2
and the diffractive lens
3
; thus definite conditions for the phase relations of the diffractive lens are guaranteed. The lens
3
has to be made, of course, from a material whose index of refraction is different from the refractive index of the tear fluid. Although in such a design comfort may be compromised by the presence of circular grooves on the backside, such a design is presently the only one which has obtained practical importance in ophthalmic optics. Putting the saw tooth profile on the front surface results in smaller acceptable machining tolerances, since abs(n
L
−1) is usually larger than abs(n
L
−n
T
), wherein n
L
is the refractive index of the lens and n
T
the index of the tear fluid. Also, a tear layer on front grooves of a diffractive lens can compromise the optics of such a lens, since the tear layer thickness will most likely be non-uniform.
In the second principal design of prior art diffractive lenses, the optical path length differences between the first and second zone is +t; between the second and third zone is −t; between the third and forth zone is +t; etc.
FIG. 2
is a schematic sketch of the central portion of a contact lens according to this prior art design, in comparison with FIG.
1
. Although it would seem that such a lens rests more comfortably on the eye, contact lenses of this design have not gained major practical importance. The reasons for this are likely to be of practical nature, since it is difficult to cut such lenses or molds for such lenses. More specifically, two adjacent comers
5
and
6
of any zone would have to be cut by diamond tools of different orientation, since the groove cross-section should be rectangular and not trapezoidal.
Combinations of the aforementioned designs are possible and occasionally mentioned in the patent literature.
The drawbacks of any of the presently known diffractive lenses can be summarized as follows:
1) Diffractive lenses or molds for diffractive lenses are difficult to machine since such lenses require exact grooves on at least one surface with groove depths in the order of microns only.
2) Due to machining imperfections—caused by the non-zero diamond tool radius—the theoretical profile cannot be machined to exactness. As a consequence, practical embodiments of such lenses exhibit a sizeable portion of non-optical surfaces.
FIG. 3
compares the ideal theoretical zone profile with its corresponding practical embodiment for a lens according to the first prior art design. In
FIG. 4
the comparison is for a lens made according to the second prior art design. Non-optical surfaces result in stray light, loss of in-focus light intensity and reduced contrast.
3) In ophthalmic lenses, grooves on the surface give rise to accumulation of debris, which compromises optical performance of the lens.
4) The flanks of the grooves—labeled
7
in
FIG. 3 and 8
,
9
in FIG.
4
—which are essentially parallel or slightly inclined to the lens axis tend to reflect incoming light. Such reflected light is lost in the foci and leads to the experience of halos by the lens user.
5) Diffractive lenses—even if manufactured to near perfection—exhibit relatively high longitudinal chromatic aberration in at least one of the diffractive powers. This holds true in particular for lenses according to the first prior art design discussed above. Although some authors describe such chromatic aberration as beneficial in ophthalmic applications, the magnitude of this chromatic aberration should be maintained within certain limits, since sizably different powers for blue and red light may compromise visual resolution in the case of multi-colored objects (e.g. color prints).
6) In diffractive lenses according to the above designs, it is difficult to provide more than two main powers. Lenses with more than two main powers require peculiar zone blaze designs which are difficult to fabricate in practice.
The principal inventors of diffractive lenses of the prior art embodiments discussed above are Cohen and Freeman. The Cohen pat
Bifocon Optics GmbH
Sugarman Scott J.
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