The present invention relates, according to a first aspect, to a method for designing a spectacle lens for a user, the spectacle lens comprising a first lens element and at least one second lens element, the first lens element having a front surface and a back surface that comprises a first back surface region and a second back surface region, the at least one second lens element comprising a front surface and a back surface, the second back surface region of the first lens element and the front surface of the second lens element being contiguous, a front surface of the spectacle lens being formed by the front surface of the first lens element and a back surface of the spectacle lens being formed in a base region by the first back surface region of the first lens element and in an achromatic region by the back surface of the at least one second lens element.
The present invention furthermore relates, according to a second aspect, to a method for producing a spectacle lens, which comprises the method for designing a spectacle lens according to the first aspect.
According to a third aspect, the present invention relates to a computer program product having program code for carrying out the method for designing a spectacle lens according to the first aspect of the invention.
The present invention furthermore relates, according to a fourth aspect, to a spectacle lens comprising a first lens element and at least one second lens element, the first lens element being formed from a first material and the second lens element being formed from a second material, the first material and the second material interacting at least partially achromatically, the first lens element having a front surface and a back surface that comprises a first back surface region and a second back surface region, the at least one second lens element comprising a front surface and a back surface, the second back surface region of the first lens element and the front surface of the second lens element being contiguous, a front surface of the spectacle lens being formed by the front surface of the first lens element and a back surface of the spectacle lens being formed in a base region by the first back surface region of the first lens element and in an achromatic region by the back surface of the at least one second lens element, the first back surface region of the first lens element having a first tangent at at least one first transition point between the base region and the achromatic region in a first cross-sectional plane of the spectacle lens, the back surface of the at least one second element having a second tangent at the at least one first transition point in the first cross-sectional plane.
The present invention furthermore relates to spectacles comprising a frame and at least one spectacle lens according to the fourth aspect.
It is widely known that, owing to the wavelength dependency of the refractive index of the optical material from which it is manufactured, a spectacle lens causes chromatic aberrations when the spectacle lens is manufactured from only one lens element. The chromatic aberrations include axial chromatic aberration, which creates different foci for different wavelengths of light. This is also referred to as longitudinal chromatic aberration. Besides axial chromatic aberration, transverse chromatic aberration occurs as a further chromatic aberration, which is manifested by colour fringes or colour edges in the image plane, i.e. on the retina of the eye in the case of a spectacle lens, which the spectacle wearer perceives and finds perturbing beyond a certain strength. Transverse chromatic aberration is also referred to as difference in chromatic magnification or as lateral chromatic aberration.
An estimate of the width of the resulting colour fringes, or the transverse chromatic aberration, can be given according to standard technical literature, for instance Diepes/Blendowske, “Optik and Technik der Brille” [Optics and technology of spectacles], Optische Fachveröffentlichung GmbH, Heidelberg, Germany, 2005, Chapter 5.3, by the formula:
      Δ    ⁢                  ⁢          δ      chrom        =            δ      e              v      e      where Δδchrom is a width of the colour fringe in
      cm    m    ,which is proportional to a prismatic power δe at the position in question and to the inverse of the Abbe number of the material in question. The prismatic power and the Abbe number in this case relate to the same wavelength, in the formula above to the e-line, i.e. a wavelength of 546.074 nm.
In the case of spectacle lenses with a low strength, the transverse chromatic aberration is not noticeable as a perturbation for the spectacle wearer. However, the chromatic aberration, in particular the transverse chromatic aberration, increases in the case of spectacle lenses with an increasing prismatic power. Furthermore, even if the ground prescription does not comprise prismatic correction, spectacle lenses may nevertheless exhibit transverse chromatic aberration owing to prismatic side-effects according to Prentice's rule for large viewing angles, even though the visual defect to be corrected is based on short-sightedness or long-sightedness.
High-index materials are often used nowadays, in particular plastics or high-index glass types, in order to keep the spectacle lens thickness as thin as possible for cosmetic reasons. Yet precisely in the case of materials with a high refractive index, the transverse chromatic aberration also becomes much stronger because an increasing refractive index generally entails a lower Abbe number.
It is therefore desirable to at least attenuate the transverse chromatic aberration which is created by a spectacle lens.
In the field of objectives, for example for cameras, it is known to correct chromatic aberration by so-called achromats. An achromat in optics means a system of at least two lenses which consist of materials with a different Abbe number and/or different refractive index and therefore different dispersion behaviour. One of the two lenses is a converging lens, which is conventionally made of a material with a higher Abbe number, for example crown glass, and the other lens is a diverging lens made of a material with a lower Abbe number and therefore higher dispersion than the converging lens, this lens being made for example from flint glass.
The two lenses are shaped and connected together at mutually complementary surfaces in such a way that the chromatic aberration is virtually eliminated for two wavelengths. The two lenses then interact achromatically.
In the context of the present invention, “interact with one another at least partially achromatically” or “interact achromatically” is intended to mean that the transverse chromatic aberration or the chromatic aberration is/are not necessarily entirely eliminated, but is/are at least attenuated.
The conventional achromats described above are not suitable for use as spectacle lenses. This is because, since these achromats are composed of two complete lenses, they also have a corresponding thickness and concomitantly a weight which is too high.
A lens is shown in the document GB 487 546 A. A lens may consist of two lens elements which essentially have the same refractive index, one of which lens elements is made of flint glass with a refractive index of about 1.61 and an inverse relative dispersion of about 36. The other lens element is made of barium crown glass with a refractive index of about 1.61 and an inverse relative dispersion of about 50. The former lens element is a diverging lens element, and the latter lens element is a converging lens element. The two lens elements are connected together at mutually complementary surfaces.
The lens produced in this way has a back surface, i.e. a surface facing towards the wearer's eye, which is formed entirely by the diverging lens element, while the front surface of the lens, i.e. the side of the lens facing away from the wearer's eye, is formed partially by the surface of the converging lens and in its edge region by the surface of the diverging lens. This lens furthermore suffers from the disadvantage that it consists of two lens elements and is therefore relatively thick and of high weight.
The technical article: “Hybrid diffractive-refractive achromatic spectacle lenses”, W. N. Charman, Ophthal. Physiol. Opt. 1994, Vol. 14, pages 389 to 392 also relates to the reduction of chromatic aberrations in spectacle lenses. It is emphasised therein that achromats which comprise a lens with a low refractive index and a high Abbe number and a lens with a high refractive index and a low Abbe number, of which the first lens is divergent and the other convergent, are not practicable as spectacle lenses since they conflict with the desire for small thickness and low weight of spectacle lenses. In order to overcome the difficulty of achromats, it is proposed therein to combine a refractive lens with a diffractive element, in which case the combination of the refractive lens and the diffractive element can have essentially the same thickness and the same weight as the refractive lens on its own.
However, a spectacle lens which is composed of a refractive lens and a diffractive element is very elaborate in terms of its production, since the diffractive element has to be produced with high precision in order to prevent other imaging defects from being induced by the diffractive element.