When developing new front surfaces for progressive lenses in the past few years, ZEISS has had two main objectives in sight: the ongoing optimisation of the power design of multipurpose progressive lenses and the development of special purpose progressive lenses, e. g. for use at computer workstations.
A spectacle lens in which the power continuously changes is known as a progressive lens. Unlike bifocal or trifocal lenses, progressive lenses ensure that the presbyopic spectacle wearer finds the right dioptric power for every distance, guaranteeing smooth, high quality vision.
The power increase is achieved by constantly decreasing radii of curvature in the vertical and horizontal directions.
In the most used zones of vision, virtually aberration-free vision is possible, as here the radii of curvature are almost identical in the vertical and horizontal directions.
Peripheral Zones of the Progressive Surface
In the peripheral zones of the lens, on the other hand, the difference between the radii of curvature in the horizontal and vertical directions grows. This affects both direct and indirect vision. The wearer’s vision is blurred when he looks through the peripheral zones of the progressive lens. In indirect vision, annoying "rocking" effects were a drawback of earlier progressive lens designs.
Using complex mathematical computations and state-of- the-art production techniques, these aberrations have now been successfully minimised by optimising the power design of the lens. This has considerably increased wearer tolerance and has markedly enhanced wearing comfort.
A PAL designed for all-round use has three zones of vision: the distance, progressive and near zones. The transitions between these zones are smooth and invisible.
The illustration shows these zones by means of a diagram. The peripheral areas marked in grey limit the zones of the progressive lens used for direct vision. In the peripheral zones the deviations from the required prescription are so pronounced that the wearer can no longer use them for direct vision.
The top area of a progressive featuring the dioptric power required for distance vision is known as the distance zone. In the distance zone the lens displays the power needed to correct the wearer’s ametropia, or no dioptric power if he is emmetropic.
The transition between the distance and near zones in which clear vision is possible is known as the progressive zone. Here, the spherical power increases constantly in a downward direction until the addition is reached. In the progressive zone the lens displays the power required to correct any ametropia present and the additional power needed for vision at intermediate distances.
The width of the progressive zone is dependent on the design of the progressive lens and on the power of the addition. One of the factors determined by the design chosen for a PAL is the "distribution" of the areas of blurring on the lens and the length of the progression zone. The following rule applies: the shorter the progressive zone and the higher the addition, the narrower the progressive zone becomes.
The near zone contains the power required for near vision and reading. The near power consists of the distance power and the addition.
Progressives specially designed for vision at intermediate and near distances (e. g. ZEISS officelens) do not display a fully correcting distance power in the upper area of the lens, but the dioptric power required for intermediate distances. For special visual requirements, this special type of progressive lens is superior to a PAL designed for all-round use.
A special progressive of this type provides the wearer with markedly larger zones for middle distance and near vision.
To meet the demands now being made on our eyes not only at the workplace – and at computer workstations in particular – but also in leisure activities, special purpose progressives are now gaining increasing popularity.
The design of horizontally symmetric progressive lenses was implemented for the first time in the form of the progressive lens Gradal HS. Now, the top-of-the-range progressive, ZEISS Progressive Individual 2 also offers the discerning spectacle wearer the many benefits of horizontal symmetry.
For extra comfort all ZEISS progressive lenses are available with variable inset and thinnig prism.
Symmetrically designed progressives were the forerunners of today’s progressive lenses. The progressive surface is structured symmetrically, i. e. the major reference points for distance and near vision lie vertically on a line one below the other on the mean perpendicular of the lens.
To obtain the decentration of the near zone required for convergence, symmetrical progressives have to be rotated by 8° to 10° before being inserted in the frame. This is the only way of ensuring that the eyes can fully utilize the progressive and near zones when they converge.
A major drawback of rotated progressive lenses is the different visual clarity experienced by the right and left eyes when they change their line of vision. In peripheral vision it may happen that the two eyes look through areas of the lens with different image quality, hence severely limiting the field of view usable for binocular vision. This can be particularly noticeable when driving.
In an asymmetric progressive the near zone is nasally displaced with respect to the distance zone (nasal inset), with the result that no rotation of the lenses is required during glazing. This design ensures improved binocular utilisation of the zones of vision. In lateral vision the two eyes look through areas with similar image quality.
To ensure maximum wearer acceptance and optimum visual quality, ZEISS goes one step further by incorporating the principle of horizontal symmetry in its progressive lenses.
Horizontally symmetric progressive lenses, as all ZEISS progressive lenses are the result of the on-going, systematic development of the asymmetric design principle and are a speciality of Carl Zeiss which can only be produced by using sophisticated, computer-aided techniques.
Apart from a larger, binocularly usable field of view, horizontally symmetrical progressives offer the following benefits:
The near zone of progressives are nasally displaced to ensure that the fields of view of the two converging eyes coincide. This lateral displacement of the near zones is known as inset.
In near vision the horizontal prismatic effects of the lens require deflection of the lines of fixation, the extent of which is dependent on the dioptric power. The higher the vertex power of the lens, the greater the convergence required by hyperopic eyes in near vision, while less convergence is needed by myopic eyes.
ZEISS gives special consideration to this by the principle of variable inset. The inset of ZEISS freeform progressives varies from 0 mm (e. g. if patient only uses one eye without convergence) to 4.5 mm, depending on the distance power and the addition. Variable inset ensures that the wearer always experiences the largest possible binocular field of view and is an important component of horizontal symmetry.
The curvature of the progressive front surface increases constantly below the major reference point for distance vision. Without the use of a thinning prism, this would mean that the edge thickness in the upper area of the progressive lens would be greater than that in the lower area.
To reduce the weight of the lenses, a base-up prism is ground into the back surface of the lens. A base-down prism is then effective in the finished progressive lens. The magnitude and direction of this thinning prism is equal on the right and left lenses with the same addition and is not therefore effective for the spectacle wearer.
The thinning prism can be measured in the prism reference point. In a prismatic prescription the value measured is the prism resulting from the powers of the thinning prism and the prescribed prism.
Modern freeform progressive lenses have individualised thinning prisms, taking into account all given order parameters such as prescription, fitting data, etc.
This thinning prism is optimised for the interaction of binocular vision. Due to the individualised thinning prism a subsequent order for a single progressive lens is only possible if the data of the partner lens is specified, as unintended prismatic effects could otherwise impair wearer tolerance.
All ZEISS progressive lenses are provided with permanent engravings. With the aid of a special chart, these can be used to reconstruct all measurement and reference points as well as the horizontal axis of the lens.
The code number engraved directly under the temporal permanent engraving is used to determine the addition. A symbol for the lens type (e. g. a "I2" for Individual 2) is found under the nasal engraving and, where necessary, also a figure indicating the refractive index of the material used (e. g. "67" for organic material with the refractive index 1.67).
The idea of creating lenses without dividing lines to provide presbyopes with smooth vision has existed since the beginning of the century. Initial attempts were made to produce progressive lenses as long ago as 1909, but without success due to the large number of aberrations. The real breakthrough was achieved in 1956 by Grandperret of the Société des Lunetiers when he applied for a patent for a lens, the surface of which formed the basis of today’s progressive lenses.
Since then, there have been dramatic developments in the field of progressive lenses which are continuing to this very day. The visual acrobatics and long periods of adaptation required by early progressives are now a thing of the past. They are now firmly established on the market and enjoy a high level of popularity thank to their sophisticated power design and excellent cosmetic benefits.
In 1983 Carl Zeiss set new standards in the field of progressives with the introduction of Gradal HS. Horizontal symmetry (HS) guarantees identical visual impressions for both eyes when the wearer changes his or her line of vision and ensures optimum binocular vision. Needless to say, the proven concept of horizontal symmetry is also a feature of the latest PAL generation from Carl Zeiss.
In Gradal Top, the ranges of vision, and the intermediate range in particular, were considerably widened and further adapted to physiological requirements.
Ongoing product enhancement incorporating the experience of our customers – and of you, the optician – is a way of life at Carl Zeiss. All processes from initial product development to the finished spectacle lens are constantly rechecked and re-optimised. Gradal Top E marks a new milestone in the evolution of progressive lenses and is replacing the entire Gradal Top product family.
Today’s development work is focused not only on the ongoing enhancement of multipurpose progressives, but also on the inception of special-purpose progressives to meet special visual requirements, e. g. computer workplaces.