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FREE-FORM TECHNOLOGY. ADVANTAGES AND BENEFITS  

2012-11-24 05:43:12|  分类: ophthalmology 眼 |  标签: |举报 |字号 订阅

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A S Carlson. Optometric Science Research Group, Department of Optometry, University of Johannesburg, P O Box 524, Auckland Park 2006.

Great strides have been made in progressive lenses since Essel (now Essilor) developed the first progressive lens, Varilux 1 in 1959. This lens was a mono and symmetrical design, meaning the reading areas were not inset and therefore could be used for either a right or left eye by rotating it nasally for right and left eyes respectively and one design for all prescriptions and reading additions. This is shown in Figure 1. The distance and near portions of the lens was spherical in design with only one corridor length. Not much attention was paid to the peripheral vision. This, however, did not produce good binocular vision through the corridor because of variable viewing fi elds being different for every prescription and inconsistent tolerances over consecutive pairs of spectacles1. Moreover, the lenses were hard in design.

FREE-FORM TECHNOLOGY. ADVANTAGES AND BENEFITS - movie6521 - have a good time !!

Figure 1. The first progressive lens, Varilux 1 from Essel in 1959. The lens could be used for either a right or left eye by simply rotating the blue line anti-clockwise for a right eye and clockwise for a left. The broken line indicates the amount of rotation (the horizontal position) for the respective eyes.

With the introduction of asymmetrical designs, the lenses are no longer rotatable. The lenses are designed for right and left eyes independently. The front surfaces are aspherical in design and more attention is now paid to the peripheral regions of the lenses. The lenses are designed for each prescription with variable insets for the reading adds thus providing better binocularity when viewing through the corridor. Softer designs and shorter corridors are now available. Figure 2 shows a field cylinder map for a left eye of an asymmetrical design lens.

However, these lenses do have their disadvantages. The corridor narrows as the reading add increases and sometimes it is diffi cult to produce the same binocular viewing fi elds for all prescriptions. Figure 3 shows the profi le of three lenses made up with different prescriptions. It can be seen how the prescription affects the viewing field. There are variable performances with different prescriptions, no individualisation for frame fit, the viewing fi elds are different for every prescription and there is an inconsistent tolerance over consecutive pairs of spectacles.

FREE-FORM TECHNOLOGY. ADVANTAGES AND BENEFITS - movie6521 - have a good time !!
Figure 2. Full cylinder map for a left eye of an asymmetrical design lens.

FREE-FORM TECHNOLOGY. ADVANTAGES AND BENEFITS - movie6521 - have a good time !!

Figure 3. Profile of lenses made up with different prescriptions.

Lately many modern progressive lenses, although not all, have standard parameters. They include the following:
  • Fixed front surface geometry. The progression from the distance portion to the near is on the front surface of the lens and aspheric in design.
  • A symmetric in design. Each lens is designed for a right and left eye respectively with the near zones already inset. Both eyes encounter similar amounts of astigmatism at each corresponding point as they look from side to side, thus optimizing binocular vision through the lenses.
  • Variable inset by addition and power. The near portion inset varies according to the add power, prescription and front surface power (base) of the lens. The higher the hyperopia and/or add power or base, the greater the near inset. The higher the myopia or fl atter the base, the less the inset. The insets may vary between 2-5 mm
  • Inter-pupillary distance (IPD). The majority of progressive lenses are designed for patients with an IPD of 63-64 mm.
  • Position of wear / as worn. Very few manufacturers take this into account. The lens design and prescription is compensated for the pantoscopic tilt, face form tilt and vertex distance.
Freeform technology.
Modern lens design technology is improving progressively each year and improvements are being made with just about every new design manufactured. This has lead to the free-form optical lens design technology that can actually optimize, or optically refi ne, the design for a given prescription by applying appropriate asphericity at various points across the lens surface2. If for example a wearer's prescription is 3 - 2 x 65 with a reading addition of 2 D, the manufacturing process begins by determining an ideal or target optical performance of the lens design, for example, a Plano with an add of 2 D. The actual optical performance of the lens for the prescription and fi tting parameters is then calculated for this starting lens surface using computer ray tracing and lens-eye modelling. Finally, sophisticated computer algorithms repeatedly manipulate the design of the surface in order to minimize the difference between the actual optical performance of the design and the target or ideal performance2. This process continues until the actual performance of the lens resembles as close as possible the target or ideal performance2. This is illustrated in Figure 4.

FREE-FORM TECHNOLOGY. ADVANTAGES AND BENEFITS - movie6521 - have a good time !!

Figure 4. The manufacturing process for a right lens using Freeform Optical Optimization lens technology.

The advantage of free-form optimization design lenses is that the geometrical inset is no longer dependant on the convex surface geometry when the inset is fi xed by the progressive surface geometry, but can be calculated individually for the lens prescription. These designs are individually computed for each lens from the knowledge of the prescription data and base curve of the lens. Concave atoric surfaces allow wider corridors and less unwanted peripheral astigmatism1. Atoric concave surfaces combined with low-base aspherical progressive power surfaces are computer generated to produce optimal performance in each zone of the lens. If astigmatic compensations are required, the astigmatic compensations are also incorporated on the concave surface. It must be emphasized that Freeform is a technology and not a design and what makes a good design lens is not necessarily the freeform technology, but the software that goes with it.

In order to achieve optimization using Freeform technology, the following must be adhered to3;
  1. Individual Parameters.
  2. PD Optimized Inset.
  3. Position Of Wear / As Worn.
  4. Unique Customization and Wave Front Optimization
  5. Wavefront and Higher Order Aberrations (HOA).
  6. Design Parameters
Individual parameters.
PD - pupil distance
CVD - cornea vertex distance
PT - pantoscopic tilt
FFA - face form angle

FREE-FORM TECHNOLOGY. ADVANTAGES AND BENEFITS - movie6521 - have a good time !!

Figure 5. Diagram showing all the fi tting parameters. All of the parameters mentioned above must be measured and submitted to the laboratory so that custom designed lenses can be manufactured according to one's own prescription.

PD-optimised inset
The convergence effort depends directly on the PD. Figure 6 shows the convergence effort for two different subjects viewing a near object at the same distance.

It can be seen that Subject 1 with PD1 must converge more than Subject 2 with PD2. Freeform technology considers the convergence effort that is PD dependent. This results in a free inset adapted to the specifi c PD of the wearer, harmonic and natural near vision, perfect location of intermediate and near zones and an ideal binocular overlap of vision fields in the near and intermediate zones.

The conventional progressive lenses, however, do not take this into account. Although the lenses do have variable insets, they do not account for all cases.

FREE-FORM TECHNOLOGY. ADVANTAGES AND BENEFITS - movie6521 - have a good time !!

Figure 6. The convergence effort depends directly on the subject's PD. The convergence effort for two different subjects viewing a near object at the same distance. It can be seen that Subject 1 with PD1 must converge more than Subject 2 with PD2.

Position of wear - As worn

FREE-FORM TECHNOLOGY. ADVANTAGES AND BENEFITS - movie6521 - have a good time !!

Features & benefits position of wear / as worn
When the spectacles are positioned on the face, the position of wear or as worn position is not the same as when positioned on the vertometer. The ordered (untilted power) of the lens may differ from the focimeter (tilted) power. Freeform technology takes this into consideration. This is shown in Figure 7.

FREE-FORM TECHNOLOGY. ADVANTAGES AND BENEFITS - movie6521 - have a good time !!

Figure 7. Figure (a) shows the untilted refraction as recorded with a trial frame or phoroptor. The power ordered equals the power recorded with the focimeter power. After optimization, Figure (b) refers to the tilted power not being the same as the ordered power.

Unique Customization.
Once all the parameters are entered, the desired vision performance is determined and then the fi nal lens design is calculated and optimized with the individual data. No fi xed or frozen design is used. This is illustrated in Figure 8.

FREE-FORM TECHNOLOGY. ADVANTAGES AND BENEFITS - movie6521 - have a good time !!

Figure 8. Flow chart for unique optimisation. (Courtesy Rodenstock)

Wavefront and Higher Order Aberrations (HOA)
Minimisation of 1st and 2nd order aberrations (prism, astigmatism and defocus) over the whole lens surface is obtained for the highest level of visual acuity. In addition to the 1st and 2nd order aberrations, higher order aberrations (HOA) optimization such as "coma" and "trefoil" are also minimized. Higher order aberrations only appear within the lens where they are physically indispensable. Lenses are aberration-free along the main viewing line and the main vision zones as far as physically possible for the best and almost unlimited vision fi elds. Figure 9 shows diagrams of the different lower and higher order aberrations that are minimized for optimization.

FREE-FORM TECHNOLOGY. ADVANTAGES AND BENEFITS - movie6521 - have a good time !!

Figure 9 shows diagrams of the different lower and higher order aberrations that are minimized for optimization. Minimisation of 1st and 2nd order aberrations (prism, astigmatism and defocus) over the whole lens surface is obtained for the highest level of visual acuity. In addition to the 1st and 2nd order aberrations, higher order aberrations (HOA) optimization such as "coma" and "trefoil" are also minimized.

Design parameters
Every lens is designed and created for personal visual needs. This provides individual accentuation of the vision areas in the near, intermediate and far zones of the lens. It also provides individual adjustments of the lens design to every spectacle shape and individual optimization to the requested near distance and individual optimization to the lightest and thinnest lens design possible. This is illustrated in Figure 10.

FREE-FORM TECHNOLOGY. ADVANTAGES AND BENEFITS - movie6521 - have a good time !!

Figure 10. Every lens is designed and created for personal visual needs

Discussion
It must be emphasized that not all manufacturers apply the same principles as mentioned above. Not all Freeform progressive lenses take position of wear/as worn into account or design parameters. The majority compensate for lower and higher order aberrations and some manufacturers apply Freeform to both the front and back surfaces of the lens while others only apply it to one surface, usually the concave surface.

To mention a few4,5, ImpressionsILT from Rodenstock use a convex spherical surface and concave atoroidal progressive surface for their lenses. Gradal Individual from Carl Zeis Vision apply a convex progressive surface and an aspherical/ atoroidal concave surface. Hoyalux iD apply an integrated double surface progression4. The near power is partly on the convex surface and partly on concave surface. The vertical change in power is on convex surface and the horizontal change on concave surface. See Figure 11. Ipseo from Essilor determine the design to be used on whether the patient is a head or an eye mover. This is shown in Figure 12. Seiko P-1SY use a convex spherical or aspherical surface and a concave aspherical/ atoroidal progressive surface. SolaOne HD from Carl Zeis Vision use a convex spherical surface and a concave aspherical / atoroidal progressive surface. Free Sign from Rodenstock use a convex spherical surface and a concave atoroidal progressive surface. See fi gure 12. GT2 and GT2 3D, the very latest designs from Carl Zeiss Vision uses a convex aspherical progressive surface and conventional concave surface and aspherical convex and aspheric convex and aspheric/atoroidal concave progressive surface respectively. The GT2 is available in semi-fi nished blanks and manufactured locally while the GT2 3D is only available ex Germany.

Hoyalux iD
FREE-FORM TECHNOLOGY. ADVANTAGES AND BENEFITS - movie6521 - have a good time !!

Figure 11. The Hoyalux iD applies an integrated double surface progression. For example, if the lens has a power of Plano with an add of 1.00 D the near power is partly on the convex surface and partly on concave surface. The front atoric surface has a reading add of 3 D along the vertical meridian. The atoric concave surface combines with the front surface to give the power. The vertical change in power is on convex surface and the horizontal change on concave surface.

FREE-FORM TECHNOLOGY. ADVANTAGES AND BENEFITS - movie6521 - have a good time !!

Figure 12. Varilux Ipseo. Subjects are asked to look into the machine and observe what is going on. If the subject tends to move their eyes more than their head, as shown in (a), a harder design is recommended because good acuity is required over a wider fi eld. If the subject tends to move their heads more as shown in (b) a softer design is recommended because good central acuity is required.

FREE-FORM TECHNOLOGY. ADVANTAGES AND BENEFITS - movie6521 - have a good time !!

Figure 13. Free Sign from Rodenstock use a convex spherical surface and a concave atoroidal progressive surface. Every lens is designed and created for personal visual needs. This provides individual accentuation of the vision areas in the near, intermediate and far zones of the lens. (Courtesy Rodenstock)

As previously mentioned, Freeform is a technology and not a design. Although many manufacturers use the technology, not all the lenses are of the same quality. It is in the author's opinion that Freeform lenses do offer more comfortable vision (less peripheral distortions). With the right design Freeform can be custom made to suit one's needs. From a laboratory's point of view, the stock inventory can be reduced as laboratories do not need to stock all bases and reading adds. The disadvantages of this technology are, perhaps, more information and parameters are needed and the costs involved. The lenses are more expensive than the conventional ones. However, once this technology has become more popular, it may become more cost effective.

References.
  1. Mc Carthy P. Progressing with Progressives. Dispensing Optics. Aug/Sept 2007 22 6 6-12
  2. Meister D. Free-form optical optimization. Zeiss, Technical Communication, 2006.
  3. Zimmerman M. Rodenstock. Personal communication
  4. Jalie. M. Ophthalmic Lenses and Dispensing. 3rd Ed. Butterworth Heinemann Toronto 2008.5
  5. Carlson AS. Free-form Optical Design Technology. Eyesite Apr 2007 pp 56-58
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