Why Free-Form Makes More Sense
The curve change, from distance to near, creates unwanted astigmatism to the right and left of the intermediate and near. It’s unavoidable and increases as a patient’s add power increases. This causes distortion and blur. It is noticed by patients, is sometimes complained about and may have been a problem when changing a patient from one lens design to another. Reduce the off-axis, unwanted astigmatism and it reduces the blur and distortion that patients see.
In fact, when significantly reduced, it produces a wow reaction because patients immediately notice the visual freedom that this provides when compared to their previous lenses. In fact, if you were able to describe the previous lens and problem, you could better identify the free-form lens fix.
Prescriptions, in standard progressives, are fabricated from specified base curves. That’s because a variety of design improvements, that are Rx related, are included in the various base curve lens blanks. For example, in modern steeper base curve progressives, the near zone is typically inset farther than in flatter base curve designs. This is called variable inset and is determined by the horizontal prism induced by the distance prescription. It’s a feature of most modern designs. So, being able to include features like variable inset places the near zone correctly in front of the converging eye. It makes for better intermediate and near vision.
Lastly, prescriptions, especially with cylinder power, change the intended design of a standard progressive. The only time that the progressive is effectively that of the intended design is when the prescription ordered is similar to the target sphere Rx, typically only a very narrow range of powers in the full range of prescriptions produced within a base curve.
The combination of the back surface cylinder with the single base curve progressive front, and its unwanted cylinder, creates a new Rx that alters the final design. That means that as cylinder Rx’s get stronger and axes shift, the design can be further corrupted resulting in significantly reduced clear areas of vision.
What are my Free-Form Lens Opportunities?
Therefore, the limitations of front surface progressives create an opportunity for free-form. As you probably know, in a free-form system, progressives can be made by placing the progressive surface on the back of the lens, or by using both surfaces to create the final lens. That means that there is an opportunity, at the time of lens manufacture, to consider fitting and lifestyle requirements beforethe final lens is completed.
To make better progressives, optimization is required i.e., an integration of all the characteristics of each wearer, in real time, in the lab, when the lens is being produced. This manages the design, lens prescription and the way it’s worn to produce a lens that is in fact still the design that was intended, but tuned to one wearer. The result is a lens that provides clearer vision over more of the lens than previous lenses. The goal is to reduce current progressive lens limitations and improve patient satisfaction and deliver better vision that patients really want. Progressive designs can be delivered as they were intended. It’s up to you to communicate the benefits. However, not every manufacturer optimizes lenses in the same way.
Definitions
Aspheric
Not spherical but has a curve that increases or decreases radius from lens center to edge
Atoric
Not toric (cylinder); different asphericity in each of the principle meridians
Bitoric
Both sides of a lens are toric
Diamond Turning
Single point method of cutting a complex surface
Distortion
Altered shape of an object seen
Effective Power
Lens power as influenced by fitting distance and tilts
Free-Form
Lens cutting method using a 3 or more axis generator and a mathematically derived software file that directs the cutter
Optimization
The integration of Rx, lens and fitting characteristics to create a cutting file that delivers a lens design as it was intended
Irregular Atoric
A surface with varying amounts of atoricity and directions
Personalized
Lenses integrating actual fitting measurements rather than an assumed vertex, tilt and faceform
POW
Position of wear, the actual lens fitting measurements
Rotational Asphere
A lens surface that has the same asphericity, from center to edge, in all meridians
Toric
A surface with two different curves, perpendicular to each other
Unwanted Astigmatism or Blur
Lens cylinder power that is not the patient’s Rx, affecting clear focusing
Variable Inset
Different decentration of the intermediate and near zone based on the effects of the prism created by the distance Rx
Optimization
Standard prescription lenses have front surfaces that are typically spherical (or in aspheric lenses are a rotational asphere) and the back surface is spherical for sphere Rx’s and toric for cylinders. That means that a progressive moved to the back lens surface must be combined with the “Rx” i.e., the toric surface. Therefore, the designer requires some thought about how to combine Rx and progressive and how the areas of blur and distortion are controlled. So, it’s possible to create a back surface progressive that is not any better than the standard progressive that you now use. In fact, it’s possible to make it worse.
Optimization combines the Rx and progressive together considering the specified base curve, corridor length and add power. Their interaction with each other is considered so blur and distortion is reduced in the final lens. Optimized lenses are typically better – they more precisely reduce blur and address the power needed by the patient. The results are, of course, dependent on the lens designer’s/manufacturer’s philosophy. So, choosing the right source is important.
Optimization can improve the field of view in both meridians of an astigmatic Rx also. It therefore answers the single base curve issue of standard progressives. Prescriptions with cylinder power have a base curve chosen to best correct the stronger power meridian at the disadvantage of the other meridian. Optimizing the distance Rx with atoricity and considering the unwanted front surface astigmatism bordering the intermediate and near, for each meridian independently, ensures that all patients with a cylinder Rx see better in all zones. This is especially evident in cylinders of 1D or more.
Optimization can be also used to alter the size and shape of the near zone and/or change corridor length. The result is a lens tuned to the segment height ordered for the frame chosen. The optician can select the corridor length wanted or let the lab provide the best choice for the height and frame “B” fit. The result is an optimized intermediate and near zone by frame.
Optimizing a progressive is complex and requires each order to be considered differently. You can include as little or as much as the patient will agree to. The difference is that the progressive is being manufactured completely by your laboratory, in real time, rather than at the factory. Therefore, the order is manipulated by the lab’s software to make the lens design as intended.
That means that each lens is individually optimized for the data received for that patient. That’s also the way to talk about these lenses to patients.
“Unlike standard progressives, which worked well for you, they were still a compromise for your prescription and frame due to the limitations of that technology. Technology continues to improve. These new techniques allow us to address each of your visual demands and produce a lens with better vision for the way that you wear them. So, vision will be sharper and comfort greatly improved for all tasks. They are worth the extra cost.”
Personalization
To further improve a progressive, and make it more personal to the way that a wearer uses their lenses, the power can be further adjusted for the position of wear (POW), the way that the patient actually wears their lenses, in the frame that they chose. This is typically different from the way that the patient was examined i.e., the way that the refractor (phoropter) positioned trial lenses in front of the patient’s eyes. When a prescription sits in front of the eyes differently from the lenses in the refractor, it has a different effective power. Progressives can be personalized for POW. POW values are vertex distance, tilt (pantoscopic) and faceform.
That also means that the lens Rx received back from the lab (as measured on a lensmeter) will not be the same as the one that was ordered (the prescribed Rx). However, when worn, the lenses will have the same effective power as the prescribed Rx. This is because the Rx has been compensated for distance and tilt angles.
Other Benefits
Moving control of the progressive, all or in part, to the lens’ back surface also reduces the magnification, distortion and keyhole effects. First, front surface curve changes, from distance to add, increases the magnification effects to the left and right of the intermediate and near. This adds to the swim that some patients see. Moving the addition to the lens back reduces these effects.
Second, move a keyhole closer to the eye and the world on the other side is more visible. So, moving the add zone to the lens’ back surface can improve the size of the intermediate and reading area.
Choices and Tiers
So, if optimized lenses and personalization can better meet my patients’ needs, what are my choices?
Concave Only Progressives – In this case the designer considers all components of the Rx and its’ fitting to replicate the intended design on the concave surface of the lens. Depending on which lens is ordered, some values may be assumed or the optician has to supply them. The Rx, progressive design, PD, base curve, fitting height and corridor length will be iterated to reach the cutting design.
There is great variety in what is produced. Choose between lenses with fixed corridor lengths (most designs) or those that morph the corridor length for the fitting height needed and the frame’s “B” dimension (Zeiss Individual, Essilor Accolade, Kodak Unique, etc.). Only a few incorporate optician measured position-of-wear values (Shamir Autograph II, SOLAOne HD and Zeiss Individual and others), the others use an average assumed set of values (Shamir Element, SEIKO Succeed and others).
The availability of design choices, at different price points provides tiers of benefits and therefore, an improved lens for any budget. Patients today still don’t necessarily want “cheap” but will scrutinize the value of the products for which they are spending. Having tiers for them and being able to explain the advantages is key to being successful with the full complement of the possibilities of freeform.
Both Surfaces and Progressive
Working with both surfaces, the software can be used to solve for progressive limitations in a variety of different ways.
(1.) Use a standard progressive front and optimize the back for the final lens. In this case, Essilor applies an irregular atoric to the back of Varilux Physio, Varilux Comfort and Varilux Ellipse to make 360° optimized lenses. This considers the front surface design, unwanted astigmatism, Rx, base curve, and an assumed set of POW characteristics to create the final design. In Additionly, the Varilux Physio is also available in a Short, reduced corridor length design. Essilor calls the technique DDV:Dual Digital Vision.
(2.) In the Essilor DEFINITY 2.0, the add power is split between the front and back surfaces. Optimization occurs in the way that the back addition power and Rx is combined and the way that the back is rotated to the front. The shift of cylinder axes reduces blur and distortion. The lens incorporates variable inset to place the near zone correctly for any distance Rx prismatic influence. It is also available in a short corridor version for small frames.
(3.) HOYA uses two different non-progressive surfaces (horizontal and vertical bitoric surfaces) that when combined, form a progressive. In this unique way, HOYA delivers a progressive in which the vertical characteristics of the corridor is controlled by the front surface and the width of the intermediate and near are controlled by the back. They call this Integrated Double Surface Technology. This method also allows the designer to adjust for blur, magnification and distortion, zone size, PD, height and corridor length, an assumed set of fitting values, base curve and material for the final result.
In HOYALUX iD, both surfaces are cut during manufacture, the front as a vertical bitoric, and the back as a horizontal bitoric. On the front, this shortens the vertical progression, reduces vertical eye rotation for a faster transition between far and near. Getting to the reading area quickly is especially appreciated by higher add wearers. A uniform front surface curvature can ensure the same visual perception for every stage of presbyopia. On the back of lens, horizontal control enlarges the usable areas to create wider, clear visual fields at all distances.
The application of double asphericity using freeform techniques provides for no restriction of frame choice, an optimization of off-axis errors and skew distortion to maximize the clear areas of view.
(4.) As a lesser-cost tier, HOYALUX iD Lifestyle utilizes a cast front surface vertical bitoric in a standard or compressed design (HOYALUX iD Lifestyle cd) and freeforms the horizontal bitoric on the back. Therefore, a choice of a 18mm or 14mm minimum fitting height is required. The benefits of progression control and zone size remain.
Summary and Conclusion
Free-Form is a terrific tool when used properly. Optimizing all the data at the time of manufacture of the final lens, allows the lens’ design to be delivered as it was intended. It increases precision and accuracy because it can be used to deliver the patient’s Rx faithfully in 0.01D increments, integrate fitting requirements, corridor length, and any “B” frame size. It can reduce the magnification and distortion effects and improve zone width control. Freeform is used to answer the “base curve dilemma” problem by addressing the correction of each meridian separately to better manage off axis errors. Lastly, personalization, by also considering the actual fitting characteristics, the effective power can be adjusted so that it replicates the Rx that the patient was prescribed.
The Future
Since free-form allows progressives to be manufactured in real time with all patient and frame data, what else is possible?
There will be an increased use of position of wear measurements so while there is little new to learn in fitting most freeformed progressives today, expertise in measuring vertex, tilt and faceform will be required. Therefore, consider purchasing digital camera measuring systems. They add accuracy, consistency and speed to any practice.
However, you can expect new lenses that will require ergonomic considerations; eye or head tracking values and/or habitual lens or wearing data that can be used to better adjust the lens design. The results will be a lens that is physiologically customized to the wearer rather than expecting a patient to have to learn to, or “get used to” wearing their new lenses. Thanks freeform! It won’t necessarily seem easier at first, but it will be better for patient and optician alike. Over time, like any new product, it always gets easier.
by Mark Mattison-Shupnick, ABOM
2020mag.com
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