Double Asphericity, A Free-form Solution for Single Vision
By Mark Mattison-Shupnick, ABOM
Release Date: December 1, 2014
Expiration Date: January 1, 2017
- Understand how free-form SV is an untapped opportunity.
- Learn how double asphericity as a free-form technology improves vision.
- Know how to discuss the advantages of double asphericity for single vision lens wearers.
Mark Mattison-Shupnick, ABOM, is currently director of education for Jobson Medical Information LLC, has more than 40 years of experience as an optician, was senior staff member of SOLA International and is a frequent lecturer and trainer.
This course is approved for one (1) hour of CE credit by the American Board of Opticianry (ABO). Course STWJHI10-2
This course is supported by an educational grant from SEIKO Optical Products of America
How many reading this are promoting and selling free-form progressives? Come on, put up your hands and keep them up.
Good, now keep your hands up if you are also promoting and selling dual aspheric and free-form single vision. Just as I thought, not enough of you have realized the opportunity and benefits of these powerful free-form based single vision lens technologies. For those with your hands up, congratulations in recognizing the advantage of these lenses.
Oh, you can put your hands down now.
THE SINGLE VISION OPPORTUNITY
It’s time to consider changing the way we think of single vision and shift even the basic SV order to one that includes more technology in the way that the lens surfaces are designed. That means the regular use of aspheric, atoric (Dual aspheric) and free-form surfaces.
Asphericity, dual asphericity, atoricity and free-form are better choices for a clearer field of vision. Each provides an improvement in the way that a patient sees. Asphericity has been typically used to change the cosmetic form of plus lenses so that the wearer gets a flatter lens and reduced magnification. All this while retaining the optics of the steeper ‘best form’ or ‘corrected curve’ lens that previously would have been used. Patients are thrilled when a 4 base lens rather than a 8 base lens is used to make that +3.00 prescription, especially in a 1.67 or 1.74 index material (Fig 1). Why?
First, technology of material (1.67 and 1.74 high index) provides a thinner lens form. Then, reducing the front surface bulge of a +8 base to that of a +4 with the accompanied aspheric correction in the periphery of the lens makes the lens thinner and better looking, yet delivers reduced magnification and off center clarity comparable to its steeper form. That’s merchandisable.
Now take two-steps forward, but pause to use each step separately when the right circumstances present (Fig. 2). First, recognize that aspheric lenses are only a partial design solution. The lens’ front surface is a rotational asphere i.e., has the same amount of asphericity in all directions. As a result, the amount of asphericity can only correct for one lens power, typically the sphere power. That’s OK if the prescription is a sphere or a cylinder of 0.75D or less.
However, if the cylinder is 1.00D or more; the asphericity is not correct for the cylinder meridian. For example, SEIKO double aspheric lenses solve the problem by using the aspheric lens front, and then cutting a free-form back so that there is a different aspheric correction applied to each meridian of power. There are two ways to do this, either atoricity (spherical front/free-form back) or double asphericity (aspheric front, free-form back). Both are created using a free-formed back lens surface.
The amount of asphericity needed in each meridian is determined by calculating the improvement of the errors that affect vision. For example, marginal astigmatism (off center errors) causes peripheral blur, increasing as the eye turns and the angle between the lens surface and the eye changes. To illustrate this, maps of the astigmatism can be created using various surface designs. The difference between the horizontal and vertical errors (sagittal and tangential) is the marginal astigmatism.
In Fig 3, the top row describes the errors that combine to create marginal astigmatism (blur). The first is a standard toric lens, next a simple front surface aspheric, the third a free-form double aspheric. The blue color, used to fill in the difference between the sagittal and tangential lens errors (errors in the two principle meridians of a lens -6.00D sphere power with a -4.00 D cylinder) represent the amount of blur (marginal astigmatism) away from lens center. Asphericity reduces blur, while double asphericity reduces it further. The graph suggests that out to 60 degrees or about 30mm, there is little variation in lens power. As you can see, the errors can be significantly reduced using double asphericity. This results in an improvement to the clear field of view. As a result, lenses, even in large sizes return better optics. The middle row shows that there is also reduced distortion.
The bottom row of Fig 3 illustrates the application of correcting the lens surface for power error and the resulting the size of the clear field of view. As you can see, reducing the power errors increases the clear field of view in both meridians of power. A power error map illustrates each of the lens forms, spherical, aspheric and double aspheric design. These lenses use the default or average fitting characteristics of tilt, wrap and vertex distance. Of course, as you learned in progressive lens fitting, decreasing the vertex distance also makes the size of the clear field larger (keyhole effect).
Now, take the calculations of full lens prescription requirements (their surfaces) to their logical next step considering lens tilt and the lens can be further improved. Lens tilt affects the delivery of the lens’ power as the eyes saccade normally i.e., scan the field of view. Input that information and using free-form surfacing, improve the prescription everywhere on the lens when lenses are worn differently than average. Review the bottom row of Figure 1. It describes the two methods of improving vision in all prescriptions with spectacle lenses while flattening also improves looks.
This single vision lens improvement is so good that it now competes for the contact lens wearer. For a long time, contact lens wearers saw better through their contact lenses than spectacles. That was because the off-axis blur in a cylinder spectacle prescription was significant in comparison to the tear film correction provided by a gas permeable contact lens. In addition, the contact lens center moved with the eye. However, with the popularity of soft contact lenses, in a patient population with cylinder correction between 0.75D and 1.50D, an optimized or a customized free-form SV lens will actually deliver better vision. If that makes a difference to the wearer, then spectacles become a better solution. The spectacles will be worn more waking hours and the patient actually cares more about the way that the frame looks. As a result, these patients purchase frames based on the usual drivers i.e., trends, brand, style, color and demographics rather than cheap glasses, just to wear with their pajamas.
The Flip Point
Borrowing a term from Barry Santini (Long Island Opticians), double aspheric and customized free-form single vision lenses have a more accessible flip point when considering the price sensitivity of patients. The flip point is the point of no price resistance i.e., an add-on or new product, that when considered for its benefits, becomes a no-brainer for the patient.
There are at least two considerations here. The first is, how much is the add-on and what’s the best way to bundle and present the option. The second is can I use finished stock lenses? After all, about half the offices in the US do their own in-office finishing. Stock lenses typically provide a savings over surfacing, both in time and money.
Considering stock and surfaced, single vision and progressives (of course no stock progressives typically), toric, dual aspheric and free-form, the free-form flip point of SV at $35 to $65 vs. $75 to $150 of progressives is easier to up-sell. The SV free-form add-on is easier to digest for both the patient and the ECP. And, the overall optical benefits for SV are better than in progressives since its easier to fix the off-axis optics of a lens without an add. Moreover, high index double aspheric lenses (1.6 SEIKO DA) are available as stock, finished.
Lots of ‘m Out There
You might be asking how many single vision wearers are there? That’s a reasonable question since we usually talk about boomers, the aging population and their problems with flat tops, and the opportunity for free-form progressives.
Turns out that more than 60 percent of the US population is under age 44, making them potential single vision lens wearers (Fig 4). In addition, the portion of the eyeglass wearing population that wears single vision is over 50% (Fig 5.). Now I agree that some of those single vision pairs are for reading glasses but that means half of the people we make eyewear for require us to know more about the options for single vision. Consumers, 18-44 years old are a great target market. They are driven by brands and trends, in fact a portion of this population purchase a new pair of glasses as often as every 1.9 years (Fig. 6). Single vision sales are an opportunity filled part of the business.
The single vision wearer lives in a world of technology (mobile devices, communications, heads up display in the car, computing power) and that drives many of their purchase decisions. Therefore, always use the fact that ‘technology’ is driving improvements when describing lens choices. Thinner and lighter lenses i.e., using the technology of better material indices still has appeal, the ability to significantly improve vision opens a new merchandising opportunity using free-form and the technology of lens design.
Moreover, if you’re also concerned about the way that the Internet, is or will affect your business, it is critical that you use products that are not available online. In this way it is possible to separate yourself from brick and mortar competition as well as the Internet. In both cases, improvements in lens technology provide a visual benefit that can be seen by the wearer just as you sold thinner and lighter lenses.
Prescription and Lens Size as Reasons
However, we so often just check the single vision lens box on the order form and pay little attention to the lens’ design. The lab will most often choose a finished toric lens, out of the envelope. So, which lenses should be different for which wearers?
From an Rx perspective (in my view) double aspheric and free-from lenses, in high lens powers (greater than -2.00D and +1.50D) and, prescriptions with cylinders of 1.00D or greater will deliver improved vision success. Moreover, as the lens size gets bigger (think large retro style frame) or as the decentration increases, more of a lens’ periphery is visible to the wearer. As a result, better treatment of vision in the periphery leads to patients saying, “I see better”. As I’ve said before, the larger the clear field of view, typically, the happier the patient.
In an analysis of about 29 thousand single vision lab prescriptions, almost 45% of the prescriptions were beyond -2.00D and +1.50. More importantly, an analysis of the cylinder prescriptions shows that about 46% had cylinders of 1.00D and above.
Therefore, consideration of lens surface design, to deliver better vision, should be important to about half of all the single vision lens patients seen. What can you do about that? Consider replacing ordinary single vision lenses with aspheric, double aspheric and free-form single vision lenses.
To be effective for the variety of patients that come through an optical office, a double aspheric lenses prescription range must be very wide. The SEIKO MV Double Aspheric lens range in 1.67 is available in -10.00D to +6.00D sphere to a -5.00D cylinder. From the analysis done, cyls over 4.00D accounted for 2 percent of the Rx’s processed.
Ideal for higher Rx’s and high cylinders is the 1.74 index SEIKO Super MV AR, which further improves the cosmetics of higher powered prescriptions. In a range of +10.00D to -20.00D to a -5.00D cylinder (total power -20.00D), these lenses also deliver double asphericity.
While double aspherics can be lab ordered at any time, finished lenses provide fast turnaround. For in-office finishing practices, a stock lens series in 1.6 high index, double aspheric (SEIKO DA) is available for the myope with prescriptions from plano to -10.00D, cylinders to -2.00D.
Of course, aside from the larger field of view aspheric and double aspheric lenses look better too. Fig 8 illustrates the improved thinness of lenses as the front base curve design is flattened (plus lenses) or steepened in the periphery (minus lenses) and index is increased. More than a 20 percent reduction in edge thinness is possible.
Double-aspheric free-form design lenses are thinner & lighter with flatter curves on both sides. The back surface free-form compensation virtually eliminates power error, distortion and marginal astigmatism. In addition magnification is reduced for hyperopes, making more of the field available within the opening of eyeglass eyewire.
This significantly improves peripheral vision clarity and size of field, making them ideal for difficult Rx’s, especially high cylinder. In Fig 9. I edged two +5.25 – 1.75 x 170 lenses, one in hard resin from the stock lens drawer (70mm diameter), the other surfaced to the PD, frame size and shape. Of course this isn’t an apples and apples comparison but does show the extreme benefit of doing this prescription using the way that this lends applied double asphericity to the visible result.
In addition, all 1.67 and 1.74 lenses include a super hydro- and oleophobic AR for easy cleaning, further enhancing the lenses overall clarity.
Free-form and double aspheric lenses are best fit considering lens tilt. As a result, request spectacle orders with the optical center or point or prescribed prism placed 4mm below pupil center. This assumes that the frame has about 8-9 degrees pantoscopic tilt. Some teach fitting aspheric lenses with the optical center at pupil center. This is incorrect since lens performance is reduced when not considering tilt. For even more precision in peripheral lens design using free-form, specify the lens tilt measurement when ordering.
In addition, unique to SEIKO free-form lenses is a patented spherical, 10mm ‘fitting’ button. This ensures a small area of uniform power for verification using all aperture lensmeters and ease of fitting.
A double aspheric low base curve design compensates for distortion, peripheral power error and astigmatic aberration in a balanced manner. Using free-form technology, the result is exceptional clarity of vision throughout the entire lens, a significant improvement in the effective viewing area, and better cosmetics, especially in prescriptions with high powers and/or high cylinder. A free-form back surface provides precise secondary aspheric/atoric compensation for each individual prescription, for exceptionally wide peripheral vision with stable, comfortable vision throughout the lens.