Advances in Progressive Lens Design
Welcome to our continuing series of Credit Educations Courses for Opticians.
This course has been approved for one hour of credit by the American Board of Opticianry. No fee is required for ABO credit.
Learning Outcomes:This course is presented at an intermediate level and is designed to help eyecare professionals be more knowledgeable in the area of lens dispensing. At the conclusion of the article, participants should be able to:
1. Better understand the evolution of progressive lenses and the advantages internal progressive lens design.
2. Understand what kinds of measurements are required in order to provide patients with these newest designs.
3. Understand the importance of internal design and how it differs from other progressive lens designs.
4. Increase customer service by matching lens product to individual patient needs and prescriptions. Test procedures: Read the article and then click on the "Take The Test" button at the bottom of the page. This will open a new window with a test consisting of 15 questions. To receive ABO continuing education credit, respondents must correctly answer 12 of 15 test questions. Simply click on the best answer for each question and click the submit button at the end of the test. Your test answers will be automatically sent to Seiko Optical and we will send your CEC or notify you of test failure within 7 to 10 business days.
Note: Some states do not accept home study courses for continuing education credit. Check with the licensing board in your state to see if this course qualifies.
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Advances in Progressive Lens Design
Introduction
This course takes a look at a groundbreaking development in progressive lens technology, the internal progressive. Seiko is proud to have designed and patented the world's first internal progressive.
This continuing education course takes you through the history of progressive lenses, and then focuses on this new technology and how it differs from the conventional PAL designs available today. As you will see, the internal progressive is a big step forward in overcoming the limitations of progressive lenses widely available today.
This course will explain in detail the differences in "external" and "internal" progressives, and provide you with the information necessary to properly fit and dispense these premium lenses.
While premium designs also come with premium price tags, today's savvy patient can easily recognize the cost versus benefit of the internal progressive lens. As an eyecare professional, it is vital to keep abreast of the latest lens technologies available. This way, you can always be sure to offer your patients the best lenses available for their visual needs.
Progressive addition lenses (PALs) represent the most advanced technology used in ophthalmic lenses. The first PAL was patented in 1907, but it would take another 52 years before manufacturers were able to commercially produce them. In the years since their introduction in 1959, there has been a steady progression of design improvements.
Progressive Design History
The first progressives were symmetric in design, with the left design identical to the right. Labs merely twisted the lens right or left to create inset required for the reading portion. This worked reasonably well, but lens designers soon realized there were advantages in individualizing the design of right lenses from left lenses. All modern PALs are now considered to be asymmetric, in that right lenses differ from left lenses and progressive lens inventories require matched pairs. Check the contour plot for most PALs and you'll see that the blur area, composed of unwanted power changes, is greater on the nasal side of the lens and encroaches slightly higher into the distance nasally than on the temporal side.
Early progressive designs were also identical for each add power. As PALs grew more sophisticated, designers began to see advantages to changing the design slightly as add power changed. All progressives share a common trait in that the reading area becomes smaller as add powers increase. Changing design parameters according to the size of the add area, designers realized, could make it possible to maximize the efficiency of each lens, regardless of add power. Designs that do not change for different add powers are considered to be "mono" designs. Those that do are called "multi" designs.
A further refinement came as wearer tests revealed that PALs for hyperopes would be enhanced if their design differed from that used for myopes.
Frame Sizes
Meanwhile, a rather dramatic swing in frame fashions was taking place and created a new issue for progressive designs. Frame styles began shrinking in size, and dispensing progressive lenses in shallow modern frames became difficult, sometimes impossible. Until then, most progressive designs required a minimum fitting height of 22mm to 26mm, impossible to achieve in modern vertically shallow frames. This led to development of short channel designs.
With all the advancements, many eyecare professionals assumed that PALs had come relatively close to perfection. In fact, however, there are four basic limitations in conventional PALs created by the compromises inherent in current designs:
1. Differing magnification throughout the lens
2. Restricted visual field
3. Compromised optics
4. Off-center astigmatism and power errors that increase with refractive index
The following summary looks at the effect each problem has on the efficiency of PALs and how these difficulties can be minimized.
1. Differing magnification throughout the lens. This problem arises from the fact that a viewed image is magnified to various degrees throughout the progressive channel and reading portion of a progressive (see Fig. 2 below). This varying magnification is created by two factors. One comes from changing curves on the front lens surface and the second is due to changes in power throughout the channel and reading portions of the lens. This size distortion creates a skew distortion (see Fig. 4 below) which contributes to the swaying or swimming complaint sometimes experienced by progressive patients.
2. Restricted visual field. The progressive curves are always on the front side of a conventional PAL, positioned a considerable distance from the eye and restricting field of vision, particularly for intermediate or near areas. If the progressive channel could be positioned closer to the eye, side-to-side vision throughout the channel would expand and so would the reading area. With conventional PAL designs, the field of vision is somewhat restricted in all visual areas.
3. Compromised optics. Conventional PALs are produced in varying base curves with each base curve averaged for a wide range of prescriptions. This represents a compromise from the best possible optics. Ideally, the base curve selected would be matched to the exact curves used on the backside. This simply isn't possible in a conventional PAL. As a result, acuity is compromised to some extent in all visual areas, depending on how close the patient's Rx is to the correction the front base curve was averaged for.
4. Off-center astigmatism and power errors increase with refractive index. Unfortunately, the reading area in a progressive is effected the most regarding astigmatism and power errors simply because that portion of the lens is located so far from the optical center of the lens (often 20mm or more). Examples: In a conventional CR 39 PAL on a 6-base with +1.00D distance correction and a +2.50D add, off-center errors would induce 0.07D astigmatism and -0.17D power error in the near area. If we take the same lens with +3.00D distance and +2.50D add, the errors increase significantly to 0.59D astigmatism and a power error of +0.16D.
These off-center astigmatism and power errors increase as the refractive index of the lens increases. Because the add curves are on the front surface, making the lenses listed above in a 1.67 index material, increases errors because of the 0.17 increase in refractive index. The +1.00D lens with the +2.50D add produces astigmatism of .20D and power error of -0.08D in the add. The lens with +2.50D distance now shows .70D astigmatism and +0.21D power error. In each case, we have an inconsistent near correction. The relationship of front base curve to power curves on the backside of the lenses also becomes more critical as refractive index increases.
Overcoming these problems
The inherent design problems reviewed earlier arise from the fact that the curves producing changing power in a PAL are traditionally positioned on the front lens surface. If it were possible to position progressive curves on the posterior surface, extensive visual improvements would result. Fortunately, this has become possible through the recent development of a new type of progressive called "internal." The term internal was coined to indicate the progressive curves are positioned on the back surface of the lens. Conventional PALs have their progressive curves on the front or outside surface, so conventional progressives are considered to be "external" PALs.
Two companies are producing internal progressives. The Rodenstock ILT is being test marketed in the U.S. and Seiko Optical's Internal progressive is now in general distribution in the U.S. To explain this new category, the following information about internal PALs and their fitting requirements is based on the currently available Seiko patented, third generation 1.67 Super Proceed Internal progressive. Seiko included factory-applied coating on all internal PALs.
Internal PALs have a spherical front surface. The backside is a free-form surface with progressive curves on the bottom half merged into prescription-specific aspheric curves throughout the back surface. When the correction includes cylinder, the toric cross curves on the backside are each aspherized precisely to the front curve. This new internal design provides significant improvements throughout the lens.
Internal PALs
Let's re-examine the four inherent problems of external PALs and see how they are affected when the progressive curves are positioned on the backside, as in an internal progressive.
1. Differing magnification throughout the lens. Magnification differences between various areas of the lens are greatly reduced. This reduction occurs because variations in the front surface are now eliminated, as it becomes a spherical surface. Moving the progressive curves to the backside produces considerable reduction in size distortion. Another benefit is that moving the progressive surface closer to the eye also reduces the skew distortion common to external PALs (See Fig. 3). As a result, patients experience less swaying and swimming sensation.
2. Restricted visual field. Moving progressive curves to the backside produces a very significant expansion of the visual fields for all distances (see Fig. 1). Another factor affecting field of vision comes from the fact that back curves are atoric or aspheric (depending whether there is cylinder in the Rx). The internal progressive manufacturer has designed aspheric compensation that is Rx specific, meaning each curve has been aspherized to the patient's exact correction. Aspherizing curves creates a flatter lens positioned closer to the eyes, and this also expands the fields of vision. Expansion of the visual fields benefits all areas of view: distance, intermediate, and near.
3. Compromised optics. In this regard, an internal PAL produces the maximum efficiency because, unlike external PALs, no compromise is required in selecting base curves. Both surfaces of the lens have been optimized to the patient's specific correction, and the posterior surfaces are either aspheric or atoric (depending on whether there is cylinder in the correction), providing the most exacting acuity possible.
4. Off-center astigmatism and power errors that increase with refractive index. In this problem area, changing to an internal design produces total control of off-center astigmatism and power errors common to external PALs. In addition, any small amounts of off-center astigmatism and power errors that remain are not increased when higher index materials are used. This consistency, regardless of index, results because there is no lens material between the progressive curves and the wearer's eyes. The near correction experienced by wearers is consistent and identical to that prescribed by the refractionist. There is accurate power throughout an expanded reading area. Unwanted cylinder power to the sides of the intermediate area (progressive channel) is controlled and greatly reduced. In the distance portion, wearers experience a remarkably wide, distortion-free view. There has been marked improvement throughout the entire lens. To provide a clearer idea of what this means to wearers, in a prescription calling for plano distance with a +3.00 add, wearers will experience 30 percent less distortion coupled with a 20 percent wider visual field.
With the powerful computing capability now available from modern computers, designing a comprehensive internal progressive with an Rx-specific back surface map has become possible. This left two major hurdles. First, there was the difficulty of transferring computer designs to the back of the lens. Once that was accomplished, there was the complexity of manufacturing a progressive lens specific to each patient's prescription. Equipment to produce these sophisticated aspheric/ atoric surfaces only recently became available, and creating this complex surface with a mirror-like finish is beyond the capability of conventional laboratory equipment. The internal progressive manufacturer has developed such equipment and can fill individual prescriptions for their distributor labs.
Fitting Internal Progressives
Practitioners now have the ability to take a quantum leap forward in fitting progressive patients. Fortunately, fitting this complicated technology remains relatively simple. Like all PALs, PDs for internal PALs should be determined monocularly by using an electronic pupilometer. The fitting cross (also known as the eye point) is positioned exactly in the center of the pupil. The lab needs to know the frame PD, the vertical dimension of the frame ("B" measurement), and height of the fitting cross. When the correction is plus, Seiko Optical stresses that fitters must also provide the ES measurement, which is the distance from the fitting cross mark to the furthest point of the lens shape. This ensures a lens with the thinnest possible edges.
Depending on the brand used, internal PALs can be fit as low as 16mm, useful for today's shallow frame styles. Use the manufacturer's cut-out chart to ensure that the lens blank size will fit the frame selected by the patient.
Due to the aspheric compensation in the internal progressive, eye power perception may vary from the actual lensometer reading in the near portion of the lens. Distance power is verified the same as on an external progressive, and the add power can be verified by checking the engraved markings on the lens.
Lenses with this advanced internal progressive technology, combined with Rx-specific aspheric/atoric curves for all portions of the lens, are more expensive than conventional PALs. When the benefits of these lenses are properly explained to the patient, there should be a substantial number who will want the finest available progressive technology for their new glasses.
This concludes the course. Click the button below to take the test.