# Optical Forums > Ophthalmic Optics >  Blended Executive and the Optical Constraints of Progressives

## Darryl Meister

*A "Blended" Executive Bifocal and the Optical Constraints of Progressive Lenses*

By Darryl J. Meister, ABOM
Copyright 2005

Periodically, while giving presentations on progressive lenses, I am asked several questions that are all similar in nature and speak to the optical and physical requirements of progressive lens designs. Does a progressive lens really need to have unwanted astigmatism? Can you make a progressive lens with no corridor length? Is it possible to make something like a "blended" Executive bifocal? Why is prism-thinning used? And so on.

I am providing, below, a short discussion on my method for constructing a _blended Executive-style bifocal_, which I will use by analogy to illustrate some of the inherent optical and mechanical limitations of conventional progressive addition lenses. Hopefully, this will provide those interested with a solid intuition when it comes to the nature of progressive lenses.

Most opticians are (painfully) aware of the geometrical aspects of Executive- and Franklin-style bifocals. A flatter distance curve meets a steeper near curve, resulting in a prominent ledge-like junction on the surface. At the center of the lens, the two front surfaces meet (or at least nearly so) at a single point, and are only contiguous (unbroken) at that point. (In reality, Executive-style bifocals actually have a tiny "lip" at this point, but imagine the lens without one.) Away from this point, the surfaces gradually break farther and farther apart as the near curve steepens more quickly than the distance curve, resulting in the infamous edge profile of these bifocals.



So, how could we go about "blending" these two curves together in order to produce a smooth, continuous surface? How could we "fill in" the region beneath the ledge that exists between the flatter distance curve and the steeper near curve? For simplicity, let us visualize an Executive-style bifocal with a Plano (flat) back curve and a Plano front curve in the distance (or _major portion_). In this case, the near (or _segment_) of the lens will have a front surface roughly equal to its Add Power, while the distance zone will be perfectly flat.

We will now remove a 90-degree wedge from the side of this lens. One edge of the wedge will be 45 degrees above the bifocal ledge, while the other edge will be 45 degrees below it. The cross-section of the lens formed by this missing wedge is now similar to a _plano plus-cylinder_ that has been cut in half. Also note that if we do this to both sides, the distance (or major) portion and the bifocal segment will both be 90-degree wedges (360 = 90 + 90 + 90 + 90).



Now visualize a _plano plus-cylinder_ lens equal in power to the bifocal segment (or Add power). Such a plus-cylinder lens will be flat (Plano) and produce no power along its _axis_ meridian, while it will produce its maximum plus power through its _power_ meridian. We will take this plano plus-cylinder, cut it in half along its axis (flat) meridian, and then insert it into the space left by our missing wedge. It should be a perfect fit. Essentially, we are showing that it is possible to "blend" the flatter distance portion to the steeper bifocal portion with the use of cylinder power (at an oblique axis).



As previously noted, the cylinder power of this plano plus-cylinder will be equal to the power of the bifocal segment, since the power curve of this cylinder is in fact an extension of the curve of the segment. Moreover, it should be apparent that this cylinder has no power along the axis meridian (that is, it's a _plano_ cylinder) because this meridian is an extension of the Plano distance curve. Further, this cylinder will be oriented at axis 45 (the angle the wedge makes in the distance).

This cylinder has seamlessly and smoothly joined the distance curve to the near curve. We now have a "blended" Executive-style bifocal lens, with 90-degree wedges of distance and Add power. Of course, we will also want to trim away the excess lens material from the portion of the plano-cylinder that is protruding from the side of the lens blank. Once we have removed this excess material, also note the thickness profile of the lens blank.



Now, what are some of the pertinent features of this "blended" Executive-style bifocal? For one, it has a smooth surface. Secondly, it has large "wings" of unwanted cylinder power in the periphery, roughly comparable in magnitude to the Add power of the bifocal segment. This cylinder power is also obliquely oriented (i.e., at axis 45 on the right side and at axis 135 on the left). Lastly, the top of the lens blank is thicker than the bottom, so _prism-thinning_ would balance this thickness difference. These are all also features of _progressive addition lenses_.

Is this lens design as good as a progressive addition lens? No. First and foremost, progressive lenses distribute the change in Add power over a larger region of the lens, which results in smoother and better-behaved cylinder power. This means less blur, distortion, and image swim for the wearer, as well as a larger "effective" field of vision in the distance. The blended Executive goes directly from _no_ cylinder power to a very high level of constant cylinder power at a particularly disturbing axis. The unwanted cylinder power in the "blending" regions of progressive lenses is also more carefully controlled than the periphery of our simple blended Executive.

Progressive lenses also offer good intermediate utility. The blended Executive has no intermediate (it has no _corridor_), and instead produces an abrupt change in vision between the distance and bifocal regions. Progressive lenses also have surfaces that produce smooth changes in power and magnification (mathematically speaking, the surfaces have continuous second derivatives). The blended Executive has a smooth surface, but produces abrupt changes in power and magnification between its "wedges."

With the right mathematical considerations, we could refine our blended Executive surface in order to smooth the transitions in power, add some intermediate functionality, achieve the right balance between the "wedges," reduce the levels of unwanted cylinder distortion, and improve its overall optical performance. Of course, we would then have a progressive addition lens.



Best regards,
Darryl

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## drk

Mind blowing!

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## JRS

Very well explained Darryl (as expected from you). The graphics certainly enhance the detail.

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## Darryl Meister

I hope you guys found it an interesting read...

Best regards,
Darryl

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## OPTIDONN

Thanks Darryl!:cheers:

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## edKENdance

Thanks!  I need more coffee though.

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## vikramg

Dear Darryl ,

Beautifully explained .

About the Progressive constraints part :

Is it possible to have the "plano cylinder wedge" 

that is much smaller ? 

Say 10-20 degrees instead of 90 ?

Will that give a blended look ?

That would give a lot more usefull area on the lens .

Thanks once again .

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## Darryl Meister

> Is it possible to have the "plano cylinder wedge" that is much smaller ?


While the surface would have to be slightly more complex than the lens I've described here, it would certainly be possible. The magnitude of the cylinder will be inversely proportional to the size of the wedge (or at least nearly so). For instance, our 90-degree wedge is equal in power to the Add (2.00). A 45-degree wedge, for instance, would have twice as much cylinder power as the Add (2 x 2.00 = 4.00), though this could be improved some with the use of more complex surfaces.

However, as a general rule for progressive lenses, as the size of the "wings" of cylinder power _decreases_ in order to produce larger viewing zones, the magnitude of the unwanted cylinder power will _increase_. This is exactly why _hard_ and _soft_ designs look the way they do.



Best regards,
Darryl

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## HarryChiling

Your graphics are very impressive Daryl.  Great explanation of the design.

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## vikramg

Dear Darryl ,

Are soft designs the defacto philosophy for progressive designs of the future ?

Or are their people who still say " I dont care for High Astigmatism on the

edges , Just give me the maximum usable areas " ?

Regards

Vikram

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## Darryl Meister

Many designs nowadays fall somewhere in between; they are not "super soft" or "super hard." From the AO*SOLA camp, I can tell you that AO progressives are generally on the softer side, since their lens designer feels that this is the best approach for comfort and adaptation (though their newer designs aren't as soft as something like the Omni). SOLA progressives, on the other hand, are now designed with softer boundaries in the distance zone and harder boundaries in the near zone, since SOLA's clinical trials have demonstrated that this seems to represent the best overall compromise.

Best regards,
Darryl

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## culland

Darryl,

Amazing explanation!  The graphics help greatly to explain everything.
:)

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## drk

Darryl,

What I can't get is why this theoretical lens-blending exercise so accurately reflects/illuminates progressive design.  I think the underlying answer is that we are dealing, ultimately, with geometry more than anything else.  That may be a big "well, duh" to you, but that's what I know I get from your example.  Cool stuff.

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## Darryl Meister

> What I can't get is why this theoretical lens-blending exercise so accurately reflects/illuminates progressive design


Keep in mind that, ultimately, a progressive lens is fundamentally a lens design that uses surface astigmatism to blend a flatter distance curve into a steeper near curve. The blended Executive-style bifocal example I've described above just demonstrates how and why this possible.

Best regards,
Darryl

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## AWTECH

This illustration was very helpful.  There are some advantages to backside bifocal using freeform technology and the blend line can be very small.  I have not seen a design review of the Essilor product they promote as a liberty which I assume is a very very small mid power area that is almost useless for vision.

Any thoughts on the backside bifocal?

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## Darryl Meister

> I have not seen a design review of the Essilor product they promote as a liberty which I assume is a very very small mid power area that is almost useless for vision.


Varilux Liberty is similar to other conventional progressive lenses, and Essilor does not advertise a "short corridor" for this lens. It actually has a rather generous intermediate zone. I'll leave it to Pete, our resident Essilor guy, to describe the lens in more detail.




> Any thoughts on the backside bifocal?


Are you referring to a back-side progressive or just bifocals in general? With progressives, you can probably expect a small increase in the field of view and a small decrease in magnification differences that occur across the front surface. Slightly more depression would be required to reach the full Add power, however.

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## AWTECH

I am talking about a true bifocal power on the back side, either with a round, oval or flat top area.  Naturally the bottom is cut out at edging.

This is for people who just need to be able to read and have a great distance lens.  No problem finding the zone to transition from far to near.

Looks are great with this also, with a great AR you can not see the change in zone.

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## Darryl Meister

I don't know that you could make a flat-top bifocal using a free-form process, or really any well-defined segment since these machines work best at cutting surfaces without abrupt changes in curvature, but you could certainly make a blended bifocal. Though I don't know whether this would be any better or more economical than ordering a semi-finished blended bifocal.

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## AWTECH

We have actually been working on this technology and it seems to produce a nice product.  We are now working on the close to flat top section with as little of a line which I estimate will be 1mm to 2.5mm in total.

The great advantage over front side is no visible hard line.

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## Darryl Meister

> We are now working on the close to flat top section with as little of a line which I estimate will be 1mm to 2.5mm in total... The great advantage over front side is no visible hard line


This isn't really a benefit of a back-side product though; it's a mechanical limitation of the free-form process, and you could accomplish the same thing on the front. The "no visibile hard line" is a consequence of the fact that the machine can't actually make a hard line. Also keep in mind that the region around the segment of these blended bifocals has unwanted cylinder power (just as a semi-finished blended bifocal does).

When you say "we are now working," do you mean that you have some control over the optimization process or geometrical configuration of the product coming off the machine?

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## AWTECH

Yes we have complete control and that is where the choices become almost endless.  Obviously much of what we are working on is confidential however I know from the work I have seen that you have produced that you have a great deal of knowledge and open mind regarding where the industry has been and where it can go.

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## Darryl Meister

Certainly, the full potential of free-form manufacturing can only truly be realized with the right tools driving it. And, of course, it's important to have a well engineered process that accurately and consistently produces the results you want.

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