Base Curve Help

[silver_chick_83]

Hi All,

It's been a while since I've been on, so I'm not sure if this has been discussed yet.

The Dr. that I work for wants to know what really determines the front base curves of lenses these days. About a year ago we switched to Hoya from Essilor, and lately we have been having a lot of issues with our single vision lenses. So the doctor is trying to figure out why the horizontal and vertical base curves are different on Single Vision Lenses.

Case in point:
Rx in 2011 (Works great) (Done at a Essilor lab)
OD -4.75 -2.25 x 018
OS -5.00 -2.00 x 155
Base curves Horizontal (OU) 2
Base Curves Vertical (OU) plano
Lenses are in Hi-Index 1.67 w/AR

New Rx 2013 (Seeing through a small spot only) (Done at a Hoya Lab)
OD -5.00 -2.50 x 018 Horizontal BC 4 Vertical BC 2
OS -5.00 -2.25 x 155 Horizontal BC 5 Vertical BC 2
Lenses are Hi-index 1.67 with AR again.

Called a different lab then the one that made the new pair, and the said that a typical BC for that Rx would be a 4. (I didn't ask if that would be both horizontal and vertical).

I've personally never really paid attention to vertical base curves before, but the doctor is curious.

Neither pair were ordered as Aspheric, or BC specified, or Digitally surfaced, or Free form.


Please Help Me. Thanks.

[sharpstick777]

SV? Or progressive? dot the OC, then I would take the lenses out and remeasure BC. It sounds like warpage to me if its vertically different than horizonatal BC.


I would have ordered this RX Aspheric, generally it will improve peripheral vision over a -2.50 total power, and you are way over that. But then again, I think most 1.67 Non-Aspheric is gone from the US market now, its possible his old pair is non-Aspheric though.. To verify if its aspheric, dot OC, then measure BC at OC, then to each side across the same meridian. if it varies its probably aspheric.

Did you use mono PD's and OCs? is the Vertex, panto and tilt the same between pairs?

[silver_chick_83]

These are both SV Lenses. PD's were mono both times. No OC was used either time. (I'm new to the office and trying to get people to use OC's more. But in WY they don't like change, and most of the people that work here are CPO's not ABO's I'm the only ABO here.)

These two pairs were different frames. And they could have used Aspheric lens, but we didn't specifically ask for one way or the other. I don't personally have her old glasses, and didn't have the chance to look at it myself.

But this isn't the first pair that we've had with different vertical BC from Horizontal BC. And I'm just trying to figure out why they are different.

Thanks For the help.

[sharpstick777]

Any variation like that in Base Curve, at OC in SV would mostly likely be due to warpage.
If you don't have the old pair your limited in what you can do. Also, your lab may have ground this RX instead of using stock lenses.

Good stock lenses will offer closer to a true form (but adjusted) base curve over a ground lens. With that high of Cyl your patient will definitely benefit from SV Free-form. So if your patient is willing to upgrade, go Free-form. If not, reorder in stock Aspheric SV (I like Seiko for peripheral clarity, its the only stock lens with 2 types of aspheric curves in one lens, button and full field). using a specified OC.

[optical24/7]

I'm not aware of any essilor or Hoya SV designs that use toric front curves.

[silver_chick_83]

Thanks. I Think that will help.

[Robert Martellaro]

The Dr. that I work for wants to know what really determines the front base curves of lenses these days.

"Corrected curve" or "best form" theory.

Neither pair were ordered as Aspheric, or BC specified, or Digitally surfaced, or Free form.

Both pairs are probably aspheric. The asphericity does not correct for any inherent errors, instead it corrects for the use of a too flat base curve, which at this power would normally be about +5.00 for a 1.67 refractive index.

[mike.elmes]

"Corrected curve" or "best form" theory.

Both pairs are probably aspheric. The asphericity does not correct for any inherent errors, instead it corrects for the use of a too flat base curve, which at this power would normally be about +5.00 for a 1.67 refractive index.

Robert... a +5 BC for a -5.00-2.00???? Could you give me your reasoning. Most 167 stock lenses(-5.00-2.00) are aspheric with a +2BC certainly not 5 base.

[Robert Martellaro]

Robert... a +5 BC for a -5.00-2.00???? Could you give me your reasoning. Most 167 stock lenses(-5.00-2.00) are aspheric with a +2BC certainly not 5 base.

Mike,

Best form for that power, 1.67 index, in spherical form, is about +5.00.

[Barry Santini]

Mike,

Best form for that power, 1.67 index, in spherical form, is about +5.00.

+1!

[MakeOptics]

Best thread on base curves:

http://www.optiboard.com/forums/show...ght=tscherning

Or download spectacle optics from the file directory. I think you may have warped lenses or measuring an aspheric lens with a spherical clock. Look to see if the lens has laser or watermarks, you'll find higher end lenses will often have markings even in SV form that may give you a place to start your search.

[MakeOptics]

Augen Optics has an HD SV Double Aspheric that comes in a SF if you see lines along the 180 that may be your culprit.

[Robert Martellaro]

Best thread on base curves:

http://www.optiboard.com/forums/show...ght=tscherning

Or download spectacle optics from the file directory. I think you may have warped lenses or measuring an aspheric lens with a spherical clock. Look to see if the lens has laser or watermarks, you'll find higher end lenses will often have markings even in SV form that may give you a place to start your search.

I don't believe the asphericity will show up with a lens clock- I have a mounted pair of +12 D Welsh four drops that only show about .5 D change from edge to edge. The OP's lenses are warped, or the clock pins are not perpendicular to the surface.

Augen Optics has an HD SV Double Aspheric that comes in a SF if you see lines along the 180 that may be your culprit.

I'm pretty sure that design only has about .5 D of non-rotationally symmetric cylinder on the front, and doesn't come in 1.67. Really nice guess though!

[Darryl Meister]

(I didn't ask if that would be both horizontal and vertical).

Assuming that you are measuring the front curve properly, 2.00 diopters of difference in horizontal and vertical curvature on the front surface in two different sets of lenses might suggest that the frame is causing the lenses to warp, particularly if the curvature of the eyewire does not match the curvature of the bevel profile on the edged lens.

This type of warpage would occur on both the front and back surfaces of each lens, so the net optical effect would be minimal. Initially, the wearer may observe a very slight difference in vertical versus horizontal magnification, sort of like playing with the "stretch" mode on a widescreen television, but wearers often adapt to these effects very quickly.

a +5 BC for a -5.00-2.00???? Could you give me your reasoning. Most 167 stock lenses(-5.00-2.00) are aspheric with a +2BC certainly not 5 base

As Robert pointed out, the "best form" base curve for this power would actually be closer to 5.00 diopters in 1.67 high-index when measured using a lens clock, assuming a spherical front curve. But this is a theoretical base curve, not necessarily the value that any particular laboratory would use for this prescription.

Lens manufacturers over the years have come to reach a compromise between purely optical base curve selection and cosmetic base curve selection, systematically reducing front curves by 1.00 to 2.00 diopters. Further, many finished and semi-finished 1.67 high-index lenses are actually aspheric, which are generally much flatter.

I like Seiko for peripheral clarity, its the only stock lens with 2 types of aspheric curves in one lens, button and full field

I think you will find that most single vision aspheric lenses within the range of Rx powers that we are normally deal with are practically indistinguishable from a sphere in the central region of the lens. Also, if you maintain a large region of "spherical-ness," you will need to compromise optical performance in the vicinity of the boundary of that region.

With that high of Cyl your patient will definitely benefit... If not, reorder in stock Aspheric SV

Or, better still, consider an atoric lens design for higher cylinder powers, if you do not go with a free-form customized single vision lens.

Or download spectacle optics from the file directory.

The Spectacle Optics program will allow you to calculate best form base curves from a Tscherning's ellipse and, perhaps more importantly, allow you to evaluate the actual optical performance of a single vision lens for any specified front curve. But you should be reasonably comfortable with basic optics before using the software.

It would probably also be worth mentioning that many of these principles are discussed in detail in the OptiCampus.com article on Ophthalmic Lens Design for those interested.

Best regards,
Darryl

[silver_chick_83]

Thank you so much Darryl. That answered my questions. I greatly appreciate the info. :)

[sharpstick777]

Robert... a +5 BC for a -5.00-2.00???? Could you give me your reasoning. Most 167 stock lenses(-5.00-2.00) are aspheric with a +2BC certainly not 5 base.

Most finished and ground lenses today do not conform to best form optics, esp in plus, because it makes the lens thicker and degrades (to some) the cosmetics.

[sharpstick777]

Atoric lenses are very rare now, I used to dispense the Vizio a ton, but like so many great Sola products it has disappeared.

Although the Hoya Nulux lens is Atoric in Europe and Asia, the version here in the States is not. No one at Hoya could explain why.

That only leaves the Resolution Poly that I know of in stock Atoric SV, but since their parent company went under, the future of the product is uncertain.

Is there an Atoric product that I am missing?

Or, better still, consider an atoric lens design for higher cylinder powers, if you do not go with a free-form customized single vision lens.

[Randle Tibbs, ABOM]

If I'm not mistaken since this will be surfaced lens, aren't some of the FF single vision lenses atoric.

[Speed]

Take them out of the frame before you put a clock on them. The stress from mounting is probably warping them. It is common in MOST jobs.

[Darryl Meister]

Atoric lenses are very rare now, I used to dispense the Vizio a ton, but like so many great Sola products it has disappeared.

I can't speak for the other lens manufacturers, but Carl Zeiss Vision offers a 1.74 high-index, double-sided aspheric (that is, atoric) finished lens in minus Rx powers. I think companies like Seiko, Nikon, and Optima also marketed "double-sided aspheric" lenses that were probably atoric in cylinder powers, although I have no idea whether they still offer them.

Best regards,
Darryl

[RT]

Although the Hoya Nulux lens is Atoric in Europe and Asia, the version here in the States is not. No one at Hoya could explain why.

NULUX is indeed aspheric everywhere in the world. There is a different product that is called "NULUX EP" in some world regions. NULUX EP is atoric. In the US, this product is called "ID SV", but is only available as Rx, not in stock form. There are many instances where similar lens products have different names in different regions (due to Marketing, trademark issues, local translations, etc.). In some instances, products are different in US and Canada. HOYA US personnel are not typically trained on Canadian, European or Asian versions of the product, nor are European personnel trained on US/Canadian versions.

[optical24/7]

Darryl, please correct me if I'm wrong. The definition of an atoric corrected lens is that a different amount of asphericity is used in the toric (cyl) meridian than in the base meridian. I know of many designs (and use them) that are atoric. PAL's and SV's alike.

[Darryl Meister]

The definition of an atoric corrected lens is that a different amount of asphericity is used in the toric (cyl) meridian than in the base meridian. I know of many designs (and use them) that are atoric. PAL's and SV's alike

Yes, that's pretty much a good working definition of "atoric." In progressive and non-stock single vision lenses, the application of atoricity typically requires free-form surfacing in conjunction with lens design software. For finished (stock) single vision lenses, the atoricity is typically molded on the back, although there are considerably more standard aspheric stock lenses out there than atoric or double-surface aspheric stock lenses...

Best regards,
Darryl

[optical24/7]

Thanks Darryl for the affirmation. I disagree with the statement that atoric lenses are rare now. Just the opposite. They are even adding it in traditional designs like FT's now with digital surfacing. Both Essilor and Hoya have these available right now (not sure of others). And there should be more expansion of the use of asphericity/atoricity in the future as programs are written for our traditional lenses designs/materials.

[Darryl Meister]

I disagree with the statement that atoric lenses are rare now.

I think that statement may have been made in the context of single vision finished (stock) lenses, only.

Best regards,
Darryl