1. ## Base Curves

Hi This is the first time I've posted , but I've been an avid reader
of Optiboard for awhile. That said, I have a question. It's almost too basic and I'm embarassed. I recently went to a continuing ed program in my home state (NY) and during a lecture the speaker asked where we would take a base curve measurement, front or back surface of the lense. No one answered and she went on to say great! of course we all knew it should be the back. Since you now know that I've read this web site for months and never posted, you know I didn't dare ask why all these years I thought base curves were taken from the front. Can someone please explain. I'v done a little research on my own. My notes from school say front , and do several published books. What I'm I missing?

2. Reading base curves from the back side would be news to me... unless the lens was ground using plus cylinder instead of minus. Maybe y'all do things different up there in New York :bbg:
(just kidding).

Blake

3. Well, Blake I'm willing to admit that maybe I was starting to zone a little... I did travel two hours to get to the conference. I was sure she said back side. It's stuck with me so much I did an informal survey of opticians to see which side they read- all front side but I guess I need some input from the board.

4. Hi Kahlua,

You were right to be confused. With very few exceptions, including the rare one mentioned by Blake, the base curve is on the front surface of the lens. You can remember it like this... The base curve of the lens is the curve that becomes the basis from which the remaining curves will be calculated. Remember that lens manufacturers generally supply semi-finished lens blanks with optically-finished front curves, which may also have a multifocal, aspheric, or progressive lens design on them. The surfacing lab will then grind the remaining curves appropriate for a given Rx onto the back of the lens blank. Consequently, the front curve has to be the base curve in these situations.

Having said that, the back surface curve containing the sphere power of the Rx (in minus cylinder form) is sometimes referred to as the "back base curve" during the surfacing process. However, when someone simply says, "base curve," they are referring to front surface (except in very rare circumstances, or when the person doesn't understand the difference).

Best regards,
Darryl

5. kahlua:
Since you now know that I've read this web site for months and never posted, you know I didn't dare ask why all these years I thought base curves were taken from the front.
Actually, I asked that same question of Bob Rihl ahwile ago and was given a bit of history lesson. Back in the land before CR-39 all lenses were glass. The base curve (occular curve) was actually on the back and the front curve was ground to the Rx. Today, it is rare but you are left with the same issue that you run across with certain MD Rx's that are written with plus cylinder. The MD requests that you match a Base Curve. You think front curve, the doc means back curve.

Why clocking a lens this way ended up in your class, in the present day - that one I cannot help you with. Maybe Bob Rihl will run across this post and fill us in.

6. Thanks for your posts. School was actually 10 years ago for me and I do remember a lecture on surfacing lenses and the optics of glass lenses. I'm still convinced that this speaker said back surface for base curves. It really woke me up. You know how some continuing eds can be . I've thought of all possibilities.
Maybe she just wanted to see who was awake (none of us)or to maybe we all failed!! ... It does bother me though because I do remember a post from Bob Rhil about base curves and back surface. Was he talking about glass? I work for MD's and I"m not sure they know about base curves at all , front or back (Just kidding in case they're a subscriber)By the way Kahlua is my dog's name, not my drink!!

7. ## Kahlua: The Dog Not You ...

You better click on the following link and go to the dogs, cats, birds, rats......... thread in Just Conversation and tell us what kind of dog Kahlua is and anything neat about him.

:D

8. Originally posted by Jo
The base curve (occular curve) was actually on the back and the front curve was ground to the Rx. Today, it is rare but you are left with the same issue that you run across with certain MD Rx's that are written with plus cylinder. The MD requests that you match a Base Curve. You think front curve, the doc means back curve.
Hi Jo,

I would certainly agree that many years ago (and I emphasize many -- probably close to 30) single vision and Ultex-style glass lenses were made in plus cylinder form. (Fused glass bifocals were made in minus-cylinder form.) I believe that the front surface would still have had a "toric base curve," which would have been the flatter curve. However, I would question how many ophthalmologists actually think of the back curve as the "base curve" when specifying, "Match the same base curve." I'd like to think that most understand the difference between the cylinder form of the prescription and the cylinder form the lens is made in. If they haven't figured that out by now, I doubt that they would understand the use of "base curve" to describe the flatter curve of a toric surface in the first place. ;) Besides, MDs and ODs generally write "Match the same base curve" to reduce magnification differences and adaptation problems. Magnification is affected by the front curve.

Best regards,
Darryl

9. Hi Darryl:

Fill in one last thing for me. Is it only the front surface that magnifies on a bi-convex lens?

10. Opticaly the base curave is the posterior curve. Mechanicly (lab rats only) the base curve is the anterior curve.

Front is used almost universally now as most bifocals are on anterior and all lab grinding is done on the back.

Chip

Chip (Old enough to remember when + cylinder was on the front and minus cylinder was on the back) Anderson

11. Hi Jo,

There are two components to magnification: magnification by power and magnification by shape. Spectacle magnification is the product of these two individual components. Magnification by power is the component contributed by virtue of lens power alone (sort of like assuming the lens is really thin), including the vertex distance. Magnification by shape is the component contributed by the form and thickness of the lens. This includes the front (base) curve, center thickness, and refractive index. Essentially, changing the magnification by shape affects the "equivalent power" of the lens, which is ultimately responsible for magnification.

Consider a typical meniscus hard resin lens, for example, with a +8.00 front curve, 6 mm center thickness, and a power of +3.00 D -- fit at a typical 13 mm vertex distance. This lens will produce a spectacle magnification of about 7.5%. Now, if we flip that very same lens around, so that the rear concave curve is now the front curve, the lens will produce a magnification of only 1.9% -- since the concave front curve produces less shape magnification than the convex front curve.

Hi Chip,

You sure you're not talking about contact lenses with that "base curve is the posterior curve" stuff?!? ;)

Here is the modern definition, which is applicable to most lenses made since the Nixon administration: ;)

Front curve = Base Curve
Weakest back curve, producing sphere power = Back Base Curve
Strongest back curve, producing cylinder power = Cross Curve

In the absence of cylinder power, the Back Base Curve may also be called the Sphere Curve.

Best regards,
Darryl

12. I'm curious about what the subject matter was that was being discussed when this was presented. I've been know to discuss base curves and ocular curves during a course to get audience participation. It's interesting to see what people come up with as answers.

Diane

13. Hi to all,

I myself at this particular moment have forgotten about base curves.

Is it always in back surface of meniscus lens??
Front surface always being sphere curve and back surface always base curve and cross curve with toric meniscus lenses.

sphere curve/base curveXcross curve= +6.00/-8.00x90-10.00x180
= 2.00DS/-2.00DCx180.

Funny how soon I forget about base curve application
:hammer:
Thanks,
Sara

14. I'm getting in rather late in this discussion, but perhaps the speaker was trying to get everyone to understand that taking the old base (front) from a patients lenses was not a good idea - all the time.
With aspheric lenses on the up-swing, telling the lab to "match base curve" when the patient is going from simple spherical SV to an aspheric lens (SV or not) is not always in the patients best interest. What was on a 6.25 base may now be using a 4.00 base in the new lens (by default). Manufacturers go to a lot of trouble sending a 'recommended base curve chart' data.

All of this is pure speculation seeing as how only 1 person was actually there.... but I thought I'd throw it out in the mix as another possible reason for the mention.

15. Thanks Darryl !

16. Curious about the subject of your class. In many surfacing laboratories, the front curve is called the front curve, and the back curve is comprised of the base and cross curves. Base and cross are used to set the generator and pick the tool.

17. Originally posted by Cj Eggbeer
In many surfacing laboratories, the front curve is called the front curve, and the back curve is comprised of the base and cross curves. Base and cross are used to set the generator and pick the tool.
Hi CJ,

They might also call an "eighth" diopter (0.12) a "twelfth" in many labs, too. ;) Rest assured, when a laboratory calls up a lens manufacturer, they order a base curve -- not a front curve. If someone called up a lens manufacturer asking for a "6 front" instead of a "6 base," they'd probably just confuse the customer service person! However, to your point, the back base curve and cross curve are in fact used to set the generator. You just might not see the "back" labeled on anything in the interest of space.

Best regards,
Darryl

18. Darryl,

Yes, I agree with you. I was just wondering if the lecture kahlua attended was related in some way to backside surfacing. If so, it might explain the speaker's comment. I didn't see the lecture topic anywhere in this thread. Do you recall, kahlua?

Cj

19. I'm sure different labs do it different ways, but when we speak of base curves, we mean the front curve of the lens. The terms "base" and "cross" refer to lap tools. But we do suffer the curse of the "twelfths". You should see the look on my coworkers' faces when I talk about "an eighth of a diopter".

Blake

20. Hi Cj,

The surfacing discussion could be a possibility... Though the speaker was referring to a base curve measurment. If I had to wager on it, I would say that the only surface curve an optician really has any need (however remote) to measure on a regular basis is the front base curve. There are very few occasions that call for a measurement of the back curves. I would guess that the speaker either A) inadvertently miscommunicated exactly what she was trying to say, B) doesn't know enough about base curves, C) misunderstood the incorrect response from the audience, or D) knows about base curves and understood the incorrect answer, but didn't want to embarrass anyone by correcting them with the right answer so she played along with the incorrect one. Or, alternatively, Kahlua was really tired and didn't hear her correctly. ;) In any event, we should probably give the speaker the benefit of doubt.

Best regards,
Darryl

21. Hi Darryl,

Thanks for acknowledging that my scenario is a possibility. I brought it up not because I am convinced it is the absolute explanation, but precisely because I was giving the speaker the benefit of the doubt.

I have attended a few ce lectures on backside surfacing and the "base" curve nomenclature inevitably does cause confusion. I've seen a couple of these lectures disintegrate into a 15 or 20 minute argument over exactly this term. To their credit, educators in this field generally do use the term "back base curve"
in their presentations, but the expanded name is still shortened in the lab and on the input screens of the generators.

Back base curve sag measurement is a part of daily maintenance in many labs, and is performed to check the accuracy of the generator curves.

Best Regards,
Cj

Talking about meniscus lens, front surface is always sphere curve,back surface is base curve.Front surface is always + surface and back surface is - surface,correct? I am talking about meniscus lenses.
Toric meniscus lens front surface is always +surface=always spherical surface.We don't work cyls on front surface right?
Back surface is base curve & cross curve again in - form.That is why we indicate -cyl in hyperopic prescriptions.
I lazy going to my first year notes,answer here will be easy for me;)
Thanks,
Sara

23. Hi Sara,

Here are my definitions, from the earlier posts in this thread; these are applicable to the vast majority of modern spectacle lenses:

Front curve = Base Curve
Weakest back curve, producing sphere power = Back Base Curve
Strongest back curve, producing cylinder power = Cross Curve

In the absence of cylinder power, the Back Base Curve may also be called the Sphere Curve.

And, in those very rare occasions when the cylinder power is on the front,

Strongest front curve = Cross Curve
Weakest front curve = Toric Base Curve

I describe the reasons for this more thoroughly in the earlier posts, so be sure to review the rest of this thread.

Best regards,
Darryl

24. Hi Darryl,
Thank you for your explaination,now I understand that there is little difference between American and British optics text, which I learn.
I understand very well now.Thanks again,
Sara

25. ## Base curves and balancing

No I did not have the base curve of a contact lens in mind. I ment that the curve that influences what the patient sees (assuming that power is accomplishe) is the Posterior Curve(s) . If one wants to accomplish image balance with dissimilar powers, one should attempt to accomplish it with curves that are most similar on the posterior surface!

The front curve is just along for the ride to give differences in thickness (which what actually slows the the light down to begin with).

Po ole Uneducated Chip

I might mention that despite some earlier controversy in another thread, the posterior curve of a contact lens is there for the fit, the anterior curve is the one for power.:idea:

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