Base Curves

[kahlua]

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?

[Blake]

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

[kahlua]

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.

[Darryl Meister]

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

[Jo]

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.

[kahlua]

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!!

[Jo]

You better click on the following link and go to the dogs, cats, birds, rats......... (http://machol.com/forums/showthread.php?s=&threadid=1670&perpage=25&pagenumber=1) thread in Just Conversation and tell us what kind of dog Kahlua is and anything neat about him.

:D

[Darryl Meister]

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

[Jo]

Hi Darryl:

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

[chip anderson]

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

[Darryl Meister]

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

[Diane]

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

[Sara]

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

[JRS]

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.

[Jo]

Thanks Darryl !

[Cj Eggbeer]

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.

[Darryl Meister]

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

[Cj Eggbeer]

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

[Blake]

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

[Darryl Meister]

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

[Cj Eggbeer]

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

[Sara]

Please correct my knowledge about base curves.
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

[Darryl Meister]

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

[Sara]

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

[chip anderson]

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:

[Jo]

Chip,

A Px has just enough of a slight Rx increase to bump them from a 6 to a 7 front curve (BC) on the charts. If you keep them on the 6, you will be changing the posterior curve and causing a change in reflections and how an image looks to the Px. If you change the front curve but match the posterior curve you minimize the amount of change that the Px will notice. Am I following you?

My next question for you guys ... I know that Base Curve charts at least take a combination of optics and cosmetics into consideration. What ever happened with trying to keep a 6 base back curve for optimal optics? Or was this just one person's opinion back then and not a rule of thumb?

[Jeff Trail]

I think it is a case of "apples and oranges"... In a lab setting, we use Base curve in refrence to calculating the amount of curvature needed to be ground to get a set power (not forgetting to take into account index of refraction) WHILE on the dispensers side of the coin the "base curve" they would tend to need to deal with is the ocular side (base/cross).. if the RX changes would it be time to "bump" the "front" curve (base to the lab rats) to make sure we have no fitting problems. (reflections,distortion,tunnel vision etc. etc.)
Same thing goes for the phrase 12th's and diopter.. it's a "buzz" word that, depending on which end of the optical spectrum you were working in, has numerous meanings :rolleyes:
The most important part is to make sure the "dispensing" end is on the same wave length as the "lab rats" then it should never be a problem. Chances are if you are using the same lab then they have gradually changed "base" curves over the RX changes from RX to RX...
One thing is certian, if you start delving into telling me (lab rats) about base curves you better have the charts handy since they change from material and design and manufacture..you would have to know how to compensate for index change, design change (spherical to aspherical) change of design depending on brand.... I guess I have it easier then most since the majority of my account have been dealing with me for years and I usually have changed it as I went along.. that and if the OD or MD says match curves I just clock the old lens and make the changes I need to match the "new" ocular curves... or if it's uncut then the opticians all have gotten into the habit of giving me the "old" RX as well as the new RX and the "old" base curve and let me do the math:rolleyes:
BUT I still think the person doing the speech was trying to use a term that is changed depending on which end of the optical line you are working from and didn't make it's use (term) clear in the context it was being used.

Jeff "sometimes it pays to be able to communicate" Trail

[Duane Metcalfe]

OK, lotsa years lost on my end, but somehow me thinks the term meniscus lens is being used incorrectly or used to illustrate a point. Can we all have the book definition of a meniscus lens, please.

CJ-Glad you joined the conversation. How ya' doin?

Duane

[chip anderson]

A miniscus lens is a lens having both a concave and a convex surface. As opposed to a plano/+or- surface. Or a bi-convex or bi-concave.

Now to the relationship of base to the eye. The idea behind the six base (in glass) was to have an extension of the orbit of the eye as measured from the piviot point in the center of the globe during rotation. Lots of complicated math by people smarter than I did this, but this was the bottom line. The idea behind the aspheric surfaces (I think) is to compensate for flatter bases not doing this an to compensate for inherent distortions as great angles are reached at the outer extremities of spherical surfaces, i.e. spherical aberrations.

Could be wrong, but it's my shot trying to recollect forgotten yore from many, many years ago.

Chip

P.S. Damn I miss spell check.

[Darryl Meister]

Hi Chip,

I have to disagree with you on a couple of points...

First of all, it is the front (anterior) curve that affects the image, not the posterior curve (though the two are obviously intimately related for a given power). Spectacle magnification is given by:

M = [1/(1-d*F)][1/(1-t/n*B)]

where B is the front (base) curve of the lens and F is the power. See my earlier post in this thread, which provides a practical example of this.

Secondly, the 6.00 base toric wasn't designed to follow the rotation of the eye. You would need something closer to a 18.00 D base for the lens to follow the spherical surface described by the eye at the vertex distance as the eye rotates. (By the way, this is similar in principle to SOLA's new Enigma eyewear). The 6.00 was empirically chosen because, like the 18.00 base, it eliminates a great deal of oblique power error. However, unlike the steeper base, it does so by producing a lens form whose back surface neutralizes the oblique astigmatism created by the front surface. Over the years, in the early 1900s, manufacturers began putting a great deal of effort into more exact lens design, which more completely eliminated these oblique aberrations. These became known as "corrected curve" (or best form) lenses. AO, for example, introduced the Tillyer series while B&L introduced the Orthogon series. Their standard, non-corrected toric lenses became known as "Centex" and "Balcor" lenses (if memory serves me correctly -- I may have spelled those wrong).

Best regards,
Darryl

[Darryl Meister]

And I'd also like to again assert that dispensers and refractionists have no need to measure the back surface (except in very rare circumstances). It's the front surface that affects magnification. It's the front surface that the lab will have to order. Even if matching the ocular curves served some useful purpose, which it generally doesn't (unless you're maybe worried about eyelash clearance or something), it would be almost impossible to do. Base curves are not made in small enough increments of power to allow one to match the back curves as the Rx changes. Manufacturer typically provide base curves in only 2.00 D steps. Which means that there would have to be a huge change in the Rx (if any) before you could select a new front curve that would match the previous back curves.

Best regards,
Darryl

[Pete Hanlin]

Just to put my two cents in, here's a quote from Ophthalmic Dispensing, 2nd edition, Borish & Brooks, pg. 404:
In constructing an ophthalmic lens, one of the lens curves of one surface becomes the basis from which the others are determined. This beginning curve, on which the net lens power is based, is called the base curve. In single vision prescription ophthalmic lenses, the base curve is always found on the front surface.
In the case of spherical lenses, the front sphere is the base curve.
If the lens is in plus cylinder form, there are two curves on the front. The base curve is the weaker, or flatter, of the two curves. The other curve becomes the cross curve. The back surface is quite naturally referred to as the sphere curve.

Although I can't find the reference, I believe selection of base curves by the manufacturer usually chooses to eliminate (or minimize) either oblique astigmatism, power error, or distortion. As I recall, a particular spherical curve can be found that will eliminate either of the first two mentioned- but not both simultaneously...

Pete

[Jeff Trail]

Pete,

Going along Darryl's thoughts some of the most informative things I have read was written and published by and from Tillyer,
But I may part ways, when it comes to curve selection with Darryl, not because it maybe the "best" selection but we are limited by the industry standards on what "types" of curves are available in designs and materials.
I might also tend to move away from the fact that back curves are not important in adaption, I have run into it a number of times where people pick up on the way the ocular reflections have "moved" because of the way the new curves interact with light. Maybe I do a lot of work for MD's and OD's who have people with a little more visual problems than an average RX (ret. Pig., WMD, DMD et cetera) and they do tend to pick up these changes more so then an average RX person.
One thing I have gotten accomplished is the people who tend to be sensitive the opticians and techs have really started to understand how important a good AR coat helps out
;)
I do think it's nice to see that people are actually delving into this here, better to pass this information back and forth and have people learning something then just using "cookie cutter" optics like I see done so often now a days where the object is "least amount of time most amount of money" is the theme of the day:o
Amazing how many people read these posts that do not contribute but DO gleam some useful information, it maybe that you might only use this stuff in a "blue moon" but still somewhere you get to use all this stuff, it sure makes an impression on the patient as well as the OD or MD they maybe working for.... To prove it, I have probably gotten 15 or so EM's over the years from people who "lurk" and seen a post and took that information and a chance came up where they used it and were treated with some respect from the OD or MD..I wish the "lurkers" would contribute more but am happy if they take some of the info we all bounce back and forth and use it. I know that the guys who work for me and a number of ones I have trained over the years love to take my posts and do some digging to see if they can correct me in my mistakes. I like that mainly because they are digging into the theorectical as well the physical side and they may find a portion of something I posted and found tons of other information that they didn't know and got that extra bit of education out of the "digging" they were doing to "correct" me...
Now If I could just get them to return half the books they borrowed I would be happier... so guys who has my B&L Coachmans mannual? all by books by Tillyer? My Anatomy and Physiology books? Last but not most borish (get it?) ..my big blue book? ;)
Here is to all the guys who are do the research and the ones who take our tirades and actually put it to use:cheers:

Jeff

[Darryl Meister]

Originally posted by Pete Hanlin
Just to put my two cents in, here's a quote from Ophthalmic Dispensing, 2nd edition, Borish & Brooks, pg. 404

I agree completely with that definition. That's why Cliff Brooks is "The Man."

Best regards,
Darryl

[kahlua]

Thanks for all the interesting replies! I've been out of town for a week and just caught up with the board. I wish I could report that the topic of the lecture was in line with some of your ideas, but the subject was ophthalmic dispensing tips- kind of an odds and ends course. I must of misunderstood the speaker. I do feel a lot better for having asked and gotten all of your input, I was a little worried that a major shift in optics theory had occurred and I was busy changing yet another pair of nosepads!! Thanks again:)

[Cj Eggbeer]

Hi Kahlua,

Thanks for coming back. This has been a good discussion, with lots of information being presented and an actual dialogue between laboratory and dispensing opticians.

I guess I have been one of Jeff's "Lurkers". In my case, I just haven't had the patience to deal with my slow internet connection. I just fired my dial-up ISP and installed DSL. With my new lickety-split connection, the internet is actually fun again.

Duane, I'm doing well, thanks. And yourself?

Cj

[chip anderson]

Now:

Is the patient with an Rx imballance more likely to experience visual disturbance if the front or or back curves of his lenses are more nearly in sync?

Second part of similar question:

If one tries to ballance thickness by using high index with lower index materials, will the patient have more distortion with the anterior or posterior curves more nearly in agreement?

Chip

[David Wilson]

Hi Chip
You've thrown in quite a few things for consideration here. First, it is my feeling that anisometropes are more concerned with differential prism (particularly when presbyopic and their prgressives or bifocals force them to read at a distance from the OC) and differential image sizes. Between 1 and 3% image size difference will cause discomfort for many, between 3 and 5% will disturb most. As Darryl pointed out in an earlier post, it is the front curve that affects spectacle magnification. SM is created by two factors; power (Lens power and BVD) and shape (front curve, thickness and index). It is possible to reduce the magnification differential while maintaining the same front curves. That is by selecting a flatter front curve, higher index, thinner lenses and smaller BVD, magnification will be reduced in both eyes and the differential will also be reduced. It is also, of course, possible to produce an isogonal pair but they won't look too flash.
But this is only part of the story. I assume that you were also concerned with aberrations. You mention distortion, but the most troublesome is oblique astigmatism (although varying degrees of distortion would also be worrying). It is oblique astigmatism (marginal astigmatism) that designers are mainly concerned with and are addressing with aspheric cuves which allow us to use flatter curves that Tscherning's ellipses would suggest. If you are using aspheric curves and the same front curve for your anisometrope then the asphercity can only be ideal for one of the lenses (given the obvious variation in the back curves). So, for off-axis viewing, our anisometrope will experience oblique astigmatism in one eye and nore, or little, in the other.
In short, anything you do for your anisometrope, short of surgery or contact lenses, will involve a trade-off of some kind. Anisometropes learn to use the central part of the lens, where differential prism is minimal, until forced to use the reading zone in presbyopic corrections. Fortunately, the central area will also exhibit less of the Seidel aberrations (and transverse chromatic aberration for that matter). Which brings me back to spectacle magnification, since it will be present for on-axis viewing: unlike differential prism and off-axis aberrations, they cannot avod it.
I'm not sure whether this helps or confuses, Chip. Incidentally, I seem to recall an anecdotal self study of Pete Hanlin where he tried varying indices in a number of spectacles.
Regards
David Wilson

[harry a saake]

:bbg: From one who was there in the labs in the sixties on up. First of all not all glass lenses were ground on the front, only the ones that Darryl stated. Actually we ground minus and plus cylinders, and in fact it was kind of a nusiance having to change the position of the generator when you had to go to plus. All exec,s or dualens as they were known at the time and kryptoks were ground on the back. While base curve was normally referred to as the front curve, when the back curve was toric in nature the term back base curve was used, but only to reference it from the cross curve, as no matter what the cross curve was, the back base curve was the same.

[harry a saake]

:eek: For those history buffs, i remember when i was with B&L, the meniscus lens was thought of so highly, that B&L had the formula for how they came up with the 6 base curve, locked up in the main vault at B&L headquarters in Rochester, N. Y.