True curve - why is it different than nominal curves?

[snowmonster]

This has been bothering me for a little while. I want to know why there is a need for a True Curve (TC).

Why not just make the actual curve of the lens mold the same as the nominal curve (NC)? For example, if Sola's 8.00 base CR-39 SV semi-finished lens is really an 8.01 TC, why not just make the TC the same as the NC?

Thanks in advance!

[lensgrinder]

The nominal base curve was more of a reference curve that the laboratories used to calculate needed curves without the need of computers.

Keep in mind that the True Curve or Refractive Curve that you see on your box is the curve of the lens taken with a device that has an index of 1.53 so the actual curve or refractive curve is different depending on what material you have.

[snowmonster]

Again, why not make it the same though?

[AWTECH]

Again, why not make it the same though?

I think this curve complexity is a result of legacy issues with lens production. From a processing of a lens, (machining), the curves are best described by a radius of a sphere for a spherical lens front curve or a spherical back curve.

Now there are three ways to describe the base curves, Nominal (estimated) TC at 1.53 index calculation and TC calculated on actual index. Some calculations are made with 1.53 index as a reference index and others are made based on true index.

[HarryChiling]

I think this curve complexity is a result of legacy issues with lens production. From a processing of a lens, (machining), the curves are best described by a radius of a sphere for a spherical lens front curve or a spherical back curve.

Now there are three ways to describe the base curves, Nominal (estimated) TC at 1.53 index calculation and TC calculated on actual index. Some calculations are made with 1.53 index as a reference index and others are made based on true index.

That pretty much sums it up, the older tools were based on old crown glass index of 1.53, so all the tools in our industry were calibrated to this index, lens clocks and surfaceing tools. So instead of having tools for every single index material surfaced they came up with a compensation. Now some labs use surfaceingn tools in increments of 1/10^th Diopter so that the error present from rounding out the curve to the nearest tool is minimized, other use tools calibrated to the index of their most heavily used material, others have multiple sets of tools, some use foam blocks which can be cut to more exact curves (higher processing costs, truer prescriptions), and others like Allen may have conformable laps.

[manish]

Why not just make the actual curve of the lens mold the same as the nominal curve (NC)? For example, if Sola's 8.00 base CR-39 SV semi-finished lens is really an 8.01 TC, why not just make the TC the same as the NC?

there are many manufacturers of SV semi finish located in one or different countries...to cut the cost involved a manufacturer may use the closest possible available tool or curve or it could also be to make one different to simply have a identity like sola makes 8.01 TC.. some other may make them as 8.10 etc...further all TC are finally recognised by their Nominals.....only on the floor we compensate depending on the TCs. to produce the required Rx.

[Dougfir8]

Nominal curves are helpful when picking lenses for a specific job and general discussion. I can tell an employee, "use an 8-base curve", and not worry about which company's lens is used. I don't want to say, "Give him an 8.2 if we have Sola or a 7.9 if we have X-cel or a 8.31 if we have a Silor", etc.

[snowmonster]

I guess my problem is that I'm using an old school 108 CT generator (for now). It just seems odd to me that they would make a 2.50 base lens have a TC of 2.24, but I guess it really doesn't matter for the people with fancy dancy generators since it's just all in the software.

[Laurie]

Hello,

The true curve and its nominal power depends on the material's index of refraction (n).

The effective (true) curvature of a higher n will be higher.

The effective (true) curvature of a lower n will be lower.

As we advance in design and production, all optical factors considered, stating the nominal and true curves helps us know exactly what we are dealing with, optically speaking.

: )

Laurie

[Darryl Meister]

Why not just make the actual curve of the lens mold the same as the nominal curve (NC)? For example, if Sola's 8.00 base CR-39 SV semi-finished lens is really an 8.01 TC, why not just make the TC the same as the NC?

In the US, at least, the "true" curve represents the 1.530-based surface power, regardless of the refractive index of the actual lens material. Someone else already mentioned the history behind the use of this particular refractive index, but there are certain advantages to using a single reference index, either way (you can use the same 1.530-calibrated sag gauge to measure all of your lens blanks, regardless of lens material, for example). As for the differences between the "true" curve and the "marked" or "nominal" curve, there are several reasons for using a different nominal curve; some of these reasons are historical, some represent modern manufacturing considerations, and some are solely for the sake of convenience.

For instance, some lenses, many years ago, were supplied with a "compensated" base curve that was marked with a different surface power. The difference in power between the actual surface and the labeled base curve was a vertex power allowance that was meant to simplify surfacing calculations by reducing the actual surface power by an amount equal to the gain in plus power caused by the assumed center thickness of the lens.

Today, since plastic lenses shrink up to 15% during the molding process, it is difficult to achieve exact curvatures consistently, especially after some form of post-processing (such as Transitions imbibing). Consequently, it is not uncommon to cast and measure a sample population of lens blanks, and then to supply the average value as the "true" curve for that particular lens type. This value will usually be rounded to 0.01 diopters. The lens manufacturer may have been shooting for a nominal 6.00 Base, for instance, but the average value may actually be something like 5.98 D, which then becomes the value supplied to laboratories for surfacing calculations.

In some cases, the lens manufacturer may have been legitimately shooting for a seemingly odd value in the first place. For instance, all of SOLA's "6 Base" lenses in hard resin, whether in single vision, bifocal, or progressive, once had a target true curve value of 5.92 D.

Of course, it is not especially practical to refer to a base curve or SKU as the "5.92 Base" or the "5.98 Base," so nominal or marked base curves today frequently represent the true curve rounded to either the nearest diopter or nearest quarter-diopter. This also allows the lens manufacturer to refer to the base curves used for the same prescription ranges across different lens designs or different lens materials by the same nominal curve for simplicity. For non-aspheric products at least, the "6 Base" generally referred to the median base curve, covering the plano and low-power prescription range, regardless of whether the actual true curve was in fact 5.92 D, 5.98 D, or whatever.

You might also want to refer to these two OptiBoard threads on the same subject: Base Curves and Progressives and Base curve - true curve - refractive index.

[Cherry Optical]

Allen may have conformable laps.

Here is some info on a new conformable pad polisher from Schneider. Got mine on order yesterday!

http://www.schneider-om.com/fileadmi...f/CCPswift.pdf

This type of grinding and polishing completely eliminates any rounding and reduces fining/polishing times to just a few minutes!

Adam

[gemstone]

Here is some info on a new conformable pad polisher from Schneider. Got mine on order yesterday!

http://www.schneider-om.com/fileadmi...f/CCPswift.pdf

This type of grinding and polishing completely eliminates any rounding and reduces fining/polishing times to just a few minutes!

Adam

Wait til the docs figure out they can start ordering powers like
-3.42 - 1.14 @ 62.5.

[snowmonster]

Since I'm in the wrong forum, can somebody please PM me as to what something like this costs?