Help on a few questions!!!

[Student21]

Hi is there anyone who can help me out on the following questions:

Why is it desirable to use the smallest available blank size? Are there any circumstances when it wouldn't be that important.

How do you take account of the geometrical inset of the lenses when using a Lens Blank Pre-determinator or the Ditest?

How is the procedure/results different when using say a Glass solid 38mm downcurve bifocal compared to that of a CR39

What extra infromation needs to be specifies, when just ordering one new lens for the patient to match an existing bifocal?

Thanx:)

[HarryChiling]

Hi is there anyone who can help me out on the following questions:

Why is it desirable to use the smallest available blank size? Are there any circumstances when it wouldn't be that important.

How do you take account of the geometrical inset of the lenses when using a Lens Blank Pre-determinator or the Ditest?

How is the procedure/results different when using say a Glass solid 38mm downcurve bifocal compared to that of a CR39

What extra infromation needs to be specifies, when just ordering one new lens for the patient to match an existing bifocal?

Thanx:)

Useing the smallest available blanks size in the lab means you put less wear and tear on both your edgers wheel and your generators tool, giviung you more life therefore bringing down the processing costs per lens. When edging finished stock plus lenses tend to be thinner f you get a smaller blank size so thickness in this case would be the benefit.

The inset can be taken into account by usin ght efollowing formula for minimum blank size:

MBS=minimum blank size
ED=effective diameter
d=decentration
MBS=ED+2*d (thanks for noticeing tmorse)

Also it would be prudent to add 1mm to this size for cutout. You could get more accurate by taking the decentration in the (x,y) and you ED and ED angle and further compute that to (x,y) coordinates where the logest radius lies and then use those measures to compute your Min Blank Size, but that would be overkill unless you were to make it part of a program then the coding would only be done once where the accuracy would be forever. :D

When ordering just one lens I like to know the frames measurements and the Distance OC above seg so that I do not create any vertical prism imbalance in the distance.

[Student21]

Are there any circumstances when it wouldn't be that important...anyone know the answer to that, for the first question?

[lensgrinder]

Are there any circumstances when it wouldn't be that important...anyone know the answer to that, for the first question?

You can use any size blank for a minus lens that you want, because the center thickness will be predetermined and will not change the edge thickness. For example a -2.00 DS with a 2.0mm ct in a 70mm blank put into a frame will produce the same edge thickness as a -2.00 DS with a 2.0mm ct in a 75mm blank put into the same frame with the same measurements, it does not matter what size blank you use.
If you do not use the proper size blank on a plus lens then it will be thicker or thinner on the edge than it was intended, because as the diameter of a plus lens increases so does the center thickness which will cause thicker edges.

[AdmiralKnight]

It also depends on what you mean by important. Chances are both the 70mm and 75mm blanks are going to work fine, it's just one will be a better thickness.

As for your first question, the smallest blank is more important for plus rx's because the smaller the blank, the smaller the center thickness... as for when it's not important, like Lensgrinder said, with minus rx's the edge thickness will be the same either way.

[HarryChiling]

It also depends on what you mean by important. Chances are both the 70mm and 75mm blanks are going to work fine, it's just one will be a better thickness.

As for your first question, the smallest blank is more important for plus rx's because the smaller the blank, the smaller the center thickness... as for when it's not important, like Lensgrinder said, with minus rx's the edge thickness will be the same either way.

From a lab's point of view the larger than necessary blank sizes increase both blank costs and processing costs, but from a patients perspective or dispensers perspective the minus lens doesn't need to have a specific blank size just something bigger than MBS.

[AdmiralKnight]

*nod* That's exactly what I meant when I said it depends on what he meant by important

[Student21]

How do you take account of the geometrical inset of the lenses when using a Lens Blank Pre-determinator or the Ditest?

Is it different when using one of the above two?

[lensgrinder]

I am not sure what a Ditest is, but if it is similar to a minimum blank size chart (this is used with the frame to find the minimum blank size needed for that frame with a certain PD and OC ht).

All charts are different though, some you will "dot" up the demo lens where you want the OC or, for progressives, where you want the fitting cross, some you will have marks indicating the PD and seg ht and you will place the frame to correspond to those measurements.

What type of lens are you trying to find the minimum blank size for?

[Student21]

ummm basically we are trying to fit a pair of 38mm CR39 solid downcurve bifocals so i'm assuming we talking about CR39 lenses here.

[Darryl Meister]

I am not sure what a Ditest is, but if it is similar to a minimum blank size chart

It's an Essilor gizmo that you would use in conjunction with a PAL centration chart. I think they only use it in Europe though.

[Fezz]

It's an Essilor gizmo that you would use in conjunction with a PAL centration chart. I think they only use it in Europe though.

Ditest, huh? I don't think I have ever seen one. Do you have any pictures? I will go thru some of my old textbooks and see what I can find.

[bostian]

Yes, that is for sure that is used in Europa, di -test helps for diameters, multifocal lenses, biofocal lenses to check corect high, pd, and axis devitation. Also easy way to how big size od diametwer would you like to order. It's a good and helpfull tool for a many optician.
I'm sory if my english is not so well but I try.
Thank you
:D

[lensgrinder]

It's an Essilor gizmo that you would use in conjunction with a PAL centration chart. I think they only use it in Europe though.

Thank you! I looked for a long time yesterday trying to find out what that was.

[tmorse]

The inset can be taken into account by usin ght efollowing formula for minimum blank size:

MBS=minimum blank size
ED=effective diameter
d=decentration
MBS=ED+d

Harry... you do mean MBS =ED + 2(d);) (... +1mm)

[HarryChiling]

Harry... you do mean MBS =ED + 2(d);) (... +1mm)

Thanks.

[Kyle]

Regarding minimum blank sise, I agree with the reduction of wear and tear on equipment but there are also additional considerations.

First, a smaller blank with a relatively clean, cribbed, pin beveled edge will create less of an opportunity for particles to enter between the fining pad and the lens, thus reducing the likelihood of swirls when producing an uncut.

Additionally, the greater the lens surface to fine, the greater production of heat. Heat and lenses don't get along well with the former causing the latter to microscopically buckle and warp, creating cumulative abberations and effectively reducing the optical "clarity" of the lens. Cribbing a lens reduces the overall heat production and increases effectness of the polshing slurry in the final process. You get better optics.

Finally, in the finishing process, the equipment wear is definitely a major consideration but heat/energy again is a potential problem, especially with AR coated lenses. The less grinding you have to do, the smaller the chance you will molecularly compromise what would have been a good bond between lens and coating - both at the lens edge AND at the clamp. Similar concerns exist with laminated lenses. Sure, the risk is probably neglegible but if it weren't a risk at all you'd be able to edge a mirror coated lens without problems. Ideally, all lenses would be coated AFTER edging but unfortunately too many labs overlook clamp "ghosting", considering it a passable flaw.

In a similar manner, if you start with a smaller finished/uncut blank, you'll have less "shrapnel" flying around your lens during the edging process, reducing the chance for damage. To combat this issue, there's always ye olde lens film.

Besides all that, it just shows how cool you are when you ask for a "cribbed blank".

[HarryChiling]

Besides all that, it just shows how cool you are when you ask for a "cribbed blank".

Yep, that's why I make em that way.:D