Tool curve

[incognito]

hello! this is my first post to the board. i fell upon this board while searching for some help about questions i had. i have been working in a full service lab for a little over a year now. i never really put much thought into it, but i would generate the the lenses accordingly to the work order. i figured (for a poly lense) that base curve-tool curve=rx. i would come close to guessing the right rx by looking at the lense/tool, so i guess i was correct in some form. but today, a job came through where i took a guess at the rx by looking at the lense/tool, and my guess was totally off! after searching through this board, i realize that the index of refract has something to do with it too? and maybe more? i did find steve's program that calculates the info, but i can't figure out what the formula is. any help would be greatly appreciated. thanks!

[Darryl Meister]

If the refractive index of the material is close to the tooling index of your lap tools (generally 1.530), the tool curves will get you pretty close to the final prescription (ignoring the effects of thickness). For higher index materials, you can account for the effect of the refractive index of the material using,

Surface Power = (Refractive Index - 1) / 0.530 * Tool Curve

For instance, using polycarbonate, a 6.00 D lap tool will produce an actual surface power of (1.586 - 1) / 0.530 * 6.00 = 6.63 D.

You can also just add the lap tools together to arrive at an initial prescription, as you've been doing, and then multiply this actual prescription by (Refractive Index - 1) / 0.530 in order to arrive at the correct prescription. It will save you a step.

[incognito]

thank you for the quick reply!

sorry if any of the questions are stupid, but what do you mean by surface power? you also mentioned thickness. how does that equate into the formula? i also tried what was suggested in the latter part of the reply. is there a reason for using (refractive index-1)/.530 (why .530)?

again, sorry if i'm asking alot of questions, or if these questions has been asked before or can be easily found/answered elsewhere. thanks!

[lensgrinder]

Greetings,
This forum is used to help so no question is stupid.
The 1.53 index is your tooling index. When the formula uses 0.53 that is 1.53-1. Most tools in the lab use 1.53 index (lens clock, sag gauge, laps, etc.). So like Darryl said if you grind a lens using a 6.00D lap the actual power on the back will be 6.63 for polycarbonate. Meaning if your front curve is 4.00D and you grind a -6.00D back curve(actually comes out to be -6.63) then your total power is -2.63D for polycarbonate.
Dn=D1+D2
Dn=Total power
D1=Front Curve
D2=Back Curve

Dn=4.00+(-6.63)
Dn=-2.63
Also keep in mind that when lens manufactures use True Curve this is based on a 1.53 index also. Surface power is the power of the surface, back or front. For thicker lenses you would use the thick lens formula or the back vertex formula (same)

F=((D1)/(1-(t/n)XD1))+D2
F=Rx of Lens
D1Front Surface Power
D2=Back Surface Power
t=Thickness of lens in Meters
n=Refractive Index

Because when you view a lens in a lensometer you are looking or measuring the back vertex power.

Hope this helps

[incognito]

Thanks lensgrinder. Well, it just seems as the more I read on these boards, the more confused I get. I'm not sure if it's because maybe I'm getting too far ahead of myself and there are certain things I should learn first, or maybe these things I'm asking I should already know. The knowledge found here is overwhelming though :)

as for the formula there, I tried inputting some numbers from a work order I printed up. for some reason, my powers are off by a quarter. Am I doing something wrong?

F=((D1)/(1-(t/n)*D1))+D2

F= -5
D1= 2.48 (Lens True Front Curve)
t= 1.7 mm (.0017m)
n= 1.586
D2= -7.73962 (1.586-1)/.53*7

but the result i'm getting is closer to -5.25 and not -5. am I doing something wrong? Plus, How do you determine what the center thickness should be when the lense is all done?

[lensgrinder]

Incognito,
The Lens true front curve that you listed is 2.48, was this from the box top or surface ticket? If so remember that this power is also based on a 1.53 index, which means you need to use the true power formula.
Dmarked/Dtrue=0.53/(Ntrue-1)

2.48/Dtrue=0.53/0.586
Dtrue=2.74
So then if you use your true back curve of -7.73 then your power will be -5.00, but if you use 2.48 then the result is -5.25.
Dn=D1+D2
Dn=2.48+(-7.73)
Dn=-5.25

Dn=2.74+(-7.73)
Dn=-5.00

Start off finding the true curve of the front of the lens and then use the formula above to find the needed true curve on the back then use the true power formula for the back curve to find out what lap to use.

For example:
You want to have a lens power of -2.00 put on a 6 base curve in polycarbonate. What tool would you use?

6(This is the marked curve) X 0.586 = 0.53 X True Curve
3.516/0.53=True Curve
TC=6.63
D2=-8.83

So we know we need to grind a -8.83 on the back to acheive a -2.00 power. What lap are we going to use to get a -8.83.

(8.83 X 0.53)/0.586=Tool
Tool= 7.98

So we would use an 8.00 D lap

Let me know if this helps.

[incognito]

thanks again :) i had realize my mistake while driving to work haha... so where does lense thickness come into play? and how do we determine how thick a lense is? or to grind it to?

[lensgrinder]

Well it depends on material, front curve and of course thickness. For example if you have a polycarbonate lens with 5mm of center thickness placed on a 6 base then your power will change by and 0.12D of a Diopter but if you placed it on an 8 base the power would change by 0.25D.

[incognito]

ok. a little confuse. let's say i have a polycarb semi-finish lense blank. it has a front base curve of 250. the final prescription is a -5.00. according to the work order that I got, the center thickness of the lense once finished will be 1.7mm. how did it determine that? thank you again for the help!

[lensgrinder]

Well generally speaking, a minus polycarbonate lens that is surfaced will have a finished CT of 1.5 usually not lower due to problems from surfacing. You would use the SAG (Sagittal Depth) formula to figure your CT on Lower minus powers or plus powers. Darryl Meister has a great article on this formula in the Optiboard File Directory. The name is Methods for Estimating Lens Thickness.

Hope this Helps.

[incognito]

i think it does. thanks so much for being patient with me :)