Above rim
Cyl Axis O.C Height Vertex Panto Facial Wrap OD -1 -0.5 75 34 13 12 8 10 OS -3 -0.5 45 35 13 12 8 10
Frame Details A B ED DBL
50 32 55 17
Is there a way to discover the front curve of a spectacle lens using only the following information?
Or alternatively find the radius (r1) of the individual lenses given the above data only.
Trying to design a thickness calc using excel and can't seem to work the formula to calculate the thickness.
You are going to need to define something on the form. Either a back curve so that a front curve can be determined or a front curve so that a back curve can be determined. The alternative is you define a best form lens design through ray tracing and use that curve to determine the other. A good ophthalmic optics book can help, my preference especially given your country of origin is going to be Ophthalmic Lenses and Dispensing Mo Jalie. A new revision should be coming out this new year and I have found every revision is worth the wait and price.Originally Posted by Bok
Steps for finding curves:
1 Determine design focus; minimum power error, mean power error, etc.
2 Ray trace to determine optimal back curve.
3 Use back curve to determine front curve.
I've always used "most minus, least plus" as the default, then the manufacturer's guidelines. Although with a 2 diopter difference in power, I'd split the difference.
In glass, I'd normally use a 4 base, unless a 5 base was available. In high index, go up one base curve.
With the available data, there is *no way* to determine the base curve. You have to 'punt'.
Bok, if your end goal is a spreadsheet to give you a leg up on lens thickness (without using calculus or exhorbitant amounts of trig), I have some friendly advise as a guy who's plowed that road:
You're better off measuring OC to lens edge yourself as the effective radius and using it as an input. (I find it simpler just to calculate and dot the occasional prism offset on the lens and remeasure from there.) Be sure to run two calculations: 1 for maximum radius (using lens power at that axis of maximum lens radius), and 1 for maximum lens power (using the radius at the Rx axis of greatest power.) You'll use whichever is higher. Kinda choppy, but the logic is good and fits on a single Excel page. : )
I hope you don't really need curve values to anticipate lens protrusion beyond the plane of the frame.
http://www.opticampus.com/tools/thickness.php
I use this for thickness calculation. It's pretty accurate and has lots of different materials available.
It's a fabulous tool for round or ovular lenses! It's dead-on for powers thickest on the 180 or close to it. I think you're right it works fine for most needs when you can afford a bit of rounding.
For square/sharply cornered lenses and/or strong cylinders where it counts, tho--I can get some significant variances from time to time. Sometimes there's two or three spots to keep an eye on...sometimes there's one big bulge right at the worst corner. (I like using a spreadsheet 'cos it's a lot to crunch in my head when the patient is talking when I need to listen. My ego would love to say I can juggle it all 100% of the time, but alas...good data entry habits are the closest thing I can manage.)
If a half millimeter or more really counts, or if you're just neurotic about patients who break estimation formulas...
by inputing:
Rx
index of refraction,
minimum edge/center thickness,
frame ED,
angle of the frame's Effective Radius,
and longest radius from each OC to the frame edge along each lens' axis of maximum power,
Excel can then parse out the actual maximum thickness for a patient & frame combo--and precisely where on the eyewire those 2-4 thickness peaks will occur.
Very handy when you need to quantify the differences between lens materials to a patient and bullseye it.
(Bok, I hope we didn't hijack your thread. Apologies if we did.)
Hi-jack away, it's good listening to what some have done and what I should look at with designing my own chart.
I have done an approximate thickness calc but was looking at trying toget it more accurate.
The idea was to have a sheet that has minimal info needed like OD and OS rx only including frame details that one could punch in then click a link to display the given calculation. So far so good.
Do you have an example of a sheet that measures O.C to lens edge etc?
The best way I found to calculate the needed lens diameter is the use of law of cosines and some basic trigonometry.
Let's say you have a lens that you decenter 5mm in and 5mm up and your longest radius is 25mm at 220
Determine the angle from 360.
360 - 220 = 140
Now determine the angle from the horizontal and vertical decentration.
Now get your hypotenuse from the horizontal and vertical decentration
Now add 140 and 45 = 185
Law of Cosines
So your minimum blank would be 64. You can use this to find thickness, etc.
See attached image
It's very complicated to calculate base curve when you know your prescription.
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