9 Replies Latest reply on Jun 19, 2013 11:02 AM by Carter West

# Aspheric lens - using formula in surface

Hi all,

I need to make two models of reading glass lens.  One is 1.75 diopter the other 2.5.  Client wants to use aspherical lens surface curvature due to limitataions of thickness.

-help-

He has provided the formula but I can't seem to get it to work.  I don't work often enough in the surface world and I know this is a factor in not seeing my errors.

=R-(R^2-r^2)^0.5+Br^4

Even if you can help without the formula and create 1.75 ashperical, then that would be great and using that example I can do the rest of the work.

Many thanks in adance

• ###### Re: Aspheric lens - using formula in surface

This is my work horse:  (x^2/R)/(1+(1-(1+(K))*(x/R)^2)^.5)

It is a formula for a Rho variable driven conic curve. X determines the start and end points; K is the Rho variable where 0 = circle, 0 > K > -1 = ellipse, -1 = parabolic, and K < -1 is hyperbolic; R is the radius of curvature. The formula gives me the freedom to change the curve type without deleting the sketch plus it adds hyperbolics. The formula driven sketch tool allows the curve to be constrained to construction lines in the sketch instead of having to manipilate the position in the interface.

Defining your variables would help since I don't design eyeglasses....

• ###### Re: Aspheric lens - using formula in surface

... I'm still thinking about this. If I understand the definition of diopter for eyewear lenses then it is the inverse of the focal length or for 1.75 it is about 541mm. The lens surfaces are going to be defined by the Radius of Curvature and the Conic Constant but the actual curve will depend on the lens material refractive index. Low density PC vs high density PC vs glass will make a difference. It seems like the optician/optometrist has given you a formula without defining at least a couple of the variables.

Just for curiosity, what is the end use of the files? I use native SW sketch tools and the formula driven sketch for non-imaging optics applications such as diffuse surface reflectors or lenses for illumination. For imaging applications where the bpsline surfaces in SW are unacceptable I use an optical design add-in. The package I use is OptisWorks.

• ###### Re: Aspheric lens - using formula in surface

Harold,

Thank you for the great help.  My purpose of this surface project is only for the need of prototype testing of a new frame design.  My client would like to start with the range of typcial 'reading' glasses of +1.5~+2.5 dioptre.  Using a plano convex convention we have a couple design factors that have me asking if the ashperical shape may allow me to be able to to stay within my constraints.  See the screenshot for my current limiting boundaries. As you can see, the left edge (outside of glasses) is tailed off to a weakened thickness.  The lens index for the moment is 1.53(n), but once I figure out the correct shape I can then mill it out of polycarb and put in the frames. The project will then go to the folks for the actual lens shape and final lab testing.

I'd like to upload the model, but I can't seem to find the icon to allow me to do that. Why do the simplest things always have to be changed?

Thanks again

Carter

Part file attached (thanks Patrick).  Two configurations

• ###### Re: Aspheric lens - using formula in surface

Click "Used advanced editor" in the top right corner of the reply window to add an attachment.

• ###### Re: Aspheric lens - using formula in surface

My first thought is that I really don't know the terms relating eyewear! But I have few questions from messing with your model.

• Is there a reason the the lens center lin is off axis? Wouldn't the center of the curvature and the center lens be coincident to align with the eyeball?
• Wouldn't the lens be a revolve and not an extrude? Otherwise the lens is a cylinder lens and as you look through the lens at an angle the prescription would change.
• The need for a conic/aspheric lens should be driven by the presciption and the fit to the frame shouldn't it? I did find adding a slight conic value corrected the chromatic aberations.

I know you were looking for a plano convex but I found making a meniscus with the formerly plano surface having a large radius worked well. My lens came up with a FL of 571mm (dioptre = (1/571)x1000=1.751. Curves are R1 = 294.75mm, Conic = -1.05; R2 = 2500mm, Conic = -.9; Center thickness = 1.5mm. Material is PC. Lens shape cutout is centered on the revolve.

• ###### Re: Aspheric lens - using formula in surface

- Client wants 58mm about CL from apex to apex for the mirrored pair of lens.   Should be about the distance between typical eyes.

- Correct.  But for this test purpose all I want to do is have the surface along the lens area for our testing (can't say why)

- Correct again.  But I'm gonna leave that up to the optic gurus.  For now, (I'm just the lowly ME) I'm tasked with just getting fit issues for prototyping.

I don't suppose you could throw that example in SW my way?  If I could see it in action, then I could probably but this to bed.

Many many thanks for your help Harold.  It is really appreciated

Carter