Solved

# Determine aspheric lense parameters

• 3 replies
• 53 views

• Infrared
• 7 replies

If I have some aspheric lense made from some glass, is it possible to find the equivalent lense parameters for lense made from other glass ?

The aspheric lense glass is substituted. Is possible to find its parameters to make that lense work like original lense ?

icon

Best answer by David 6 December 2021, 21:21

View original

### 3 replies

Userlevel 5
+3

I think the best way to do that is to reoptimize the lens after changing glass type. If you have the OpticStudio design for the original, including the merit function, just reoptimize radii and aspheric terms. If you are starting with a prescription, make those terms variable which appear in the prescription and use a  merit function appropriate to the intended use of the lens.

Thanks David,

But the matter is I dont know the merit function even the lense purpose.

Is it possible in the such blind case to find the lense equivalent ?

Userlevel 5
+3

This is really quite complex.

In the simple case of a spherical equiconvex lens, where both radii are the same, we could use the lensmaker’s equation. It gives the focal length in terms of the two radii, the thickness, and the index. Since the two radii are the same, we could use the same thickness and focal length and solve for the new radii in terms of the new index.

Making the problem more general, suppose the two radii are not the same. There are now more variables (two) than equations (one). There are an infinite number of solutions. We need another equation. This is where knowing the intended use of the lens can help. By inspection, we might deduce that it’s a best-form design intended to image an infinite object. There is now a known relationship between the radii. We assume a Coddington shape factor that minimizes spherical aberration. Now we have two equations and two variables. In fact, if we have the prescription or can measure the radii we could determine the shape factor of the lens and use that same factor in the second equation.

In the case that the either or both surfaces are aspheres, we likely have a reasonable solution if we solve for the radii as above, since near the axis where the aspheric contribution to the slope is small, the lens still needs to focus these paraxial rays. Then assume the same aspheric terms.

But if we have two different radii, and only the lensmaker’s equation, there are an infinite number of solutions. Lenses like this are common as elements of a compound lens, where the lens bending works in conjunction with the rest of the design to minimize aberrations.

I still think that the best method is to deduce an intended application, which gives us object and image distances, and then design a merit function to reduce spot size, in position or angle space depending on the application.

Here is a link to some useful information: hyperphysics page