Bifocal Intraocular Lens (IOL) Design using Ansys Zemax OpticStudio (AZOS)

Bifocal Intraocular Lens (IOL) Design using Ansys Zemax OpticStudio (AZOS)

By Muhammad Rizqi Nauval Afif | Application Engineer | CAD-IT 

Category: Tips & Tricks 

Intraocular lenses (IOLs) are artificial lenses implanted in the eye to replace the eye’s natural lens when it is removed during cataract surgery. Cataracts, which cause clouding of the natural lens, are a common condition that can lead to significant vision impairment or blindness if left untreated. The primary motivation for developing IOLs is to restore clear vision to individuals who have undergone lens removal due to cataracts or other lens-related issues. These lenses help patients regain the ability to see clearly, improving their quality of life by enabling them to perform daily activities with better visual acuity.

There are several types of IOLs available to cater to different visual needs. Monofocal IOLs are the most common type and are designed to provide clear vision at a single distance, typically set for either near, intermediate, or far vision. Patients with monofocal IOLs may still require glasses for certain activities. Bifocal and multifocal IOLs, on the other hand, are designed to provide clear vision at multiple distances, reducing the need for glasses by allowing the patient to see both near and far objects.

Figure 1 A bifocal IOL showing an aspheric surface and annular zones on the lens (the size of the rings is exaggerated).

In Ansys Zemax OpticStudio (AZOS), we utilise the Binary 2 surface to model a bifocal IOL. The Binary 2 surface is a diffractive surface where the phase added to each ray varies as a rotationally symmetric polynomial. Phase is delayed or advanced via the following expression:

binary 2 surface equation

where the coefficients Ai are in units of radians, N is the number of polynomial coefficients in the series, M is the diffraction order, ρ is the normalised radial aperture coordinate with coefficients represented by the A terms.

For example, assume that an IOL with the following parameters is to be modelled.

A screenshot of a computer

Description automatically generated

A pupil diameter of 3.5 mm will be used, to approximate the iris size of the human eye in moderate illumination conditions.

First, we model the human eye before implanting with the IOL as follows.

lens data editor settings
layout image

Then, we replace the lens (surface 6) with a diffractive IOL, which is represented by a Binary 2 surface.

lens data editor

To model a bifocal correctly, we must model two configurations. One for the near field, and another one for the far field. The first configuration will model the 0th diffractive order of the bifocal IOL and will require an infinite object distance. The second configuration will model the 1st  diffractive order of the lens and will require a finite object distance.

multi config editor

Before optimising the IOL, we will set some parameters as variables (V): radius, 4th order term, 6th order term, coeff. on p^2, coeff. on p^4, coeff. on p^6, and coeff. on p^8 for both surfaces 7 and 8.

far vision configuration
far vision configuration

After everything is set correctly, we run the optimisation in AZOS. The below picture is an example of what the Merit Function Editor might look like at the final stage of optimisation.

merit function editor

The optimisation will also update the variables in the Lens Data Editor directly. We can see that the values are now changed.

optimization results
optimization results

In the 3D Layout, we see that the Bifocal IOL has been successfully designed. For both far and near configurations, the image is precisely located on the retina.

AZOS comes with many types of analysis that we can use. For example, we can see the results of the Fast Fourier Transform (FFT) Modulation Transfer Function (MTF).

We can also generate the Wavefront Map. The below picture shows the Wavefront Map for far configuration.

Another interesting feature of AZOS is that a bitmap image can be used to see quantitatively the generated image in any particular surface. Below is the Geometric Bitmap Image Analysis for the far configuration.

With the current trends of personalised medicine, more people are seeking customised healthcare solutions based on the real measurements of their body condition, including for their eyes. For patients having the desire to more accurately model the wavefront following the corneal surface, they can obtain the Zernike coefficients from corneal topography measurements. AZOS has a type of surface called Zernike Standard Phase that allows Zernike coefficients to be input in the Lens Data Editor.

LDE

Below is an example of the modelling of Zernike IOL. Note that here the sample eye model used earlier remains essentially the same. The only difference is that the corneal “lens” from earlier has been replaced with a paraxial surface of the same power.

Acknowledgement: Thanks to Dr Nazirah Mohd Razali, CAD-IT’s former application engineer, for creating the first version of this article.

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