Understanding Modern IOL Power Calculation Formulas Haigis Barrett EVO and Kane Explained

In this article I will explain how the different IOL calculation formulas like Haigis, Barrett universal II, Kane, EVO and Olsen formula work.

To begin with, if you ask that what is the difference between the older generation IOL calculation formulas like SRK T or Holladay I, Haigis, and the current generation modern formulas, the first thing that comes to mind is the difference of IOL power calculation by thin and thick optics.

What is thin and thick optics formula? This is explained in detail in the article, Gaussian optics, Gullstrand eye, & Biometry ( https://www.quickguide.org/post/all-about-gaussian-optics-gullstrand-eye-theoretical-and-ray-tracing-formula ). Basically, a thin lens is such that its thickness is so negligible that all refraction is considered to happen only at one plane. However, no lens, be it IOL or any other, will have some finite thickness. A thick lens formula considers the lens optic to have separate pairs of refracting plane - one anterior and the other posterior refracting plane.

The first principal plane is where light rays appear to refract as they enter the lens or system from the object side.

The second principal plane is where rays appear to emerge after they have refracted through the system and exit towards the image side.

Older two variable formulas like the Holladay I, SRK T, or the Hoffer Q have calculated the IOL power and ELP of the IOL based on thin lens optics. The new generation formulas like the Barrett, EVO or Kane are however based on the thick optics approximation. In such formulas, both IOL power and ELP are based on the two refracting planes.

Here you can access a calculator that helps you understand IOL power based on thick lens formula. https://sheet.zoho.com/sheet/open/oxim98090819123a94bd99435f9a6bc62eed2?sheetid=0&range=F15

Screenshot of my thick lens based IOL power calculator that gives you an understanding of the first and second principal planes of the IOL, and the focal length and IOL power based on the input variables. quickguide.org

Let us know how these individual formulas are different from each other. Let us start with the modern thick lens IOL calculation formulas.

This is how Barrett Universal II formula work:

The Barrett is an unpublished formula. However, I have given a brief explanation of the formula in the below video:

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