A Beginner's Guide to the Optics of Intra Ocular Lens for Eye Care Professionals
- Jul 6, 2025
- 19 min read
Understanding the basics of optics is essential for eye care professionals because it enables them to accurately diagnose and manage various visual problems. A solid foundation in optics helps professionals explain how light interacts with the eye, understand refractive errors such as myopia, hyperopia, astigmatism, and presbyopia, and determine appropriate corrective measures like glasses, contact lenses, or surgical options. Additionally, knowledge of optics allows for the proper adjustment and fitting of corrective devices, optimizing visual acuity and patient comfort. Overall, a grasp of optical principles ensures that eye care professionals can deliver comprehensive, precise, and effective eye care services.
Cornea and human lens:
Assuming a basic understanding of the eye anatomy is already available to the reader, let us start by understanding that the cornea and the human lens are the two main organs in the eye that helps bend the light on the retina. This bending of light is known as refraction.
Refraction happens due to the light passing through two different mediums of different densities. As light travel through the air and reach the eye, it pass through the cornea and human lens that bends the light to reach the fovea of the retina. Fovea is a location in the retina that has the highest concentration of the cone cells. Cone cells are a type of photoreceptor cell located in the retina of the eye. They are responsible for color vision and function best in relatively bright light, unlike rod cells, which are more sensitive to low light.
The bending of light is a result of the difference of speed of light travelling in air and travelling through the cornea and human lens. The speed of light in the cornea is approximately 2.17 x 10^8 meters per second(m/s), while the speed of light in air is close to 3.0 x 10^8 m/s. This slowing
of light causes bending of light, a phenomenon that is defined by refractive index, which is a dimensionless number that describes how light propagates through a particular medium. It is defined as the ratio of the speed of light in a vacuum to the speed of light in that medium.
Refractive Index (R.I) of Intra Ocular Lens(IOL):
Higher the refractive index, more the light slows down while passing through the IOL material. That is, light passing through a material having higher refractive index will bend more. If you come across an IOL that has a refractive index of (say) 1.50, it would mean that light passing through the IOL material will slow down 1.50 times as compared to light in air.
The cornea has a lower refractive index of 1.376 than the human lens which has a gradient refractive index of 1.39 to 1.42 . Yet, the cornea accounts for two-thirds of refraction of light due to a very high difference of refractive index between air(1.0) and cornea (1.376), as compared to the human lens(1.39 to 1.42) and aqueous (1.336). Thus the greater the difference in refractive index of two medium that light pass through, more the bending of light. Add to this, the convex curvature of the anterior cornea also adds to its greater refractive power.
You will often hear the discussion around refractive index of a particular IOL. The refractive index of Tecnis Eyhance is 1.47 at 35 degree centigrade while that of the Vivinex IOL from HOYA is 1.548 at 23 degree centigrade. Notice, quoting the refractive index without quoting the exact temperature in which it is measured does not give us the full information.
What impact has temperature on refractive index of lens? The relationship between refractive index and temperature is generally such that the refractive index decreases as temperature increases. This occurs because increasing temperature causes the material's density to decrease, leading to a reduction in its optical density and thus a lower refractive index. Understanding this relationship is important for optical systems requiring high precision, as temperature variations can affect focusing, dispersion, and other optical properties.
It is not usually spelled out by IOL manufacturers if the IOL power is measured in room temperature or a temperature that is closer to our body temperature. Standard testing environments should be used that mimics closely the body temperature. If the IOL is measured in room temperature, or if the IOL optical power is measured in a temperature condition that is lower than the body temperature, then as the IOL is implanted in the body, the dioptric power of the IOL will change, though marginally. This is usually taken into account for IOL power labelling by applying IOL power change to temperature changes. On an average IOL power was seen to increase by .13 diopters between the IOLs measured at 22 deg and 35 deg centigrade(1).
What are the pros and cons of a higher refractive index for IOL platforms?
Imagine a patient requires a 21 diopter IOL that will lead to emmetropia (light refracting and falling on the fovea). For a 21 diopter IOL a lens with lower refractive index, say 1.47 (Tecnis) will be thicker compared to an IOL of 21 diopter that has a higher refractive index (Clareon/AcrySof- 1.55). This is because to bend the same light to the fovea, the IOL with lower refractive index will need a higher volume of the material compared to a higher refractive index IOL material. Thus an IOL with higher refractive index will be thinner than an IOL with lower refractive index, which helps the IOL to be implanted through a smaller incision.
On the flip side, it is argued that IOLs with higher refractive index has a lower ABBE value, that may lead to more amount of chromatic dispersion when light travels through the IOL optic. A detailed understanding of this concept is available in the article 'Why we should stop asking about Abbe Number with Intra Ocular Lenses' ( https://www.quickguide.org/post/abbe-number-and-iols ). In short, chromatic aberration refers to the dispersion (separation) of white light when passing through a lens.
A lens which has a high refractive index, will have a lower ABBE value, that would signify a large dispersion/separation of each of the wavelength of light. Thus when white light passes through the lens, the various components of white light (VIOLET, INDIGO, BLUE, GREEN, RED) will fall at different focal distances, leading to a drop in contrast. However, this is true for lens that are exposed to air. Our eye has an ability to correct many of the higher order errors, and chromatic aberration is one of them. Therefore, the effect of chromatic aberration on the eye is debatable.
The concept of radius of curvature of an IOL:
In the image on the left, two circles of a sphere shape are drawn. The radius is the distance from the center of curvature of the circle to its edge (dashed lines). The line drawn across the center of curvatures of the two surfaces (yellow line) is the optical axis of the lens. Every IOL has two surfaces. Each surface is part of a sphere. In reality however, IOLs have rarely same equal curvatures of their anterior and posterior surfaces. Most IOLs will have an asymmetrical distribution of curvatures of the anterior and posterior surfaces. This asymmetry in curvature may vary across the entire dioptric range.
