If you have heard of Abbe number you probably have heard of chromatic aberration already. Chromatic aberration or chromatic distortion is the failure of light to come to focus at a single point. When white light passes a glass prism, it breaks into its several components. The short wavelength blue light falls before the green light, and the green light falls before the red light. This process of white light splitting into several of its constituent colours is called dispersion of light (fig 1). Light travelling through a prism from air bends towards the base of the prism. This bending towards the base happen differently for all constituents of white light according to their wavelength.
The effect of chromatic aberration is colour fringing (seen in fig 2) that often obscures the picture by adding an unwanted colour along the edges of objects or centrally. This degrades the picture quality and contrast. Named after German Physicist, Ernst Abbe, Abbe number (denoted by VD) is the measure of this dispersion or image degrading effect of a lens. Thus Abbe number gives us an idea of the drop in contrast that a picture will undergo when taken by the lens of a camera. The higher the Abbe number of the lens, less the dispersion of light rays passing through the lens, and therefore sharper the image. The commonly used ophthalmic material for glasses would have an Abbe number between 30 and 58, the higher the better.
If chromatic aberration is detrimental to quality of image passing through a lens, then how do manufacturers of lenses in microscope or camera negate the effects of chromatic aberration? In engineered optics positive and negative powered surfaces and materials of higher and lower chromatic dispersion are utilized to create optics that would have low dispersion of light. Such lenses are achromatic in design or simply, achromats. However, unlike achromats that are a combination of positive and negative powered lenses, the human eye has only positive power lenses, like the cornea and the human lens. Despite this high positive powered lenses, studies generally show that human eye do not, or atleast suffer minimally from the image degrading effects of chromatic aberration.
Marcos S & co-authors (1) showed that with a wavelength of light between 365-750 nm, a longitudinal chromatic aberration of 3.2 diopters is obtained in the human eye. Between a wavelength of light 450-650 nm, a 1.33 diopters of longitudinal chromatic aberration is obtained. Charman and co-authors (2) shows the same as 1.4 diopters for a wavelength of light between 460-700 nm.
In light of the above information, a few questions come to mind.
The human lens has a Refractive Index of 1.39 to 1.41, with a Abbe Number of 47 which is relatively high. Why does the human lens show a large amount of chromatic aberration with a relatively high Abbe number?
If the eye has a large chromatic aberration, then what protects the eye from an overall image degradation? The perceived visual appearance does not display such chromatic aberrations. Then what protects the eye(3)?
The effect of chromatic aberration in the eye, despite its high presence is probably negated by a number of mechanisms.
First, the human lens, as well as the macula pigment filters out the short wavelength blue light. The filtration of blue light reduces chromatic aberration. The macula pigment consists of caratenoids, mainly the Meso-Zeaxanthin, Zeaxanthin, and Lutein. Maucla pigment concentration is at its peak at the Foveola, the place which is responsible for peak human vision in photopic or mesopic condition. Filtering out the short wavelength blue light thus mitigates the image degrading effects of chromatic aberration.
Secondly, human vision peaks at a wavelenth of light between 500 nm and 550 nm(fig 4). There are three types of cone cells : S-cones, M-cones, and L-cones. The S-cones are particularly sensitive to blue light. It has been seen that the fovea has less S-cones cells compared to the M-cones and L-cones. The M-cones and L-cones are sensitive to longer wavelength light and therefore foveal sensitivity shifts towards this zone. This could also explain why the effect of chromatic aberration is less in the human eye.
In figure 5 we see how the three types of cone cells are arranged in fovea. The blue wavelength sensitive S-cells are significantly less in numbers. According to one estimate in the retina the ratio of L- and M- cones to S-cones is about 100:1 (source :cis.rit.edu)
Studies like Bradley & co-authors have pointed that the effect of chromatic aberration in the human eye is no more than .25 diopters. Campbell & Gubisch in 1967 first addressed this subject. They concluded that the image degrading effect of chromatic aberration is negated by other monochromatic aberrations like spherical aberration.
Another aspect that may be considered and comes to my mind is the Duochrome test. The duochorme test is often done with green and red letters to fine tune the final prescription. We have just described the minimal presence of blue light sensitive S-cones cells in foveola. This leads to the question of longitudinal difference in dispersion of red and green wavelengths of light? When the duochrome test is done, a patient typically ends up accepting .25 diopter of glass for correcting red or green letters (-.25 for patients who say red is sharp or +.25 who say green letters are sharp). If the eye did not have its natural defense against chromatic aberration, then how would it be possible that the eye accepts only.25 diopter of glass correction to make the red and green letters equally sharp ?
In the IOL industry, a matter of huge debate has been the question of Abbe number of IOLs and its relevance in postoperative image quality that the patient gets. With this article I wanted to place things in its proper perspective. While Abbe number is important for optics engineering and its application in glasses, camera and microscope, its relevance in IOL industry, wherein the IOL seats closer to the nodal point of eye, is by far questionable.
Marcos S., Burns S.A., Moreno-Barriusop E., Navarro R. A new approach to the study of ocular chromatic aberrations. Vis. Res. 1999;39:4309–4323. doi: 10.1016/S0042-6989(99)00145-5
Charman W.N., Jennings J.A.M. Objective measurements of the longitudinal chromatic aberration of the human eye. Vis. Res. 1976;16:999–1005. doi: 10.1016/0042-6989(76)90232-7
Neuronal Mechanism for Compensation of Longitudinal Chromatic Aberration-Derived Algorithm, Yuval Barkan & Hedva Spitzer; Front. Bioeng. Biotechnol., 23 February 2018 Volume 6 - 2018 | https://doi.org/10.3389/fbioe.2018.00012
Bradley A, Glenn A. Fry Award Lecture 1991: perceptual manifestations of imperfect optics in the human eye: attempts to correct for ocular chromatic aberration. Optom Vis Sci. 1992;69(7):515–521. doi:10.1097/00006324-199207000-00002
The Impact of IOL Abbe Number on Polychromatic Image Quality of Pseudophakic Eyes, Article in Clinical ophthalmology (Auckland, N.Z.) · August 2020 DOI: 10.2147/OPTH.S233099