Optical aberrations

Optical Aberrations in Human Eye

1. Optical Aberrations Jagdish Dukre

2. Optical aberration is an imperfection in the image formation of an optical system. Aberrations fall into two classes: monochromatic and chromatic.

3. Monochromatic aberrations are caused by the geometry of the lens and occur both when light is reflected and when it is refracted. They appear even when using monochromatic light, hence the name. Chromatic aberrations are caused by dispersion, the variation of a lens's refractive index with wavelength. They do not appear when monochromatic light is used.

4. One needs to keep in mind these important points: unlike the standard eye model, an actual eye is : An active optical system, with adjustable components and aberrations varying in time, It is not strictly centered system, It is not a rotationally symmetrical system, and Final perception is the subject of neural processing.

5. WAVEFRONT ANALYSIS Aberrations can be defined as the difference in optical path length (OPL) between any ray passing through a point in the pupillary plane and the chief ray passing through the pupil center . This is called the optical path difference (OPD) and would be for a perfect optical system.

6. Wavefront aberrometer shines a perfectly shaped wave of light into the eye and captures reflections distorted based on the eye’s surface contours. Thus, it generates a map of the optical system of the eye, which can be used to prescribe a solution, correcting the patient’s specific vision problem.

7. Another way of characterizing the wavefront is to measure the actual slope of light rays exiting the pupil plane at different points in the plane and compare these to the ideal; the direction of propagation of light rays will be perpendicular to the wavefront. This is the basic principle behind the Hartman-Shack devices commonly used to measure the wavefront. Wavefronts exiting the pupil plane are allowed to interact with a microlenslet array.

8. If the wavefront is a perfect flat sheet, it will form a perfect lattice of point images corresponding to the optical axis of each lenslet . If the wavefront is aberrated , the local slope of the wavefront will be different for each lenslet and result in a displaced spot on the grid as compared to the ideal. The displacement in location from the actual spot versus the ideal represents a measure of the shape of the wavefront.

9. Wavefront maps are commonly displayed as 2-dimensional maps. The color green indicates minimal wavefront distortion from the ideal. While blue is characteristic of myopic wavefronts and red is characteristic of hyperopic wavefront errors.

10. Once the wavefront image is captured, it can be analyzed . One method of wavefront analysis and classification is to consider each wavefront map to be the weighted sum of fundamental shapes . Zernike and Fourier transforms are polynomial equations that have been adapted for this purpose. Zernike polynomials have proven especially useful since they contain radial components and the shape of the wavefront follows that of the pupil, which is circular.

11. Following the above division of the Zernike expansion adopted in ophthalmology, monochromatic eye aberrations are addressed as: (1) lower-order aberrations, with the Zernike radial order n<3, and (2) higher-order aberrations, with n≥3.

12. The important optical aberrations that affect vision are : 2nd Order optical aberrations – currently measured in all eye exams providing sphere, cylinder and axis corrections 3rd and 4th Order optical aberrations – high order aberrations currently not measured in today’s eye exams but can account for up to 20% of the eye’s refractive error .

13. 5th and 6th Order optical aberrations –also high order aberrations not currently measured in today’s eye exam. These aberrations are of less significance clinically, however they manifest in reduced vision for a small percentage of eyes.

14. The lower-order aberrations are Piston Tilt Defocus Astigmatism The 2nd order aberrations, defocus and primary astigmatism - are the most significant contributors to the overall magnitude of eye aberrations Lower-order aberrations

15. Remaining lower-order forms, piston and tilt, or distortion, are usually ignored . The former being not an aberration with a single imaging pupil, and The latter being not a point-image quality aberration).

16. Higher order aberrations Higher order aberrations are measured with wavefront aberrometers and expressed in terms that describe the shape and severity of the deviated light rays as they pass through the eye's optical system and strike the retina . Coma, spherical aberration, and trefoil are the most common higher order aberrations .

17. Coma causes light to be smeared like the tail of a comet in the night sky . Double vision is a common symptom of coma. Trefoil causes a point of light to smear in three directions, like a Mercedes-Benz symbol . Spherical aberration is characterized by halos, starbursts, ghost images, and loss of contrast sensitivity (inability to see fine detail) in low light.

18. Starbursts – Patterns of Small Lights Around Light Sources Haloes – Circles of Light Around Light Sources Ghosting – A Faint Duplicate of Each Object Similar to Double Vision Glare – Intensification of Light Sources. It's quite common for a patient to have an increase in all of these aberrations, resulting in distorted night vision when the pupil opens and allows light to enter through a larger area of the irregular corneal surface.

19. Coma A comet-like tail or directional flare appearing in the retinal image, when a point source is viewed . Because the eye is a somewhat nonaxial imaging device, and because the cornea and lens are not perfectly centered with respect to the pupil, coma generally is present in all human eyes . A large amount of coma (0.3 μm of coma alone) may point to known corneal diseases, such as keratoconus .

20. Spherical Aberration Fortunately, spherical aberration is relatively easy to understand. For a normal photopic eye, spherical aberration may vary from approximately 0.25 D to almost 2 D. Light rays entering the central area of a lens are bent less and come to a sharp focus at the focal point of a lens system. However, peripheral light rays tend to be bent more by the edge of a given lens system so that in a plus lens, the light rays are focused in front of the normal focal point of the lens and secondary images are created.

21. This is why many lens systems incorporate an aspheric grind, so that the periphery of the lens system gradually tapers and refracts or bends light to a lesser degree than if this optical adaptation was not included . The variation in index of refraction of the crystalline lens (higher index in the nucleus, lower index in the cortex) is responsible for neutralization of a good part of the spherical aberration caused by the human cornea.

22. Chromatic aberration Because the index of refraction of the ocular components of the eye varies with wavelength, colored objects located at the same distance from the eye are imaged at different distances with respect to the retina. This phenomenon is called axial chromatic aberration. In the human eye the magnitude of chromatic aberration is approximately 3 D .

23. Chromatic aberration However , significant colored fringes around objects generally are not seen because of the preferential spectral sensitivity of human photoreceptors. Studies have shown that humans are many times more sensitive to yellow–green light with a central wavelength at 560 nm than to red or blue light.

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