Biooptics - Abberations PDF

❑ Optical Aberrations Lenses do not form perfect images, and there is always some degree of distortion or aberration introduced by the lens which causes the image to be an imperfect replica of the object. Aberration generally leads to blurring of the image. It occurs when light from one point o...

❑ Optical Aberrations Lenses do not form perfect images, and there is always some degree of distortion or aberration introduced by the lens which causes the image to be an imperfect replica of the object. Aberration generally leads to blurring of the image. It occurs when light from one point of an object after transmission through the system arrives in different points. Instrument-makers need to correct optical systems to compensate for aberration. There are several different types of aberration which can affect image quality. ❑ Spherical Aberration The spherical aberration of the cornea is usually positive whereas the young crystalline lens exhibits a ❑ Chromatic Aberration negative spherical aberration. Besides, there is strong evidence of compensation for aberrations between ❑ Comatic Aberration the cornea and intraocular optics in cases of astigmatism (horizontal/vertical) and horizontal coma. The appearance of visual complaints such as halosThe appearance of visual complaints such as halos, glare. The appearance of visual complaints such as halos, glare and monocular diplopia after corneal refractive surgery has long been correlated with the induction of optical aberrations. ❑ Spherical Aberration Spherical aberration is caused because spherical surfaces are not the ideal shape to make a lens, but they are by far the simplest shape to which glass can be ground and polished and so are often used. Spherical aberration causes beams parallel to, but away from the lens axis to be focused in a slightly different place than beams close to the axis. This manifests itself as a blurring of the image. Lenses in which closer-to-ideal, non-spherical surfaces are called aspheric lenses. They have less peripheric curvature compared to central curvature. Spherical aberration can be minimized by using aspheric lenses. Cemented an additional aspheric lens (weaker lens of different refractive index) can minimize spherical aberration Spherical aberration is an optical effect observed in an optical device (lens) that occurs due to the increased refraction of light rays when they strike a lens near its edge, in comparison with those that strike close to the centre. Using a «stop» in front of the lens such as iris can minimize Spherical aberration Spherical aberration can be minimized by using aspheric lenses. The effect of spherical aberration in the human eye is reduced by several factors: Corneal surface is flatter peripherally than its center, therefore acts as aplanatic surface Center of the eye lens has higher refractive index than the periphery, so axial zone of the lens has greater power than the refractive periphery Iris acts as a stop to reduce spherical aberration Pupil constriction Retinal cores are much more sensitive to light which enters the light paraxially than the light which enters obliquely through the peripheral cornea Spherical aberration for a normal photopic eye may vary from 0.25 D to 2 D. Spherical Aberration-correction Spherical aberration in the human eye is reduced by the aspheric shape of the lens and the cornea ❑ Chromatic Aberration Chromatic aberration is the problem of different colors of light being focused at different points along the optical axis. It is caused by the dispersion of the lens material, the variation of its refractive index n with the wavelength of light. Since the focal length (f) of a lens is dependent on n, different wavelengths of light will be focused on different positions. Chromatic aberrations can be longitudinal, where different wavelengths are focused at a different distance from the lens; or lateral, where different wavelengths are focused at different positions in the focal plane. ❑ Chromatic Aberration Chromatic aberration can be further minimized by using an achromatic doublet or achromat in which two materials with differing dispersion (usually crown and flint glass) are bonded together to form a single lens. This reduces the amount of chromatic aberration over a certain range of wavelengths, though it does not produce perfect correction. In humans, the eye lens exhibits chromatic aberration (approximately 3D). A yellow pigment in the fovea called the macula lutea helps to protect us from this problem. Sensitivity of human photoreceptors is different, so coloured fringes around objects are not seen by the human eye. Human eye is more sensitive to yellow-green light with central wavelength at 560nm compared to red or blue light  North AJ. J Cell Biol. 2006 http://www.ncbi.nlm.nih.gov/pub med/?term=10.1083%2Fjcb.200 507103 The effect of chromatic aberrations on images. A-D show Multicolor beads: A) The lateral shift between blue and green signals is minimal. However, when rotated to view from the z-axis (B), the green and blue images appear separated by ∼600 nm, due to chromatic aberrations. After applying a 600-nm corrective z-axis shift to the blue image the two colors now appear superimposed (C and D). E–F show images of some nuclear structures. After applying the chromatic correction (G and H), the majority of spots are seen to be nuclear. Thus, chromatic aberrations can lead to incorrect conclusions by uninformed users.  ❑ Comatic Aberration Coma is an image degrading aberration associated with a point, even a short distance, which causes rays from an off-axis point of light in the object plane to create a trailing "comet-like" blur directed away from the optic axis. In general, a bundle of parallel rays passing through the lens at a fixed distance from the center of the lens is focused on a ring-shaped image in the focal plane, known as a comatic circle. The sum of all these circles results in a V-shaped or comet-like flare. As with spherical aberration, coma can be minimised (sometimes eliminated) by choosing the curvature of the two lens surfaces to match the application. Lenses in which both spherical aberration and coma are minimised are called best-form lenses. Off-axis: Looking at a point which is not aligned with the optical axis Coma is not well compensated for in the human eye. ❑ Refractive error: Astigmatism Astigmatism usually is caused by an irregularly shaped cornea or lens. Instead of the cornea having a symmetrically round shape (like a ball), it is shaped eliptical, with one meridian being significantly more curved than the meridian perpendicular to it. Distance and close Vision will both Appear blurry corneal astigmatism or lenticular astigmatism  Oblique Astigmatism This aberration primarily influences the image quality of spherical lenses. When‫ﺮاف‬the ‫اﻧﺤ‬ wearer looks at an angle through the lens, there is a deviation which he perceives as blur. The higher the dioptric power of the lens, the more pronounced this error becomes.  Oblique Astigmatism A dot is no longer imaged as a dot but as two image lines. ❑ Refractive error: Hypermetropia ‫دورﺑﯿﻨﯽ‬ Hypermetropia (long-sightedness) is a common eye condition where nearby objects appear blurred, but your vision is clearer when looking at things further away. 'plus' or convex in shape EX: light refractive convex lens rays compared index toiscornea passing of a corrective 1.5,through what happens the lensto ❑ Refractive error: Myopia ‫ﻧﺰدﯾﮏ ﺑﯿﻨﯽ‬ Myopia is an eye disease where light focuses in front of, instead of on, the retina. As a result, distant objects appear blurry while close objects appear normal. diverging lense Image Distortion Plus lens Minus lens  Custom Lasik-Wavescan Custom can correct for – Myopia or hyperopia – Astigmatism – Spherical Aberration – Coma Custom does not correct for – Chromatic aberration – Diffraction