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Ophthalmic lens aberrations
- 1. Ophthalmic Lens Aberrations Shahana Nethradhamaschool of optoometry
- 2. Aberration: • Aberration isa property of optical systems such as lenses that causes light to be spread out over some point. • Aberration cause the image formed by a lens to blurred or distorted.
- 3. Types of aberrations: Thereare six major lens aberration: i. Chromatic aberration ii. Spherical aberration iii. Marginal astigmatism iv. Curvature of field v. Coma vi. Distortion
- 4. Chromatic aberration • Chromaticaberration is a defect in the lens in which the various colors of the spectrum are not brought to the same focus. • Blue light is refracted more than the red light when it passes through a lens the result is out of focus. • The higher the power of the lens the greater is the chromatic aberration. • Chromatic aberration depends upon the material of the lens. Since the lens materials have a different refractive index for each wavelength • The lens will have a different focal length for each wavelength. The refractive index is larger for blue than the red wavelength so focal length is less for blue than the red.
- 6. Correction: • The easiestsolution to minimize chromatic aberration is to change the lens material to higher abbe value. • Careful placement of optical centre with monocular pupillary distance and its height in a small frame may reduce the chromatic aberration • Reducing the vertex distance may also result in minimizing the effect of chromatic aberration • Anti-reflection coating with consumer education may also be tired to minimize the effect of chromatic aberration • The best solution is achromatic lens system
- 8. Ocular chromatic aberration: Refractionby the human eye is also subject to chromatic aberrations. As a result the focusing power of the eye is different for different wavelength of light. The blue wavelength focuses before the retina and red beyond the retina. The cause of chromatic aberration are dispersion in the cornea, aqueous , crystalline lens and vitreous humour.
- 9. Spherical aberration • Sphericalaberration is an axial and wide beam aberration • The light rays from the peripheral edge of the lens are refracted to a greater degree then the light rays passing through the centre of the lens. • Peripheral rays bend more than the paraxial rays this creates a slight blurring of the image that is minimized by the size of the lens.
- 11. Correction • Spherical aberrationmay be reduced by occluding the periphery of the lens such that only the paraxial zone is used. • Lens form may also be adjusted to reduce spherical aberration.aplanatic surface where periphery curves is less than the central curvature may be used.
- 13. Ocular Spherical Aberration •Refractive power of the ocular structure is greater for peripheral rays than paraxial rays.The effect of spherical aberration on retinal image is a symmetrical blur like defocus which is reduced by the size of pupil.
- 14. Marginal Astigmatism • Marginalastigmatism or oblique astigmatism aberration is a small angle aberration • When a narrow beam of light enters obliquely to lens axis of spherical lens the refracted ray become astigmatic • The emergent ray instead of uniting in a single image point form two foci at right angles to one another with a disk of least confusion
- 16. Correction • It maybe reduced by the use of aspheric surface or by a suitable choice of lens bending. • Proper use of pantoscopic tilt with optical centre height may help reducing marginal astigmatism
- 17. Ocular Astigmatic Aberration Obliquerays passing through the pupil of the eyes create astigmatic aberrations in human eye. The visual effect is not more than reduction of contrast.
- 18. Coma • Coma isa wide beam aberration and is similar to spherical aberration • Oblique rays passing through the periphery of the lens are deviated more than the central rays and come to focus near the principal axis • The result is unequal magnification of the image formed by the different zones of the lens. The composite image is not circular, but elongated like a comet and coma.
- 20. Correction • The effectof cometic aberration can be minimized by parabolic curve. • Aspheric lens design helps reduce coma in high plus power.
- 21. Ocular coma In humaneyes coma gives a comet like image because of small decentration of the cornea and lens which result in different magnification in the different parts of the pupil.
- 22. Curvature of field •Curvature of the field is a phenomenon which causes the image formation of a plane to become curved like. • The effect is largely depend on refractive index of the lens material and curvature of the lens surface.
- 24. Correction • Curvature offield affects peripheral vision. Curvature of field is minimized with corrected curve design base curvatures. As a rule if marginal aberration is corrected, the effect of curvature of field is also reduced. Good correction of astigmatism results in small curvature of field.
- 25. Ocular curvature offield • In the human eye the curvature of the retina compensates for the effect of curvature of field. So this is an advantageous aberration in the human eye.
- 26. Distortion • Distortion isanother aberration of thick lenses. In distortion the object is sharply imaged but does not retain its shape. • There are 2 types of distortion 1. Barrel distortion: it is produced in minus power lens where the rays in the centre are more than the rays in the off-axis. This is due to the minification of corner of the square grid. 2. Pincushion distortion: it is produced in plus lens where the central rays are less magnified. This is due to the magnification of the corner of the square grid
- 28. Correction • It canbe minimized by using steep back surface.