ABERRATIONS ASSOCIATED WITH SPECTACLE LENSES

ABERRATIONS ASSOCIATED WITH SPECTACLE LENSES

LENS ABERRATIONS

    One of the first decisions to be made while dispensing a new prescription is the choice of lens form.When the eye views along optical axis of spectacle lens, the form of lens does not matter and imageformed by lens is not afflicted with any defects or aberrations which might affect its sharpness or itsshape.          

    But in actual world, the eyes turn behind the lens to view through off-axis visual points and it is thenthe form assumes importance.

Ideally, the off-axis performance of lens should be same as its performance at the optical centre.But it is never so.

    The off-axis images are afflicted with various types of aberrations, which spoil the quality of imageformed by the lens.

    There are 6 major lens aberrations that work against obtaining a perfect image through theperiphery of the lens. They are :

    Chromatic Aberration     Spherical Aberration     Marginal Astigmatism     Coma     Curvature of Field     Distortion

1.CHROMATIC ABERRATION

    Chromatic Aberration is a defect in lens in which various colours 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 image.

    The wearer complains of peripheral colour fringes around object which is more pronounced off-axis. The higher the power of 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 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.

IMPORTANCE:

    Since chromatic aberration occurs because refractive index of the lens material varies with wavelength of incident light, it gives rise to what is called Abbe value of lens material which is denoted by V-vaue.

    Higher Abbe value implies low chromatic aberrations and vice-versa. So, Polycarbonate lens with Abbe value of 30 cause more chromatic aberration than CR39 lens with Abbe value of 58.

CORRECTION:

    The easiest solution to minimize chromatic aberration is to change the lens material to high abbe value.     Careful placement of optical centre with monocular pupillary distance and its height in small frame may reduce the chromatic aberration.     Reducing vertex distance may also result in minimizing the effect of chromatic aberration.     Anti-reflection coating with consumer education may also be tried to minimize the effect of chromatic aberration.     The best solution is Achromatic lens system. It uses two different lens materials - one has a regular focal length and other corrects dispersion of first lens. For this purpose one lens is made of crown glass, ie Low dispersion, while other is made of flint glass, high dispersion.

    The crown glass concentrates on optical effect and introduces some dispersion and the flint glass aims at balancing this dispersion while having least possible optical influence on the lens function.

OCULAR CHROMATIC ABERRATION

    Refraction by human eye is also subject to chromatic aberration. As a result, the focusing power of the eye is different for different wavelengths of light in real world of polychromatic objects.

    Consequently, there is no place in visual space where one can place an object and expect it be well focused on the retina for more than one wavelengths of light at a time.

    Hence, blue wavelengths focuses before the retina and red beyond the retina, ie, the far point on an individuals focus varies with the wavelengths of light.

    In terms of foveal vision, the dominant chromatic aberration is axial or longitudinal chromatic aberration.

    The causes of chromatic aberration are dispersion in cornea, aqueous, crystalline lens and vitreous humour.

    Dispersion is simply a variation in refractive index of material with various wavelengths of light and causes white light to be dispersed into various spectral colours, just as prism disperses light into a rainbow.

    Refractive surgery techniques can not correct chromatic aberration as this error is inherent to properties of ocular structures and not to shape of the ocular components.

2. SPHERICAL ABERRATION

    Spherical aberration is an axial and wide beam aberration. The light rays from the peripheral edge of the lens are refracted to a greater degree than the light rays passing through center 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.

CORRECTION:

    1. Spherical aberration may be reduced by occluding. Periphery of the lens such that only paraxial zone is used.

    2. Lens form may be adjusted to reduce spherical aberration. Aplanatic surface where periphery curve is less than the central curvature may be used.

OCULAR SPHERICAL ABERRATION

    Refractive power of ocular structure is greater for peripheral rays than paraxial rays. The effect of spherical aberration on retinal image is symmetrical blur like defocus which is reduced by size of the pupil.

INFLUENCING FACTOR:

    The effect of spherical aberration in human eyes is reduced by several factors:

    The anterior corneal surface is flatter at periphery than at its centre, and therefore acts as applanatic surface.

    The iris acts as a stop to reduce spherical aberration. The impairment of visual acuity that occurs when pupil is dilated is almost entirely due to spherical aberration.

    The nucleus of lens of the eye has a higher refractive index than the lens cortex. Hence, the axial zone of lens has greater refractive power than the periphery.     

    Finally, the retinal cones are more sensitive to light which enters the eye paraxially than the light which enters obliquely through peripheral cornea. This directional sensitivity of the cone represents limits the visual effects of residual spherical aberration in the eye.

CLINICAL APPLICATION:

    Spherical aberration accounts for much of the phenomenon known as “Night Myopia”.

    At low light levels, the pupil enlarges and allows more peripheral rays to enter the eye.

    The peripheral rays are focused anterior to the retina, rendering the eye relatively myopic in the lower light levels.

    The typical amount of night myopia is about 0.5D, but it can be as large as 1.25D.

3. MARGINAL ASTIGMATISM

    Marginal or Oblique Astigmatism aberration is small angle aberration. When a narrow beam of light enters obliquely to lens axis of a spherical lens, the refracted rays become astigmatic.

    The emerging rays, instead of uniting a single image point, form two foci at right angles to one-another with a disk of least confusion.

    On joining the two foci, a line image is created. The plane containing the optical axis of surface is referred to as “tangential plane” and plane at right angle to the tangential plane is referred as “Sagittal plane”.

    Another way to understand effect of marginal astigmatism is tilting a spherical lens adds spherical and cylinder of the same sign as original lens and is called astigmatism of oblique incidence.

    The effect of marginal astigmatism is that it produces blurring an image due to imposition of unwanted spherical-cylinder between lens and the eye.

    It also reduces contrast, and as aperture is opened wider

and wider, the astigmatic figure of marginal astigmatic aberration becomes typical cometic figure.

CORRECTION:

    Marginal astigmatism is small angle aberration, it is very important in ophthalmic lens designing. It is considerably affected by form of the lens used.

    It is much worse in bi-convex and bi-concave lens than meniscus lens form.

    It may be reduced by use of an aspheric surface or by a suitable choice of lens bending. ie. by using corrected curve theory.

    Proper use of Pantoscopic tilt with optical centre height may help reducing marginal astigmatism.

    Marginal Astigmatism is very important in ophthalmic optics and it is the aberration modern corrected curves design seeks to eliminate.

OCULAR ASTIGMATIC ABERRATION:

    Oblique rays passing through the pupil of eyes create astigmatic aberration in human eyes.The visual effect is not more than reduction of contrast.

INFLUENCING FACTOR:

    Applanatic curvature of the cornea reduces the marginal astigmatism as well as spherical aberration. The spherical surface of retina also results in minimizing the effect of marginal aberration.

    Finally, the astigmatic image falls on the peripheral retina, which has relatively poor resolving power compared to the macular area. Visual appreciation of astigmatic image is therefore, limited.

CLINICAL APPLICATION:

    Pantoscopic tilt in spectacle creates astigmatic aberration which becomes more substantial with high power and greater amount of tilt.     Myope sometimes tilts their glasses forward to gain more minus power, indicating a need for stronger prescription.Tilting forward induces both minus spherical and minus cylinder.     Oblique astigmatism aberration can also be a problem when objective measure of the refractive error of an eye is attempted. This is especially important when retinoscopy is performed on a patient who does not maintain fixation, as commonly seen in small children.

4. COMA

    Coma is wide beam aberration and is somewhat similar to spherical aberration. It is applied to oblique rays coming from points not lying on the principal axis.

    Oblique rays passing through the periphery of lens are deviated more than central rays and come to focus near the principal axis.

    The result is unequal magnification of image formed by different zones of the lens. The composite image is not circular, but elongated like a comet and coma.

    Coma is associated with the off-axis object points. It occurs because magnification is the function of the height of the rays of the light.

    It is the worst type of aberration as it degrades and deforms the image of a point object.

    The funny part is that a lens with considerable coma may produce a sharp image at the centre of the field, but becomes increasingly blurred towards the edges.

CORRECTION:

    The effect of cometic aberration can be minimized by using parabolic curves. Aspheric lens design helps reduce coma in high plus power.

OCULAR COMA:

    In human eyes, coma gives comet like image because of small decentration of the cornea and the lens, which results in different magnification in the different parts of the pupil.

    As most of the day time undilated pupil is around 2-3 mms in size, the ocular cometic aberration does not carry much of practical importance.

5.CURVATURE OF FIELD

    Curvature of field is phenomenon which cause the image formation of a plane to become curved like the inside of a shallow bowl, preventing the lens from producing a flat image of a flat object.

    This occurs even when the spherical aberration, marginal astigmatism and coma have been eliminated.

    The effect is largely dependent upon the refractive index of the lens material and the curvature of the lens surface.

    This causes an unequal vertex distance between the centre of the lens surface and its periphery. So the image so focused is either sharp on the edges or in the centre.

    When the centre of the image is in focus, the periphery is out of focus and when the periphery is in focus, the centre is out of focus.

CORRECTION:

    Curvature of field 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.

OCULAR CURVATURE OF FIELD:

    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.

6.DISTORTION

    Distortion is another aberration of thick lenses. In distortion the object is sharply imaged but does not retain its shape.

    There are two types of distortions resulting from lateral magnification of the image that results in a lateral displacement of the image. They are:

1. BARREL DISTORTION

2. PINCUSHION DISTORTION

1. BARREL DISTORTION:

Barrel distortion is produced in minus power lens where the rays in centre are more magnified than the rays farther off-axis.

This is due to minification of corners of a square grid more from minus lens.

2. PINCUSHION DISTORTION:

    Pincushion distortion is produced in plus lens where the central rays are less magnified.

    This is due the magnification of corners of a square object more from plus lens. Distortion is of minor importance in ophthalmic lens designing.

    It is a problem mainly of high power lens. It can be minimized by using steep back surface.

SUMMARY

    In designing a spectacle lens, the designer has very limited degree of freedom. Practional conditions specify lens materials, safety considerations fix lens thickness, and fashion dictates lens position before the face, weight and cosmesis mean only two lens surfaces may be used.

With so few degrees of freedom, few aberrations may be corrected.

    The wide angle spherical and cometic aberrations are of no importance since the entrance pupil of the eye is so small.