Optical Aberration

APPLYING KNOWLEDGE OF OPTICAL ABERRATIONS IN ENHANCING PATIENT’S VISUAL CLARITY

CHAPTER THREE

RELATED TASKS

• Describe aberration , monochromatic and chromatic
• List types of aberration
• List monochromatic aberration
• Describe spherical aberration
• Describe coma aberration
• Describe astigmatic aberration
• Describe curvature of field aberration
• Describe distortion aberration
• Describe transverse chromatic aberration
• Describe tangential chromatic aberration
• Explain how to overcome chromatic aberration

Demonstrate how to overcome aberration.

INTRODUCTION TO ABERRATION

Aberration, may be referred to a defect in an optical system that causes light rays to fail to converge at a single point, leading to a blurred or distorted image. OR Is the inability of a lens to bring all incident rays to a desired image point.

Optical aberrations are defects in an optical system that degrade the quality of the dioptric image, even when the image is in focus.

It is important to have knowledge of the aberrations pf the eye because they impair the quality of the retinal image.

There are two general categories of optical aberrations ;

Monochromatic aberration that occurs when a single wavelength of light is employed.

Chromatic aberration that result from the use of mixed wavelength of light eg white light.

MONOCHROMATIC ABERRATIONS

• Also known as Von Seidel aberration.
• Are a characteristic of the shape or configuration of an optical system.
• There are five monochromatic aberrations that can affect the uality of the retinal image.
• These are;
• Spherical aberration
• Comma aberration
• Oblique or radial astigmatism
• Curvature of field

Distortion

Spherical aberration

Spherical aberration exists when the marginal rays ( rays refracted through the peripheral parts) and paraxial rays fail to come to focus in the same plane.

Occurs when rays from axial object are refracted by the periphery of the lens.

Pencil of light refracted on the large aperture of refracting system, where rays tend to refracted on diff zones which have different prismatic effects.

Maybe positive spherical aberration or negative spherical aberration ; where in positive the marginal rays comes to focus infront of paraxial rays. ( more refractive power at the peripheral) and vice versa.

The extent of aberration can be expressed in one or two ways ;

Longitudinal aberration; refers to dioptric distance between points where the marginal and paraxial rays come to a focus ( dioptric difference btn the two regions.) Lateral aberration ; denotes the radius of the patch of light produced by the marginal rays in the plane of focus.

2. Coma aberration

Coma affects the clarity of images formed for “off axis” objects.

Coma occurs when rays from an off axis point source come to focus in the same image plane, but they fail to focus at the same point in that plane.( refracted on the periphery) Variation in linear magnification cause coma that lead to asymmetry in the image.

Forms a comet shape like image of a point source.

When the rays through the paraxial region of the optical system comes to focus closer to the optical axis within the image plane is referred to as positive coma and vice versa.

3. Oblique or radial astigmatism

occurs when off-axis points are refracted near the optic axis and the image is blurred due to the resulting astigmatic effect.

also known as radial or maginal astigmatism caused by incidenting ray at an oblique angle leading the pencil of light to form two image lines at two different focal locations separated by an interval of sturm involves the part of the lens which is in use doesnot affected by the pupil

it is produced by spherical lens not astigmatic lens

• due to prismatic effect

image produced by oblique astigmatism is known as image shells this is from the set of lines corresponding to each focus so as to look like a shell structure

the shells are arranged according to their meridians which are tangential and sagittal

• tangential shells are horizontal arranged looks like a cup upper part
• saggital shells are verticlly arranged so it form an image like a saucer
• they are perpendicular to each other

by combination of these image lead to teacup and saucer oblique astigmatism can not be overcomed by any form of the lens instead is overcomed by tilting the front part of the frame here have to deal with pantascopic tilt

4. Curvature of field

• it is a curved image surface from the flat object surface

occurred when the object surface doesnot match with the image surface always the lens tends to affect the vergence of object rays which follows the curvature of the lens which is not equal to that of the cornea and retina image become out of focus and therefore blurred image surface is known as petzval's surface can be overcomed by making the lens back curvature which resemble with retina, for cameras image surface should be curved

5. Distortion aberration

Is an aberration occurred btn the ray which passes at the centre of the lens and the actual principal ray.

Is produced by unequal linear mx across the image plane. depending on the refractive nature and position of the aperture.

• Happens when magnification change through lens.
• This is regarded as a spherical aberration of the principal ray

Image produced is sharply defined

But the marginal points are the one affected by either highly magnified or minified so as to displace them far or closer than the respected points of object

Types of distortion

• Barrel distortion;- peripheral image points are too closer to centre than the ideal image
• Marginal rays are being minified
• Produced by minus lens
• Pincushion distortion;- peripheral image points are further away than the ideal image
• marginal rays are being magnified

Produced by plus lens

Chromatic aberration

Is a type of aberration which is due to dispassion of the white ray into its consistence wavelength when passed through the lens The dispassion lead to blurred image As it is due to un equal refraction of wave length through a lens Chromatism is directly proportional to the lens power and inversely proportional to abbe number

Types of chromatic aberration

Axial chromatic aberration/ longitudinal; different colors focus at different distance along the optical axis( in front or behind retina) causing focus shift and blur. The effect is constant over the entire image.

Transverse chromatic aberration/lateral ; different colors focus at different positions in the image plane.

Way of over coming C.A

• Using a lens material with high abbe number
• Using a lens with low refractive index
• Reduce back vertex distance
• Increase facial wrap

Use a small eye size spectacles

Assignment

Explain on how the eye overcome its own aberration.