Refractive Error is a problem of focusing the light on the surface of retina due to the shape of eye.
> Most common type of refractive error are
> They are usually corrected with the help of spectacles , contact lenses and refractive surgeries.
>Problems with the high refractive errors are:
• Greater thickness of lens
• Weight of lens
• Subject to aberrations
• Magnification properties
• Visual field limitation
>Problems with the aphakia spectacle wearer:
• Increased retinal image size
• Decreased field of view
• Increased ocular rotation
• Increased lens aberration
• Motion of objects in the field of view
• Appearance of wearer
• Demand of convergence
> INCREASED RETINAL IMAGE SIZE
The normal phakic eye values
Refractive power of eye = +58.64D
Anterior focal length = 17.05mm
The aphakic schematic eye values
Refractive power of eye = +43.05D
Anterior focal length = 23.23mm
The correcting lens is placed at anterior focal point of the eye
Retinal image size is directly proportional to anterior focal length and inversely proportional to refractive power of eye
In short: increase in retinal image size by 36%
Retinal image size in aphakia = 23.23 = 1.36
Retinal image size in emmetropia = 17.05
>DECREASED FIELD OF VIEW
The prismatic effect of a high plus lens reduces the size of field of view through lens
> PRESENCE OF RING SCOTOMA
The strong plus lens causes an angular gap in object space completely around the lens
The patient sees nothing either through the lens or beyond the lens unless a head movement is made
Beyond the scotomatous area wearer has uncorrected vision and centrally to scotoma the wearer has corrected vision
Jack in the box phenomenon
Lens power : stronger the lens larger the ring scotoma
Vertex distance : farther the lens from eye smaller the scotoma and moving the lens farther out makes scotoma move centrally
Lens size : the lens size increases the ring scotoma increases in size but it also moves towards periphery
Pupil size : the smaller the pupil larger the ring scotoma
Lens thickness : the lens thickness increases so does the magnification and the size of ring scotoma
Base curve : with increasing steepness of base curve both the magnification and the size of the ring scotoma increases
> INCREASED OCULAR ROTATION
When the spectacle is worn the angle of eye must turn in changing fixation from one object pt to another increases or decreases
The excursion of eye is increased for plus lenses and decreases for minus lenses
The increase or decrease of ocular rotation is caused by the presence of prismatic effect
> INCREASED LENS ABERRATION
Curvature of the image
> MOTION OF OBJECTS IN FIELD OF VIEW
When a high ‘+ve’ lens wearer holds the eyes steady and moves the head to look an object indirectly, a reversed motion (against motion) of field of view is experienced. This motion is known as “Swim”. It occurs when the field from which visual axes are moving away is seen in indirect vision
It can be avoided by moving the eyes from one fixation point to another while the head is held stationary
> APPEARANCE OF WEARER
It may convey 2 impressions
1st impression: apparent enlargement of the eyes & their unusual appearance behind bulbous, thick lenses
2nd impression: General awkwardness of the patient’s bearing and movements
Both these are come from the magnification & distortion effects of lenses
> DEMAND OF CONVERGENCE
If the distance center of aphakic lenses are placed in front of the center of the wearer’s pupil, convergence of the visual axes towards a near fixation point creates a base out prismatic effect
Because of this the eyes when reading, must converge considerably more than for a wearer of lenses of low power
> CORRECTION FOR HIGH MYOPIA
PROBLEMS CAUSED BY HIGH MINUS LENSES
> LENS WEIGHT AND THICKNESS
Lens weight is not a problem for myope as a plastic lens is used
Lens thickness is more of a problem for myope
High minus lenses causes minification
Both the apparent size of the wearer’s eyes and the retinal image size increases
> MINIMIZING THE EDGE THICKNESS
Small lens size
High index material
> SMALL LENS SIZE
Lens edge gets thicker farther away from the center
Frames with rounded corners should be used
> HIGH INDEX MATERIAL
1.64 to 1.74 are high index
Advantages of this are lens will be lighter, thinner, good cosmesis and reduced magnification.
> ASPHERIC LENSES
A lens in which one or both surfaces are not spherical
Doesn’t have same radius of curvature over the entire surface
Optically correct oblique astigmatism produced when looking through lens.
In the middle, starts out as spherical surface
> EDGE REFLECTION AND CONCENTRIC RINGS
Edges of the high minus lenses produce multiple rings seen - true images of the edges of lens.
Caused by total internal reflection.
Total internal reflection repeated and multiple ring reflections seen by observer
Rings more pronounced on oblique viewing than straight ahead position.
> WAYS TO REDUCE EDGE REFLECTION
Buffing the edge
Tinting the lens
> LENTICULAR LENSES
Used in case of very high myopes
The central area of the lens contains the prescribed refractive power of the lens. The peripheral area is the carrier of lens.
Front surface is almost flat contains the cylinder component of prescription.
A high minus “bowl” in the middle of back surface. Plano back carrier area
2. MINUS LENTICULAR
Carrier is not plano. Instead back surface of the carrier is made positive so the outer edge will thin down considerably.
3. BLENDED LENTICULAR
Optical aperture is blended into the flange eliminating the obvious line and improving the appearance
> HIGH INDEX LENSES
Normal index – 1.48 to 1.53
Mid index – 1.54 to 1.63
High index – 1.64 to 1.73
Very high index – 1.74 to above
> High index available in
Single vision designs.
Aspheric and atoric designs.
High index, especially the index of 1.67 and 1.70, is also an excellent choice for rimless frames
Polycarbonate or Trivex for children
> ADVANTAGE OF HIGH INDEX LENS
1. Power rings round the edges of the lens is
2. Reduce thickness
3. Look flatter
6. The benefit is greater peripheral vision as well as less magnification or minification of the eye
> DISADVANTAGE OF HIGH INDEX LENS
1. Reduced Low Abbe number
2. Poor light transmission and increased backside and inner surface reflection increasing
3. More expensive than normal lenses
4. Increase chromatic aberration
> WAYS TO REDUCE ABERRATIONS ARE:
> WAYS TO REDUCE CHROMATIC ABERRATION
Using an achromatic lens or achromat
The most common type is an achromatic doublet
Using apochromatic lens or apochromat.
> SPHERICAL ABERRATION
Use an aperture in front of the lens
Like spherical aberration, coma is a wide angle aberration
But it applies only to off axial rays.
The comatic blur figure is asymmetric and "comet shaped", hence the name coma.
Correction : Using bestform or aplanatic lenses
Choosing the correct curvature of lenses
Narrow pencil of ray from a extra axial object point to pass through, after refraction give two focal lines perpendicular to each other, that is the pencil has become astigmatic. This aberration is oblique astigmatism.
Occurs due to oblique incidence of light rays on spherical surface.
Use meniscus lenses, instead of biconcave or biconvex.
Orientation of lens so that astigmatism is minimized.
> CURVATURE OF FIELD
Curvature of field is another oblique aberration
In curvature of field a plane object is sharply imaged, but on a curved surface.
It may be handled by bending the lens or curving the receiving the screen.
Correction: insert a field stop (iris) in order to remove edge light rays
Another oblique small angle aberration is distortion.
In distortion the object is sharply imaged but doesn't retain its shape.
2 kinds of distortion are barrel distortion and pincushion distortion.
Distortion can be dealt with through placement of the aperture or by using lens systems, e.g. a so called orthoscopic doublet.
> ENHANCING COSMESIS
REDUCING LENS THICKNESS
Selecting a smaller lens
A thinner edge reduces light entering into the edge, reducing an additional source of internal reflections.
Extremely thick lenses for myopia can be beveled to reduce flaring out of the very thick edge
Ways to reduce the weight and thickness of corrective lenses :
Choose glasses frames with small lenses
Choose a frame that allows the pupil to occupy the exact middle point of the lens.
Choose a lens as near round as possible.
Choose as high a refractive index.