Dr. Richa Naik

How to prescribe a spectacle to a person??• Step 1:• Objective refraction1. Retinoscopy2. Auto refractometer3. Photorefraction4. Electrophysiological method• Step 2 :• Subjective refraction.

Myope

Myopia upto 6D : in children <8 years of age….1. full correction2.Constantly wearing of glasses to avoid

developing squinting and to enhance developing accommodation

Always undercorrect myopes.Always advise the patient to choose the

lens that makes the letter more clear and not the one which makes the letter smaller and darker.

in case of exophoria minus correction can be given.

in adults <30 years…full correction in adults >30 years…not able to tolerate full

correction over 3D

High myopia > 10D undercorrection is always better to avoid

problem of near vision and that of minification of images.

Pseudomyopia

Cycloplegic refraction(atropine or HA)• If tendency for accommodative spasm- Cycloplegic drops- Plus over correctionIncrease accommodative facility by exercise

Hypermetropia

Cycloplegic refraction is must.• If manifest error is small..i.e. 1D or small, correction

is given only if the patient is symptomatic.• Children <4 years- accept full cycloplegic

correction once a child reaches school age, reduce the plus

lenses to 1/3 of refractive prescription( but child is not allowed to accommodate more than 2.5D)

Older children- may not accept full correction so 1st undercorrect and gradually increase the

spherical correction at 6 month interval till he accepts manifest

hypermetropia.

If there is associated exophoria undercorrection for about 1-2 D

In the presence of accommodative convergent squint, full correction at 1st sitting

Remember_ hypermetropia may diminish with growth of child..so refraction should be carried out every 6 months.

Try to give manifest correction for adults.

Astigmatism

• Adult- 1st time diagnosed• Try optimal correction• Undercorrection is acceptable with maintaining

the spherical equivalent• Rotate axis towards 90 & 180• Check binocular vision• Check one or both axis to be parallel• adult- already astigmatic• It can be due to change in power• - see pts comfort• -may require undercorrection• Axis- try to maintain previous one• -see binocular vision

Astigmatic dial techniqueFog the eye( to relax

accommodation) with enough plus lenses by creating compound myopic astigmatism.

Patient is asked to identify darkest and sharpest line..

Minus cylinder added perpendicular to that axis

Rule of 30Switch to distance vision

chart and reduce plus lenses

Astigmatic fan testAdd plus lensRefer the patient to

the fan chart and ask which line or group of lines appear clearest & darkest

Directing attention to the maddox arrow

Directing attention to the blocks

fogging Place enough PLUS lenses to FOG vision to

~6/12 line

Slowly reduce the plus power until best VA is obtained

Remember:“Maximum plus power for best visual

acuity”

Duochrome testBased on chromatic aberration Green letters clearer = Add ‘+ 0.25DS’ Red letters clearer = Add ‘- 0.25DS’ End-point is obtained when the letters on the

RedGreen chart appears equally dark or when a reversal occurs.

Pin hole test

Pin hole is put in front after correction if patient is improving than our prescribed lens is undercorrected.

JCC used to determine the cylindrical axis and the cylindrical power for the patient.

Binocular BalancingThe technique is also known as "equalising". During the monocular refraction, a different

state of relaxation of accommodation may occur because one eye was under test while the other was not.

Thus, binocular balancing is performed to balance between eyes.

1.Fogging and Alternate occlusion method2.Duochrome test with fogging3.Prism dissociation method

Presbyopia

The amount of presbyopic correction can be calculated if the remaining amplitude of accommodation(for his near point) is determined and his working distance is specified.

i.e. A emmetropic patient has remaining amplitude of accommodation 3D(near point 33cm). In order to achieve comfortable near vision he must keep one third of this in reserve….so he must use only 2D. If he wishes to see 25 cm clearly he needs 4D of accommodation n so requires 2D of presbyopic correction.

Spectacle for presbyopicsBifocals1.Franklin split bifocals2.Franklin cemented bifocals3.Fused4.Double segment 5.Solid TrifocalsProgressive

Franklin split bifocal lensEarliest designA distance lens

whose flat bottom abuts the flat top of a separate near lens.

Franklin cemented bifocalsNear portion is

constructed by attaching supplementary lens to the surface of a distance lens of same RI.

Ulraviolet cured epoxy resin used as adhesive.

Almost obsolete.

Fused bifocals2 different material is

used…button is of flint glass and main lens is of crown glass.

Advantage- inconspicuous dividing line, mechanical stability and low cost

Disadvantage- chromatic aberrations

Solid(executive) bifocalSingle piece

constructionNear addition is

produced by a different curvature of either- back or front surface

Full width horizontal junction

i.e.Plastic bifocals

Double segment(trifocal)Close work above

eye level i.e. librarian, electrician and painter

Fused as well as 1 piece

Progressive lens Power of lenses

change gradually between the distance and near zones.

No visible interface between zones.

2 types1.Hard2.Soft (newer)

Types of lensesFlat lenses1.Biconcave or biconvex2.Plano-concave or

plano-convexCurved lenses1.Meniscus lens2.Toric lens3.Lenticular lens4.Apheric lens

Meniscus lens Base curve-semifinished lens Base curve is added

to anterior surface and optician grinds its other surface to get required power

deep meniscus lensPeriscopic lens

Lenticular lensesUsed for high powerCentral portion –

power – aperture – 30mm

Peripheral portion – carrier – 1.2-2 mm thinner than central part

Reduce the weight of spectacle and aberrations

Toric lensWhere one surface is spherical and other surface is

toroidal.What is toroidal surface? if we visualise a cylinder its one axis is curved while

other is straight which is the axis of cylinder…now straight axis is also curved then surface will become toroidal.

Spherical lens is ground on anterior surface and posterior surface is made toroidal

Base curve used 6DDifference between base curve and curvature of

toric surface equals the cylindrical power

Aspheric lenses used to make high

plus aphakic lenses by modifying the lens curvature peripherally to reduce aberration and provide better peripheral vision

Base curve for aphakic lens is relatively flat.

Lens materialPolycarbonate lensPhotochromatic lensHigh index lensPolarised lensTinted lensTrivex lensAnti reflective coating lensYellow filter

Polycarbonate lenses These lenses are impact-resistant and are a

good choice for people who regularly participate in sporting activities, work in a job environment in which their glasses may be easily scratched or broken

for children who may easily drop and scratch their glasses.

Polycarbonate lenses provide ultraviolet protection.

High index lenses

 Designed for people who require high power prescriptions, these lenses are lighter and thinner than the standard thick lenses that may otherwise be needed.

Polarised lensesLight reflected from

water or a flat surface can cause unwanted glare. Polarised lenses reduce glare and are useful for sport and driving.

Photochromatic lenses Made from either glass

or plastic, these glasses change from clear to tinted when exposed to sunlight. This eliminates the need for prescription sunglasses. These lenses may not darken in a car because the windscreen could block the ultraviolet rays from the sun.

Tinted lensDecreases transmittanceDone when patient is uncomfortable in bright

lights or exposed to UV radiation

Transmittance level Uses 75-80% Indoor uses

20-25% Mountain climbing , flying

20% sunglasses

Trivex lensesMade from a newer plastic with similar

characteristics of polycarbonate lenses. It is light weight, thin and impact-resistant and may result in better  vision correction in some people than polycarbonate lenses.

Anti reflective coating glasses for IRcopper and gold coating reflect approximately 98%

of IR above 750 nmYellow filtersShooter’s glassesIt increases contrast for longer wavelength objects

viewed against shorter wavelength background

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