OPTO 4101: Refraction1 - PowerPoint Presentation

Slide1

OPTO 4101: Refraction1

Subjective verification for refraction

MSc Mohammed A.M Aljarousha

Department of Optometry

Faculty of Allied Health Sciences

Islamic University-GazaSlide2

Verification for spherical powerSlide3

Duochrome testSlide4

Typical wavelength of colorsVIBGYOR

Wavelength/ nanometer

Color

410

Violet

450

Indigo

550

Blue

570

Green

580

Yellow

610

Orange

660

RedSlide5

Duochrome testThe

index of refraction varies for different wavelengths of light.

The index of refraction is

greater

for short wavelength light rays than for longer ones.

Green light

is refracted to greater degree than

yellow light

, which is refracted to a greater degree than

red light

, and so on.

Incoming rays of white light are

dispersed

, with blue refracted to a greater degree than red.

In an

emmetropic eye

, therefore, yellow light is focused on the retina, green light is focused anterior to the retina, and red light is focused posterior to the retina.

In

myopic eye

, red is closer to the retina, while in hypermetropic eye green is closer.

If the

red green test

is used and the patient is myopic and insufficiently corrected , then the letters on the red side will stand out

blacker

,

clearer

and

sharper

. They will require more minus power for the green to be as distinct as the red.Slide6
Slide7

On the other hand if the same

myope is overcorrected and made

artificially hypermetropic

,

then the letter against the green background appear blacker, clearer and sharper.

The

sphere

is adjusted until letters on both sides are of equal quality.

Duochrome test

is useful only in

refining spherical power, contributing nothing to the determination of cylinder power or axis.Slide8

2. Trial and error

This method is simply adding +0.25Ds

and

-0.25Ds

in a monocular fashion and asking the patient if the vision is better or worse:

If the patient report that it is better without any lens, then he is

emmetrope

, and his refractions is correct.

If the patient report that it is better in adding the

+0.25Ds

, continue adding plus until no further improvement is achieved. Then add the new addition power for his previous prescription.If the patient responds that vision is worse, remove +0.25Ds or prepare to add -0.25Ds;

ask the patient if the letters are sharper and clearer or not?If it is better, continue adding minus until

no further improvement is achieved, and then add the new addition power for his previous prescription.

- This method of verification completely depends on

patient’s answers,

so the accuracy will be determined according to the patient observations.Slide9

3. Pinhole

The

pinhole lens

is a lens that has a hole in the center of lens, this hole has the effect of reducing the width of the bundle of diverging rays (called a ‘‘

pencil of light

’’) coming from each point on the viewed object.

Normally, the full opening of the pupil

admits

light.

It is the improper bending of the outermost rays in that pencil of light which causes refractive errors such as

myopia, hyperopia, presbyopia and astigmatism

to be noticeable.Pinhole can brings about clearer vision in all these conditions.By blocking these peripheral rays, and only lettering into the eye those rays which pass through the central portion of the pupil, any

refractive error in the lens

or

cornea

is not noticed as much.The pupil may be wide open, but only the central portion is receiving light. The improvement in visual acuity can be striking.

So if the

vision

is improved when looking through the pinhole lens, that’s mean the prescription is not accurate, not completely correct and then verification is necessary to be done. Slide10

Verification for cylindrical power ‘‘astigmatism’’Slide11

1. Astigmatic fan

It is used to verify the

axis of trial cylinder lens

.

On looking at such fan if any of the lines

are seen

more clearly

than the others astigmatism must be present.

If the vertical line is

clear

more than the horizontal meridian it is the less emmetropic one (has the defect). (Inverse relation).

more clear line = less emmetropic.Less clear line = more

emmetropic.A cylinder lens is placed in front of the eye with its axis horizontal will correct the vertical meridian and when the correct glass power is to be worn all the lines of the astigmatic fan

appear equally distinct

.

The cylinder which thus renders the outline of the whole lines equally clear is the amount of astigmatism and the axis of the cylinder is at right angle to the line which was initially seen most clearly (less emmetropic) according to the sign of the cylinder lens. Slide12

2. Staenopic slit

It

is essentially an accessory in the trial set that consists of an elongated pin hole aperture as a slit cut in an opaque disc, used to detect

the axis of astigmatism if present

.

When

put before the eye it allows

only

rays of light in a particular meridian to enter the

eye.

If the slit is placed

horizontal

before the eye the vertical meridian is pin holed and produces a point image.

After correction of any

spherical ammetropia

all meridians in 360 degree can be seen equally on

rotating the staenopic slit

.

In presence of a high astigmatism and before correction , the staenopic slit can detect the axis

of correcting cylinder if it's properly positioned

.

The slit is sometime useful in determining the astigmatism when other devises are

unsatisfactory

.

When the

image

is equally clear in 360 degree in presence of the slit no astigmatism is present , all lines are equally clear

.Slide13

N.B. The main difference

between the stenopic slit and astigmatic fan is the slit of this device detect the

less ametropia

or the

emmtropic meridian

(has the defect)

and the axis of the correcting cylinder lens is put on this meridian

(at zero power).

Ametropia= more clear

Ametropia= less clearSlide14

3. Rotation of dial screwIt is simply

done by asking the patient to rotate the screw of the trial frame

to rotate the

cylindrical lens

in the trial frame, the patient will rotate the screw until he see the

best clear image.

At this point the rotation is

stopped,

and we can determine the accurate axis of the

cylindrical lens.Slide15

4. Jackson cross cylinder

Cross cylinder is

composed

of a plus cylinder and a minus cylinder of equal power that are

ground

on a lens, with their

axes

at right angles to each other.

The commonly used

cross cylinders

are of ± 0.25D and ± 0.5D.All cross cylinder lenses have a spherical equivalent power of zero (Plano).The cross cylinder may be used to refine the patient’s cylinder axis or power after estimation by either objective (retinoscope) or subjective (astigmatic dial) means.

To use the cross cylinder to refine the axis of the patient’s eye, the cross cylinder is placed over the trial lens in such a manner that the axes of the cross cylinder straddle the trial lens axis.

For example, if the trial lens has a cylinder axis at 90, the cross cylinder is placed so that one axis is at

45

while the other axis is at

135.The cross cylinder is then rotated 180 (flipped) so that the meridian was originally at 45, is now at 135 and vice versa.Slide16

If the patient reports

equally blurred vision in each of the positions of the cross cylinder, the trial lens axis is

correct

.

If however the vision is better in one position, the trial lens is moved in this direction. When using

plus cylinder

, the trial lens axis is moved toward the cross

cylinder plus axis.

When using

minus cylinder

, the trial lens is moved the cross cylinder minus axis.- Next the

cross cylinder is repositioned again to straddle the trial lens axis, and the procedure is repeated until the patient reports equally blurred vision in both positions of the cross cylinder.