Company

LOGO Dr Sneha Thapliyal

OBJECTIVE REFRACTION

CLINICAL REFRACTION

Determines and corrects refractive errors

Objective refraction Subjective refraction

Essential in:Young childrenMental disabilityLanguage difficultyDeaf or mute

OBJECTIVE METHODS OF REFRACTION

1. Retinoscopy2. Autorefractometry3. Photorefraction4. Electrophysiological method of objective

refraction

RETINOSCOPY/SKIASCOPY/SHADOW TEST

Objective method of finding out the error of refraction by means of retinoscope utilizing the technique of NEUTRALIZATION

Far point concept The far point of eye is defined as the point in space that is conjugate with the

fovea when accomodation is relaxed.

Emmetropia: Parallel rays focus on fovea. Retina conjugate with infinity. Far point is at infinity.

Ammetropias: Parallel rays do not focus on retina. Ammetropic eyes require a correcting lens to make retina conjugate with

infinity, i.e., to move far point to infinity

Hyperopia: Deficient refractive power. Parallel rays focus behind retina. Far point is beyond infinity. Plus lens converges rays on to retina and conjugate fovea with

infinity.

Far point concept Myopia: Excessive refractive power. Parallel rays focus in front of retina. Far point is between infinity and eye. Minus lens diverges rays on to the retina and conjugate

fovea with infinity. Aspherical ammetropias: This indicates different types of astigmatism. This type of errors have two far points. As a set of rays converge at one place and other at different

place due to cornea not having same radius of curvature in all the meridians.

Principle of retinoscopy:

To bring the far point of patient at the nodal point of the observer at the working distance

NEUTRALIZATION

1. No movement of red reflex = NEUTRAL POINT

2. Reversal with overcorrection by 0.25 D

3. On alternating the working distance, slight forward, ‘with’ movement and an ‘against’ movement by slight backward movement.

PREREQUISITES FOR RETINOSCOPY

PREREQUISITES FOR RETINOSCOPY

1. Dark room: 6 m long2. A trial set: 64 pairs of Spherical lenses(plus and minus) 0.12D 0.25-4.0D 4.0-6.0D 6.0-14D 14-20D

0.25 0.51D2D

20 pairs of Cylindrical lenses (plus and minus)

0.25-2.00D 2.0-6.0D

10 Prisms: ½ to 6 8,10,12

0.250.5

Accessories: Plano lenses

Opaque discs Pinhole

Stenopaeic discsMaddox rodsRed and Green glasses

3. A trail frame Light weight Aluminium alloy- 30 g Adjustable- horizontally and vertically 3 compartments: spherical, cylindrical, prisms Cylindrical compartment : smooth and accurate rotation Side pieces should be joint- tilted while testing for near vision Back lens in trial frames should occupy as nearly as possible the

position of spectacle lens i.e. around 12 mm in front of the cornea.

4. Phoropter or refractor Turn the dial – change the lens before the aperture of the viewing system

5. Distance-vision chart6. Near vision charts7. Retinoscope:

Invented by Dr Jack Copeland

REFLECTING (MIRROR) RETINOSCOPES

Priestley-Smith’s mirrorPlane mirror

SELF-ILLUMINATED RETINOSCOPES

Spot self-illuminous Streak retinoscope

HEAD

NECK

HANDLE

SLEEVE UP

PEEPHOLE

SLEEVE DOWN

STREAK RETINOSCOPE

HEAD

HANDLE

NECK

MIRROR LENSSLEEVEPARASTOP

BULB

BATTERIES

Contact point b/w neck

and handle

bulb

Dark mirror

Reflecting mirror

Sleeve

Observing the optics of retinoscope we find two main systems

- Projection system: Light source Condensing lens Focusing sleeve Current source - Obsevation system: Peep hole

RETINOSCOPE AND IT’S PARTS:

The retinoscope: how it works?

PROJECTION SYSTEM: illuminates the retina

Light source: a bulb with linear filament that projects a line or streak of light

Condensing lens: focuses rays from bulb onto the mirror

Mirror : placed in the head of instrument at 45 degree angle, it bends the path of light at right angles to the axis of the handle

Focusing sleeve controls Meridian:Turning the sleeve rotates the streak of light. Vergence:By varying the distance between the lens and

the bulb Bulb: Moved up-Plane mirror effect(Parallel rays) Moved down-Concave mirror effect(converging

rays) Lens: Moved up-Concave mirror effect Moved down-Plane mirror effect

In Copelands instrument, moving the sleeve up or down moves the bulb. Sleeve up creates plane mirror effect & sleeve down

creates concave mirror effect

Copeland type- fixed lens with moving bulb

In other Retinoscopes, the lens rather than the bulb moves on moving the

sleeve Sleeve up creates a concave mirror effect & sleeve

down creates plane mirror effect

Fixed bulb type, the lens moves

OBSERVATON SYTEM: Allows to see the retinal reflex

Light reflected by the illuminated retina enters the retinoscope, passes through an aperture in the mirror & out of the peephole at the rear end of the head.

Optics in Retinoscopy

Illumination stage: light is directed into the subject’s eye to illuminate the retina

Reflex stage: image of the illuminated retina is formed at subject’s far point

Observation stage: image of the far point is located by moving the illumination across the fundus and noting the behavior of the luminous reflex seen by the observer in the subject’s pupil

Clinician

So

S1S2

Patient

Optical principles

The light in the pupil is called the “ret reflex”

The final goal

Observer Subject

Reflex disappears

Reflex moves down,i.e. with directionof movement of light

Mirror tilts further forwards

Mirror tilts forwards

S2

S2

Far point behindobservers pupil Within pupil

Outside pupil

No S2

S1

Non-neutral point: With movement Far point behind observers

pupil.With movement of reflex, gradual change from red

reflex to no reflex

With movement

Observer Subject

Far point in front ofobservers pupil

Reflex disappears

Reflex moves up,i.e. against directionof movement of light

Mirror tilts forwards

Mirror tilts further forwards

S2

S2

Within pupil

Outside pupil

No S2

S1

Non-neutral point: Against movement Far point in front of observers

pupil.Against movement of reflex,

gradual change from red reflex to no reflex

Against movement

Reflecting mirror:

Perforated mirror

Central hole: 2.5mm anteriorly 4 mm posteriorly

By a plane mirror, rays are slightly divergent causing less illumination

Small hole corresponding to the central hole reduces illumination in pupillary area by causing central dark patch

Sides of the small hole are blackened to prevent annoying reflexes from entering the eye

Slightly concave mirror with central opening of 4mm and focal length greater than the distance between observer and patient; 150 cm

LUMINOUS RETINOSCOPE

Streak retinoscope is the most popular Both light source and mirror are incorporated Intensity and type of beam can be controlled.

Two main techniques of retinoscopy are

Static Retinoscopy: It is the refractive state

determined when patient fixates an object at a distance of 6 m with accomodation relaxed.

Dynamic Retinoscopy: The refractive state is

determined while the subject fixates an object at some closer distance, usually at or near the plane of retinoscope itself with accommodation under action.

RETINOSCOPY TECHNIQUES:

PROCEDURE OF RETINOSCOPY

Patient is made to sit at a distance of 2/3rd of a meter

Accomodation should be relaxed Use right eye for patient’s right

eye and left eye for left Trial frame is fitted Direct the light from retinoscope

into patient’s pupil Move the retinoscope in

horizontally and observe the red reflex.

Suitable lens is placed before the eyes to neutralize the band when the pupil will be filled with uniform light

Now move vertically, if still pupil is filled with uniform light, no astigmatism.

Examiner sits just enough to the side to avoid blocking patient’s fixation

Characteristics of the moving retinal reflex

Speed and brilliance

Low refractive High refractive

Bright FaintMoves rapidly Moves slowly

2.Width of reflex

Narrow WideHigh degree of ametropia Low degree of ametropia

3. Presence of astigmatism

When the axis does not correspond with the movement of the mirror, the shadow appears to swirl around.

Examiner should scope both vertical and horizontal meridian.

We correct the astigmatism with cylindrical lens.

Cylindrical lens may be plus or minus, but have power in only one meridian, that which is perpendicular to the axis of the cylinder.

The axis meridian is flat and has no power.

ASTIGMATISM

CYLINDRICAL AXIS

Break in the alignment between the reflex in the pupil and the band outside it when the streak is off the axis

Rotate the streak until the break disappears

Width of the streak appears narrowest when the streak aligns with the true axis.

STRADDLING

Confirmation of axis Retinoscope streak- 45

degree off axis in both directions

Correct axis- width of streak equal in both positions

Axis not correct- width is unequal

Narrow reflex is guide towards which cylinder’s axis should be turned

PROBLEMS IN RETINOSCOPY:

1. Red reflex may not be visible or maybe poor Small pupil Hazy media High degree of refractive error Overcome by causing mydriasis and/or use of

converging light with concave mirror retinoscope2. Changing retinoscopic findings Abnormally active accomodation Overcome by fogging retinoscopy and cycloplegic3. Spherical aberrations Dilated pupils

4. Conflicting shadows Irregular astigmatism

5. Triangular shadows Conical cornea

www.phacotube.com

Scissoring shadowsAstigmatismOvercome by: undilated pupil

Cycloplegics in retinoscopy

When retinoscopy is performed after instilling cycloplegic drugs it is termed as Wet Retinoscopy

1. Atropine 1% ointment Used in children < 7yrs age Dose:1% eye ointment 3times daily for 3days Effect lasts for 10-20days Deduction for cycloplegia with atropine is 1D

2. Homatropine 2% drops Used in hypermetropes between 7 to 35yrs Dose:1drop instilled every 10mins for 6times &

retinoscopy is performed after 1-2hrs Effect lasts for 48-72hrs Deduction for cycloplegia with homatropine is

0.5D3. Cyclopentolate I% drops

Used in persons between 7 to 35yrs Dose:1drop instilled every 10mins for 3 times &

retinoscopy is performed after 1- 1 1/2hrs Effect lasts for 6 to 18hrs Deduction for cycloplegia with Cyclopentolate is

0.75D

Only mydriatic [ 10% phenylephrine] may is used in elderly patients to enhance the pupillary reflex when the pupil is narrow or media is slightly hazy

Later it should be counteracted by use of miotic drug[ 2% pilocarpine]

Clinician

S2

Patient

Working distanceWe now have neutral

We have also introduced negative vergence due to our working distance (WD)

= 1/d (m)Where d = distance in m, measured between your ret and patient’s eye

We have added lenses

To get the right prescriptionwe need to compensateRx = lens power – 1/d

So to get neutral, we needed: lens power = Rx + 1/d

Working distance compensation

Calculation

For example, if neutrality is achieved with a +3.00DS lens and your working distance is 50cm

Rx = +3.00DS – (1/0.50) = +3.00 – 2.00 =+1.00DS

Working distance lens Before you begin, add a

“working distance lens” A +ve lens to cancel out

the negative vergence As in eg., WD = 50cm WD lens = 1/0.50 =

+2.00DS Neutrality for the same

patient is still +3.00DS WD lens = +2.00DS Lens power = +1.00DS

Rx = lens power - 1/d

AUTOREFRACTOMETRY

Alternative method of assessing error of refraction by use of an optical equipment called refractometer or optometer

OBJECTIVE SUBJECTIVE

History Collins(1937) developed the first semi automated electronic

refractionometer.

Safir (1964) automated the retinoscope and this work led to the first commercial autorefractor-the Ophthalmometron.

6600 Autorefractor was the second commercial autorefractor (1969).

Munnerlyn (1978) design using a best contrast principle with moving gratings led to the Dioptron, an automated objective refractor.

OBJECTIVE SUBJECTIVE

SOURCE OF LIGHTTIME REQUIRED

Infra-red light2-4 min

Visible light4-8 min

INFORMATION PROVIDED Less informative More informative (corrected visual acuity)

PATIENT CO-OPERATION Required less (only has to look straight at target)

More co-operation (patient has to turn knob, focus target, answer questions about appearance of target)

OCULAR FACTORS Better in macular diseases with clear ocular media

Can be done in hazy ocular media

OVER REFRACTION CAPABILITY WITH SPECTACLES, CL, IOL

Difficult No problem

EXPECTED RESULTS Preliminary refractive findings

Gives refined subjective result.

TOPCON RM 8900/8800

Monitor

Control leverwith measurementswitch

Forehead rest

Chin rest

Examination window

Power switch

1.Print switch 2.Menu switch

3.IOL switch

SPECIFICATIONS AND PERFORMANCE

OPTICAL PRINCIPLES

1. The Scheiner principle2. The optometer principle3. Retinoscopic principle4. Knife edge principle5. Ray- deflection princile6. Image size principle

Basic design

Infrared source (IR-LED)

Fixation target

Badal lens system

Polarized beam splitter

Light sensor

Autorefractors:

Primary and secondary source of electromagnetic radiation:

Primary- NIR (Near Infrared Radiation): 780-950nm

Efficiently reflected back from the fundusEssentially invisible to the patient

Secondary- back scatter from the fundus Determine sphere power, cylinder power, and

cylinder axis

Fixation targetAccomodation is most relaxed when target is- Of low spatial frequency Same as typically seen at distance

Nulling vs open-loop measurement principle

NULLING PRINCIPLE REFRACTOMETERS: Change their optical system until the refractive error

of the eye is neutralized.

OPEN-LOOP PRINCIPLE REFRACTOMETERS: Makes measurements by analysing the

characteristics of the radiation exiting the eye Do not alter their optical system

ALLOWANCE

•NIR = 800-900nm•The reflectance of the fundus increases towards the red end of the spectrum•Into the infra red, there will be multiple reflections of scattered radiation within the eye which will degrade the image•A 800-900nm will create hypermetropia of 0.75-1.00 DS relative to 550nm

Basic Principle

Infrared light

Rotatingchopper

Badallens

To PC

Polarisedfilter

Slitimagemore minus 0 more plus

Slitmask

Lightsensor

BADAL OPTOMETER:

Infrared light is collimated

passes through rectangular masks housed in a

rotating drum

beam splitter

Optometer system

Moves laterally to find the optimal focus of the slit on the retina

Optimal Focus:

Optimal focus is achieved when a peaksignal is received from the light sensor.

SCHEINER DOUBLE PIN-HOLE

The original Scheiner double pin-hole was invented in the 16th century.

In a clinical setting, the double pin-hole identifies the level of ametropia in a subject by placing it directly in front of the patient’s pupil

In a myopic eye, the patient sees crossed diplopic images, whereas in hyperopia, the patient sees uncrossed images.

Crossed and uncrossed doubling can easily be differentiated by asking the patient which image has disappeared, when either top or bottom pin-hole is occluded.

OPTOMETER PRINCIPLE

Aim: to study the number of dioptres of correction on a scale with fixed spacing when the light from the target on the far side of the lens enter the eye with vergence of different amounts-

0 - + The vergence of the light in the focal plane of the optometer lens is linearly related to the displacement of the target.

Modern optometers

Limitations of early optometers:1. Alignment problem: both pin-hole apertures

must fit within the pupil.2. Irregular astigmatism: the best refraction over

the whole pupil may be different in contrast to the two small pin hole areas of the pupil.

3. Accommodation: the instrument myopia alters the actual refractive status of the patient.

Scheiner’s Principle:

Two LEDs (light emitting diodes) are imaged to the pupillary plane.

These effectively act as a modified Scheiner pin-hole by virtue of the narrow pencils of light produced by the small aperture pinhole located at the focal point of the objective lens.

2 LEDs imaged to the pupillary plane

Ocular refraction l/t doubling of LED if

refractive error is present

Reflection of LEDs from retina back out of the

eye

Reflection by a semi-silvered mirror

Dual photodetectors

RETINOSCOPIC PRINCIPLE

Autoretinoscopes Source of optical train corresponds to streak

retinoscope. Source of light is drum rotating about a source of

NIR which produces incident rectangular beam. Neutralization is by badal optometer Unwanted reflexes filtered by polarization of

incoming NIR Meridional refractors

1. Autoretinoscope based on direction of fundus streak motion are nulling refractors.

2. Autoretinoscope based on speed of fundus streak motion are non nulling refractors.

BEST FOCUS PRINCIPLE:

Aim: to find the best focus of an image on retina through the analysis of maximum contrast that can be captured by autorefractor.

Both meridional and nulling Neutralization is by badal optometer Filters unwanted reflexes by polarization

Knife-edge principle

Aim: to evaluate the refractive uniformity of the lens. These are nulling autorefracors that are not meridional Based on principle of reciprocity All light returning through the system passes completely

back into the source. Theoretically, no light should escape past the knife-edge.

Knife edge principle

Foucault Knife edge test for an emmetropic eye. The reflex on the detectormoves over most of the surface

Knife edge test for myopic eye.The motion of the reflex across thedetector provides information on the nature of the refractive error.The speed of the reflex describes the magnitude of refraction

Ray deflection principle

Aim: to measure Linear deflection of the fundus image in 3 or more

meridia at a fixed distance from the eye Angular deflection of rays Position of far point in those meridia trignometrically

Open loop (non-nulling) meridional refractors Corneal reflexes removed by central aperture in the

plane Coaxial reflexes removed by polarization

Image size principle

Aim: to measure the size of fundus image in 3 or more different meridia and calculate the full refractive error on the basis of ocular magnification or minification of the image relative to emmetropia.

Non nulling Detection system : fundus camera- CCD camera, video

imaging of the fundus reflex, analysis by sophisticated computer programme

Conclusion Autorefraction is a valuable tool in

determining a starting point for refraction. Modern technology has resulted in

improvements in design, size, speed and accuracy.

There are primarily two principles utilised in current autorefractors

the Scheiner principle Retinoscopic principle. Improvements in target design (auto-fogging

distance targets and open view autorefractors) attempt to relax accommodation in patients.

Autorefractor should not be used as the final refractive correction without further confirmation.