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OPTICS & REFRACTION
Dr. Prabhat Kiran Devkota
MBBS(TU),MD Resident(NAMS)
Lumbini Eye Institute
1
2/12/2024
Content
• Basic Terminologies
• Types of Lens
• Refractive State of Eye
• Refractive Component of Eye
• Objective Refraction
• Retinoscopy
• Autorefractometry
• Subjective Refraction
• Lens Notation, Transposition
2
Basic Terminologies
• Light: a form of energy whose interaction with retina gives
the sensation of sight.
• Optics: the study of the behavior of light and its interactions
with matter.
3
• Reflection: a phenomenon of change in
the path of light rays without any change
in the medium.
• Refraction: the change in direction of
light when it passes from one transparent
medium into another of different optical
density.
4
Interference:
• When two or more waves of light of the
same frequency travelling in same
direction superimpose, the resultant
intensity is different from the sum of
individual waves.
• This modification in the distribution of
intensity of light in the region of
superimposition is called interference
5
Diffraction:
• Bending of light caused by edge
of the aperture or rim of the lens.
• Diffraction effects get less
obvious as the gap gets larger
6
Polarization:
• Phenomenon of confirming the vibrations of a wave in a
specific direction perpendicular to the direction of wave
motion is called polarization.
7
Plano Lens:-
• Plano lenses are ophthalmic
lenses that do not have any
power.
• Used for protection.
8
Types of Lenses
Prism:
 It is a refracting medium, having two
plane surfaces, inclined at an angle.
• Refracting angle is the angle between
two surfaces. The greater the angle, the
stronger the prismatic effect.
• Axis refers to the line bisecting the apical
angle.
• Base refers to the surface opposite to the
apical angle.
 The image formed is erect, virtual and
displaced towards the apex of the prism.
9
when the prisms are arranged with their bases together,
a convex lens is formed which converges the incident
light to a point.
when the prisms are arranged with their apices together,
a diverging effect is produced and a concave lens is
formed.
10
Spherical lens has the same curvature over its entire
surface, and thus the same refractive power in all
meridians.
11
Convex lens (plus lens)
- Converge light rays
- Thick at center & thin at
periphery
- Magnifies object
- With movement of lens, object
moves in opposite direction.
12
13
Concave lens (minus lens)
- Diverge light rays
- Thin at center & thick at
periphery
- Minifies object
- With movement of lens, object
moves in same direction.
14
Cylindrical lens have vergence power in only 1 meridian,
which is perpendicular to the axis of the cylinder. They have no
power in the meridian parallel to the axis. Cylindrical lenses
focus light rays to a line.
 Types: Convex & Concave
15
• Rotation of the cylindrical lens
causes a distortion of image.
16
Spherocylindrical Lens:(aka Compound or Toric lens)
• Spherical and Cylindrical lenses combined in 1 lens.
• Two focal lines formed by 2 radii of curvature. The 1st focal line
is nearer the cornea, created by the more powerful corneal
meridian. The 2nd focal line is farther away, created by the less
powerful meridian.
• Between the 2-line foci produced by the conoid of Sturm is a
point called the circle of least confusion, which represents the
point of best overall focus. 17
• The circle of least confusion is the circular cross-section of the
Sturm's conoid, dioptrically midway between the two focal lines.
• The goal of refractive correction is to place the circle of least
confusion on the retina. 18
Refractive Components of the Eye
• Radius of curvature: The anterior and posterior radii of curvature
of the central part are around 7.7 mm and 6.8 mm respectively.
• About 1 mm increase in radius of curvature results in ~6 D of
hypermetropia and viceversa.
• Refractive power : Net refractive power is ~+43D(anterior
surface +48.83D and posterior surface -5.88D) which is 2/3rd of
the total refractive power.
• Refractive index = 1.376
19
Cornea:
The Anterior Chamber
• Contains Aqueous Humor.
• Depth of AC = ~2.5 to 4.0 mm
• Reduction of 1 mm AC depth
forward shift of lens
increase the total power by ~1.4 D
• Refractive index of aqueous = 1.336
20
Iris & Pupil
• Regulate amount of light entering the eye
• At ~2.4mm pupil size, best retinal image obtained, as
aberration and diffraction are balanced optimally.
• Average size:2-4mm
• Small pupil: depth of focus increases (pinhole effect)
• Large pupil: more aberration occurs
21
Crystalline Lens
• Radius of curvature:Ant. surface ~10 mm
Post. surface ~6 mm
• Refractive index: Nucleus 1.41
Pole 1.385
Equator 1.375
• Accommodative power: Provides a mechanism of focusing at
different distances. At age 8yr is~14D,
age 28yr ~9D,
age 64yr ~1D
• Refractive Power:~+16 to +20 D, accounts for about 1/3rd of
the refractive power. 22
Vitreous
• Refractive index same as aqueous. ie. 1.336
• About 1 mm shortening of the AP diameter of the eye
results in 3D of hypermetropia and vice versa.
23
Schematic & Reduced Eye
24
A way to represent the complex refracting system of human eye.
Gullstrand's schematic eye:
- Two principal foci
- Two principal points
- Two nodal points
Listing's reduced eye:
- Two principal foci
- Single principal point
- Single nodal point
Optical Axis
• Optical axis: Line passing through the centre of the cornea (P)
and centre of the lens (N) and meets the retina (R) on the nasal
side of the fovea
• Visual axis(aka foveal- fixation axis): Line joining the fixation
point (O), nodal point (N) and fovea (F).
• Fixation axis: Line joining the fixation point (O) and the centre
of rotation of the eye (C).
25
Visual Angles
• Angle alpha(ONA): Angle formed between the visual axis and the
optical axis at the nodal point. Normally the visual axis cuts the
cornea nasally this is denoted as positive angle alpha.
• Angle gamma(OCA): Angle formed between optical axis and fixation
axis at the center of rotation
• Angle kappa(OPA): Angle formed between the pupillary line and
visual axis. Substituted for angle alpha as it can be measured
clinically. 26
Refractive Index
Refractive power of substance in comparision to that of air is
termed as refractive index.
27
Total Internal Reflection
• The critical angle is the incident angle at which the angle of
refraction is 90 degrees from normal.
• As rays emerging from a denser medium to a rarer medium if it
strikes the interface at an angle > critical angle it is reflected
back which is called total internal reflection.
• It occurs at the cornea:air interface, and prevents visualisation
of the AC angle.
28
• Near point:
– Closest point at which an object can be placed and form
a focused image on retina within eye’s accommodation
range.
• Far point:
– Point in space that is conjugate with fovea when
accommodation is relaxed.
– Emmetropia: Far point is at infinity.
– Myopia: Far Point is between infinity and eye
– Hypermetropia :-Far point is beyond infinity
29
30
Refractive States of the Eye
 Emmetropia: When parallel light rays are focused sharply on the
retina when accomodation is at rest.
 Ametropia: When eye is unable to bring parallel light rays from a
distant object into focus. • Myopia • Hyperopia • Astigmatism
 Isometropia:- The optical state with equal refraction in the two eyes.
 Anisometropia: When the total refraction of the two eyes is unequal.
 Aniseikonia: an anomaly of binocular vision in which the ocular
images are unequal in size or shape or both.
 Antimetropia: rare refractive condition in which one eye is myopic
and the fellow eye is hyperopic.
31
Optical Defects
Physiological Optical Defects:
• Diffraction
• Chromatic aberration
• Spherical aberration
• Decentering
• Peripheral aberrations
• Coma
32
Pathological Optical Defects:
•Myopia
•Hypermetropia
•Astigmatism
Chromatic Aberration
• Light of different wavelength gets
refracted differently. Shorter
wavelengths (blue) are refracted
more than longer wavelengths (red).
• Eye focuses light of the greatest
intensity i.e. yellow. So, images
formed by light of longer and
shorter wavelength are less intense
and are neglected.
• The eye is hyperopic for red and
myopic for blue light. 33
Spherical Aberration:
• Periphery of a lens has more
refracting power than the center.
• A convex spherical lens refracts
peripheral rays more strongly than
paraxial rays, so peripheral rays are
focused closer.(d/t the prismatic
effect).
• Seen only when the pupil is widely
dilated. Small pupil cuts off the
peripheral rays.
34
Natural mechanisms to decrease aberrations
• Cornea at periphery is flatter than centre.
• Iris blocks the peripheral rays of light.
• High refractive index of central nucleus of lens than cortex,
• Low sensitivity of the peripheral retina, and
• Stiles Crawford effect i.e. light passing near the edge of the
pupil is less efficient at evoking sensation than through the
center of the pupil.
35
Myopia (Nearsightedness):- The light rays focus anterior to
the retina when accommodation is at rest due to excessive
convergent power. A concave lens(-) is used to correct
myopia.
Types:
• Axial,
• Curvatural,
• Index,
• Positional,
• d/t excessive accomodation
36
Hyperopia (Farsightedness):- the light rays focus posterior
to the retina when accommodation is at rest due to insufficient
convergence power .A convex lens(+) is used to correct
hyperopia
Types:
• Axial,
• Curvatural,
• Index,
• Positional,
• Aphakic
• Consecutive
37
Astigmatism: type of refractive error wherein the
refraction varies in different meridian. Corrected with
cylindrical lens.
a) Regular Astigmatism: when principal meridians are
perpendicular & the refractive power changes
uniformly.
• against-the-rule astigmatism (0-30)°: Horizontal meridian steeper
• oblique astigmatism (30-60)°
• with-the-rule astigmatism (60-90)°: vertical meridian steeper
b) Irregular Astigmatism: irregular change of refractive
power in different meridia.
it cannot be corrected by spectacles 38
39
Presbyopia is a progressive loss of accommodative
ability of the crystalline lens caused by the natural process
of aging. It manifests after the age of 40years. It can be
corrected optically with use of plus lenses for near work.
40
Refraction
• Phenomenon of bending of light as it
passes from one transparent medium into
another of different optical density.
• Snell’s Law of Refraction:
– The incident ray, refracted ray and the
normal all lie in same plane.
– Ratio of sine of angle of incident to
sine of angle of refraction is constant
in any two transparent mediums,
which is termed as refractive index 41
n1 sin θ 1 = n2 sin θ 2
• Definition:-
Process of measuring patient’s refractive error & determining
the optical correction needed to focus light rays from distant
and near objects onto retina and providing the patient with
clear and comfortable vision.
42
Refraction Methods
 Objective Refraction
I. Retinoscopy
II. Refractrometry
III. Keratometry
IV. Photorefraction
 Subjective Refraction
I. Subjective adjustment of refraction
II. Subjective refinement of refraction
III. Binocular balancing
43
Retinoscopy
aka ‘Skiascopy’ or ‘Shadow Test’ or ‘pupilloscopy’.
 An objective method of assessing error of refraction.
 Utilizes technique of neutralization.
 Focault's Principle - when light is reflected from a mirror into
the eye, the direction in which light will travel across the pupil
will depend upon the refractive state of the eye.
 aim of retinoscopy is to convert the far point of the subject's eye
to the nodal point of the observer's eye.
 It is performed with Retinoscope.
44
Objective of retinoscopy
To locate the far point of the eye conjugate to the retina.
Myopia or hyperopia
Bring far point to the infinity by using appropriate lenses.
Determines amount of ametropia
45
Retinoscopy - Optics
 Part of fundus illuminated by light reflected into patient’s
eye with retinoscope.
 Illuminated area – serves as an object. The rays which
emanate from this illuminate the pupillary area and forms
its image at the far point of the eye
 When the immediate source of light is moved across the
eye, behavior of the luminous reflex in pupil will depend
upon the refractive status of the eye.
46
Retinoscopy - Stages
1.Illumination Stage:- Light is directed into the patients eye to
illuminate the retina.
2.Reflex Stage:- An image of the illuminates retina is formed at
the far point of the eye.
3.Projection Stage:- The image at the far point is located by
moving the illumination across the eye(fundus) and noting the
behaviour of the reflex in the patients pupil
47
Types of Retinoscope
• Reflecting mirror retinoscope
– Plane mirror
– Priestley-smith’s mirror
• Self illuminated retinoscope
– Spot retinoscope
– Streak retinoscope
48
Consists of:-
 Projection System:-
• light source (bulb)
• condensing lens
• mirror
• focusing sleeve
• current source
 Observation system:-
• peephole 49
Parts of Streak Retinoscope
• By sliding the sleeve up or down, the distance
between the bulb and lens is changed to
converge or diverge the light streak.
1. Plane mirror effect: apparent light source
behind retinoscope, creating parallel light rays.
2. Concave mirror effect: apparent light source in
front of the retinoscope, creating concave light
rays.
50
Pre-requisites for Retinoscopy:
1. Dark room
2. A trial set consisting of:
-32 Spherical lenses (plus & minus)
from 0.12 upto 20
-20 Cylindrical lenses (plus & minus)
from 0.12 upto 6
-Prism (1/2,1,2,3,4,5,6,8,10,12pd)
-Accessories: Plano lens, Pinhole, Occluder, Stenopaeic slit,
Maddox rod & Red and Green lenses.
3. A trial frame
51
4. Retinoscopes
5. Phoroptor (alternative to
trial set & trial frame)
6.Distance vision chart: commonly Snellen’s self
illuminated vision box.
7. Near Vision charts: Jaeger’s chart, Reduced
Snellen’s test types & Times Roman type face. 52
Trial Frame & Phoroptor:
53
Refractive States of the Eye
• Optics of reflex and projection stage when performed at 1
meter distance from subject with plane mirror effect depends
upon the refractive status of eye as:
Emmetropia or Hypermetropia or Myopia <1D: with movement
Myopia of 1D: no movement
Myopia >1D: against movement
54
55
Optics of Movement of Reflex
 Emmetropia: with movement.
Light rays emerging out of eye from
illuminated spot on fundus (F1) are
parallel to each other.
Hypermetropia: with movement.
Light rays emerging out of eye from
illuminated spot on fundus(f1) are
divergent.
56
Myopia (<1D): with movement.
Light rays emerging out of fundus are
convergent & meet behind observer
sitting at 1 m from patient & examiner
intercepts them before they meet.
Myopia (1D): no movement.
Light rays emerging out of fundus are
convergent & meet at a point 1 m
infront of patient i.e. at pupillary plane
of observer.
57
Myopia >1D: against movement.
Light rays emerging out of the patient’s eye are
convergent & meet in space between patient &
observer.
58
Procedure
• Patient made to sit at an arm length distance (harmon’s
distance) from examiner.
• The eye level of the patient and examiner should match.
• Sitting at the rightside of patient hold the retinoscope with
right hand, using your right eye look for patients right eye &
vice versa.
• Patient is instructed to look at far point (to relax the
accommodation) in dry retinoscopy or, directly into light (if
cycloplegics is used).
59
• Examiner observes red reflex in pupillary area of patient
through a peephole in retinoscope.
• Retinoscope is moved in horizontal and vertical
meridians and characteristics of moving retinal reflex are
noted.
60
Finding Neutrality
 "Against" movement
- Myopic
- neutralizes with minus lenses
 "With" movement
- Hyperopic
- neutralizes with plus lenses
61
62
Characteristics of Reflex
 Speed:
Large refractive errors have slow reflex whereas small refractive
errors have faster reflex.
 Brilliance:
-Reflex is dull when far point is away from the observer and
becomes clearer as it comes closer to neutrality.
-Against reflex are usually dimmer than with reflex.
 Width:
Streak is narrow when the far point is away, becomes wider and at
neutrality it fills the entire pupil. 63
64
Finding Cylinder Axis
• Before power the axis of meridians must be determined
• Characteristics of streak reflex :
Break : is seen when the streak is not oriented to the
principle axes and it disappears when it is rotated correctly.
Width : Appears narrowest when streak aligns with
principle axis.
Intensity: Brighter when streak is on correct axis.
Skew : is the oblique motion of the reflex. The streak
moves slightly differently than the reflex when the axes are
not aligned. It can be used to refine axis in small cylinders.
65
66
Confirmation of Axis
• The axis may be confirmed with straddling technique
• Streak is turned 45° off axis in both direction
• If axis is correct, width of reflex should be equal in each of
two positions
• If axis is not correct, widths will be unequal in two
positions
67
Finding Cylinder Power
+3.00
+2.00
68
With 2 Spheres :
 Neutralize one axis with appropriate sphere
 Keep on changing sphere till second axis is neutralized
 Difference between two sphere =Cylindrical power
 For Eg:
If 90° is neutralized with +3.00 sphere and 180° is neutralized with +2.00
sphere
Gross retinoscopy: +2.00 /+1.00× 180°
Examiner's working distance +1.50ds(for 66cm) is subtracted from sphere
Final refractive correction: +0.50 /+1.00× 180°
With a Sphere and Cylinder
– Neutralize one axis with spherical lens
– Neutralize other axis with a cylindrical lens at appropriate
orientation
– Spherical cylindrical gross retinoscopy can be read directly
from the trial lens apparatus
69
End Point of Retinoscopy:
• Neutralization point i.e. no apparent movement of
the reflex in any meridian & pupil filled with red
glow.
• Verified by following maneuvers:
- over correction by 0.25 D should cause reversal
of movement.
- on altering the working distance by forward or
backward movement of the head ‘with’ or
‘against’ movement should be observed.
70
Working Distance
• Distance between the examiner and patient’s eye must be
measured and converted into diopters.
• Subtract dioptric equivalent of working distance from
neutralization point reached in retinoscopy.
• Eg: working distance of
arm’s length (̴ 66cm) requires +1.50 D,
50cm distance requires +2D and
1 meter distance requires +1D
• Working Distance must remain constant throughout the
examination. 71
Cycloplegic Refraction
Causes: paralysis of accomodation & dilatation of pupil.
When cycloplegics is used - Wet Retinoscopy.
Atropine: < 5 yrs, 1% , TDS x 3days, Retinoscopy on 4th day,
Effect 10-20 days, PMT after 3wks, Tonus allowance 1D
Homatropine: 5–8 yrs,2% Gtt. every 10min x 6times, Retinoscopy after 90 min,
Effect for 48-72hrs, PMT after 3days, Tonus allowance 0.5D
Cyclopentolate:8–20 yrs,1% Gtt every 15min x 3times, Retinoscopy after 90 min,
Effect for 6-18 hrs, PMT after 3days, Tonus allowance 0.75D
72
Indications:-
• Childrens (<15yr)
• Patient with active accommodation
• Hyperopic patients upto 35yr (if symptomatic)
• Early Presbyopia (if using glasses)
• Variable powers on dry refraction
• Symptoms disproportionate to refractive error
• Accommodative Esotropia
• Bilateral refractive asymmetry
• Mentally disabled or uncooperative patients
• Suspected Malingering 73
74
Postmydriatic Test
• It refers to repeating the refraction after the effect of
cycloplegia has worn off.
• Ideally, it is done in all cases of cycloplegics refraction if there
is a difference in the correction in dry and cycloplegics
refraction.
• The next visit depends on the cycloplegics agent used. Eg. For
atropine it is done after 3 weeks, for homatropine and
cyclopentolate it is done after 3 days.
75
Rough Estimate of Refractive Error after
Retinoscopy:
• Gross Retinoscopy: retinoscopic value
• Net Retinoscopy= Gross Retinoscopy - [Working Distance +
Tonus Allowance(if used)]
• If retinoscopic finding is +7 Ds at 1 meter with Atropine then,
Refractive Error = +7 – (1+1) = +5 D
76
Autorefractometry
 Alternative method of detecting refractive error using
optical equipment called Refractometer or Optometer.
 Based on 2 principles:
-Scheiner Principle
-Optometer Principle
 Advantage:
-Faster
-No training required
 Disadvantages:
-Expensive
-Need electricity
-Wrong reading on irregularities & opacities 77
Lensometer
 An instrument used to measure the power of a patient’s present
spectacle lenses. Both manual and automated lensometers are
available.
 Measures 4 principal properties :
i. spherical and cylindrical power
ii. axis if cylinder is present
iii. presence and orientation of prism
iv. optical centration
78
Subjective Refraction
• Should proceed as:
A. Subjective adjustment of refraction
B. Subjective refinement of refraction
C. Binocular balancing
79
A. Subjective Adjustment of Refraction
1. Trial & Error technique
2. Fogging Technique
80
1. Trial & Error technique
• This employs use of different spherical & cylindrical lenses.
• SPHERICAL LENSES:
-the strongest convex lens and the weakest concave lens
providing best vision should be chosen for hypermetropia and
myopia respectively.
81
• CYLINDRICAL LENSES
Need verification of axis and power.
Verification of axis:
- Best done by simply rotating the cylinder in steps of 5 or 10 ˚
in either direction and asking whether acuity improves.
Verification of Power
-by changing the cylindrical lens in the trial frame.
82
2. Fogging
83
Principal: to make eye artificially myopic by increasing plus
power (or decreasing minus power).
Fog eye by adding 1 to 1.50D power more than retinoscopy
finding.
Then unfog by reducing 0.25D till best Snellen’s visual acuity
is attained
B. Subjective refinement of refraction
- First refine sphere than cylinder
1. Refining Sphere
i. Fogging test
ii. Duchrome test
iii. Pinhole test
2. Refining cylinder
i. Jackson’s cross cylinder technique
ii. Astigmatic fan & block technique
84
1. Refining Sphere
• Done by:-
i. Fogging technique
ii. Duchrome test
iii. Pinhole test
85
Duchrome Test
• Determines too much minus or too much plus
correction.
• Due to chromatic aberration, the shorter green
wavelength is focused in front longer red
wavelength
• Patient is asked to read letters with red and green
background
• Emmetropic: both colors equally sharp
• Hypermetropic: green is sharper, indicates over
minused , plus should be added.
• Myopic: red is sharper, indicates over plused,
minus should be added. 86
Pinhole Test
• Pinhole size varies 1 - 2mm (ideal size 1.2 mm).
• Can correct ref. errors between +5.0D and -5.0D.
• Pinhole admits only central rays of light which do not require
refraction by the cornea/lens, thus cutoff the scattered rays
increasing the depth of focus. So that the blur created by optical
irregularities or refractive error becomes reduced.
• It confirms whether the optical correction is correct or not.
• An improvement in vision through a pinhole indicates that more
refractive correction is still possible.
87
2. Refining Cylinder
Done by:-
i. Jackson’s cross cylinder technique
ii. Astigmatic fan & block technique
88
Jackson’s cross cylinder technique
• Combination of 2 cylinders of equal strength,
with opposite sign placed at right angles (90°)
to each other.
• Cross cylinder is placed with its axis 45° to
axis of cylinder in trial frame.
• If no change, axis of correcting cylinder is
correct.
• If improvement occurs correcting cylinder
should be rotated. Should be repeated several
times until neutral point reached.
89
Refinement of Cylinder Power
• Align the JCC axis with cylinder axis.
• Flip the cross cylinder and ask the patient “which is better?”
• Add or subtract cylinder power according to preferred
position of JCC
• For every change in -0.50 dcyl add +0.25dsph and viceversa.
90
Astigmatic Fan & Block Test
• Consists of series of radiating lines
spaced at 10o interval & arranged in a
manner of rays of a rising sun around a
central panel carrying a V and two sets of
mutually perpendicular lines ( the
blocks).
• ‘V’ & ‘block’ simultaneously can be
rotated through 180o.
91
 Patient is asked to see the fan after fogging
by + 0.5 D
• Normal patients can see all the lines
clearly.
• In astigmatism, some lines are seen
more sharply defined.
• Concave cylinder is then added with
each axis at right angle to the clearest
line until all the lines are equally sharp
92
C. Binocular Balancing
• Process sometimes known as equalizing accommodative
effort or equalization of vision
• It allows both eyes to have retinal image simultaneously in
focus
• Methods:
– Alternate Occlusion
– Prism dissociation
93
Near Refraction
• Usually done at age >40years or, pseudophakic eyes
(irrespective of age).
• Extra plus sphere is added to distant correction usually started
with +1DS
• Patient’s range of clear vision is determined by using near
vision chart
– If range is too close: power is reduced, add -0.25DS in steps
– If range is too far: power is increased, add +0.25DS in steps
94
95
Lens Notation
• A written spectacle prescription follows a
standard format
• Power of sphere(Sph) is recorded first
with sign(+/-), followed by power of
cylinder(Cyl) with its sign(+/-) and axis(°)
• Prescription is recorded for each eye
using abbreviation OD (oculus dexter) for
the RE, OS (oculus sinister) for the LE.
96
-To change the lens to another optically equivalent form.
-Usually spectacle are transposed & made as minus cylinders, even
if the prescription was written in plus cylinder form.
WHY??
• Minus cylinder (concave) lens is thinner, lighter and more
cosmetic.
• Minus cylinder reduces some amount of aberrations.
• Plus cylinder produce more magnification than the equivalent
minus-cylinder form.
• Plus cylinder is often difficult to adjust for the wearer.
97
Lens Transposition
98
Optically Equivalent Lens Form
Cylindrical power can be recorded in either plus or minus form .
3 Steps:-
• Add cylinder power to sphere power
• Sign of cylinder power is reversed
• Rotate axis of cylinder through 90°
For Eg:
+2.50 ds/ +0.50dc x 180° +3 ds/ -0.50dc x 90 °
-1.00ds /+1.50dc× 90° +0.50ds/ -1.50dc ×180°
99
 Optical Cross into Sphero-Cylinder:
• Small power (sphere)
• Big-small power difference (cylinder)
• Axis of small power
 Sphero-Cylinder into Optical Cross:
• Sphere (given axis)
• Sphere+Cylinder (opposite axis)
100
+3
+2.50
Eg:- +2.50 dsph/ +0.50dcyl x 180°
+3
+2.50
Eg:- +2.50 dsph/ +0.50dcyl x 180°
Spherical Equivalent
 It is average power of a spherocylindrical lens. It represents the
dioptric position of the circle of least confusion of the conoid
of Sturm.
-to compare or balance both eyes, and
-to reduce an excessive cylindrical correction.
 Calculated as:
Spherical equivalent = power of the sphere + (cylinder power/2)
101
References
• Borish’s Clinical Refraction 2nd Edition
• Duke-Elder’s Practice of Refraction 8th Edition
• AAO-Clinical Optics 2022-2023
• A. R. Elkington, Clinical optics (1999)
• Practical Ophthalmology AAO 8thEd
102
Thank You!!
103

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Optics & Refraction-Dr.PrabhatDevkota.pptx

  • 1. OPTICS & REFRACTION Dr. Prabhat Kiran Devkota MBBS(TU),MD Resident(NAMS) Lumbini Eye Institute 1 2/12/2024
  • 2. Content • Basic Terminologies • Types of Lens • Refractive State of Eye • Refractive Component of Eye • Objective Refraction • Retinoscopy • Autorefractometry • Subjective Refraction • Lens Notation, Transposition 2
  • 3. Basic Terminologies • Light: a form of energy whose interaction with retina gives the sensation of sight. • Optics: the study of the behavior of light and its interactions with matter. 3
  • 4. • Reflection: a phenomenon of change in the path of light rays without any change in the medium. • Refraction: the change in direction of light when it passes from one transparent medium into another of different optical density. 4
  • 5. Interference: • When two or more waves of light of the same frequency travelling in same direction superimpose, the resultant intensity is different from the sum of individual waves. • This modification in the distribution of intensity of light in the region of superimposition is called interference 5
  • 6. Diffraction: • Bending of light caused by edge of the aperture or rim of the lens. • Diffraction effects get less obvious as the gap gets larger 6
  • 7. Polarization: • Phenomenon of confirming the vibrations of a wave in a specific direction perpendicular to the direction of wave motion is called polarization. 7
  • 8. Plano Lens:- • Plano lenses are ophthalmic lenses that do not have any power. • Used for protection. 8 Types of Lenses
  • 9. Prism:  It is a refracting medium, having two plane surfaces, inclined at an angle. • Refracting angle is the angle between two surfaces. The greater the angle, the stronger the prismatic effect. • Axis refers to the line bisecting the apical angle. • Base refers to the surface opposite to the apical angle.  The image formed is erect, virtual and displaced towards the apex of the prism. 9
  • 10. when the prisms are arranged with their bases together, a convex lens is formed which converges the incident light to a point. when the prisms are arranged with their apices together, a diverging effect is produced and a concave lens is formed. 10
  • 11. Spherical lens has the same curvature over its entire surface, and thus the same refractive power in all meridians. 11 Convex lens (plus lens) - Converge light rays - Thick at center & thin at periphery - Magnifies object - With movement of lens, object moves in opposite direction.
  • 12. 12
  • 13. 13 Concave lens (minus lens) - Diverge light rays - Thin at center & thick at periphery - Minifies object - With movement of lens, object moves in same direction.
  • 14. 14
  • 15. Cylindrical lens have vergence power in only 1 meridian, which is perpendicular to the axis of the cylinder. They have no power in the meridian parallel to the axis. Cylindrical lenses focus light rays to a line.  Types: Convex & Concave 15
  • 16. • Rotation of the cylindrical lens causes a distortion of image. 16
  • 17. Spherocylindrical Lens:(aka Compound or Toric lens) • Spherical and Cylindrical lenses combined in 1 lens. • Two focal lines formed by 2 radii of curvature. The 1st focal line is nearer the cornea, created by the more powerful corneal meridian. The 2nd focal line is farther away, created by the less powerful meridian. • Between the 2-line foci produced by the conoid of Sturm is a point called the circle of least confusion, which represents the point of best overall focus. 17
  • 18. • The circle of least confusion is the circular cross-section of the Sturm's conoid, dioptrically midway between the two focal lines. • The goal of refractive correction is to place the circle of least confusion on the retina. 18
  • 19. Refractive Components of the Eye • Radius of curvature: The anterior and posterior radii of curvature of the central part are around 7.7 mm and 6.8 mm respectively. • About 1 mm increase in radius of curvature results in ~6 D of hypermetropia and viceversa. • Refractive power : Net refractive power is ~+43D(anterior surface +48.83D and posterior surface -5.88D) which is 2/3rd of the total refractive power. • Refractive index = 1.376 19 Cornea:
  • 20. The Anterior Chamber • Contains Aqueous Humor. • Depth of AC = ~2.5 to 4.0 mm • Reduction of 1 mm AC depth forward shift of lens increase the total power by ~1.4 D • Refractive index of aqueous = 1.336 20
  • 21. Iris & Pupil • Regulate amount of light entering the eye • At ~2.4mm pupil size, best retinal image obtained, as aberration and diffraction are balanced optimally. • Average size:2-4mm • Small pupil: depth of focus increases (pinhole effect) • Large pupil: more aberration occurs 21
  • 22. Crystalline Lens • Radius of curvature:Ant. surface ~10 mm Post. surface ~6 mm • Refractive index: Nucleus 1.41 Pole 1.385 Equator 1.375 • Accommodative power: Provides a mechanism of focusing at different distances. At age 8yr is~14D, age 28yr ~9D, age 64yr ~1D • Refractive Power:~+16 to +20 D, accounts for about 1/3rd of the refractive power. 22
  • 23. Vitreous • Refractive index same as aqueous. ie. 1.336 • About 1 mm shortening of the AP diameter of the eye results in 3D of hypermetropia and vice versa. 23
  • 24. Schematic & Reduced Eye 24 A way to represent the complex refracting system of human eye. Gullstrand's schematic eye: - Two principal foci - Two principal points - Two nodal points Listing's reduced eye: - Two principal foci - Single principal point - Single nodal point
  • 25. Optical Axis • Optical axis: Line passing through the centre of the cornea (P) and centre of the lens (N) and meets the retina (R) on the nasal side of the fovea • Visual axis(aka foveal- fixation axis): Line joining the fixation point (O), nodal point (N) and fovea (F). • Fixation axis: Line joining the fixation point (O) and the centre of rotation of the eye (C). 25
  • 26. Visual Angles • Angle alpha(ONA): Angle formed between the visual axis and the optical axis at the nodal point. Normally the visual axis cuts the cornea nasally this is denoted as positive angle alpha. • Angle gamma(OCA): Angle formed between optical axis and fixation axis at the center of rotation • Angle kappa(OPA): Angle formed between the pupillary line and visual axis. Substituted for angle alpha as it can be measured clinically. 26
  • 27. Refractive Index Refractive power of substance in comparision to that of air is termed as refractive index. 27
  • 28. Total Internal Reflection • The critical angle is the incident angle at which the angle of refraction is 90 degrees from normal. • As rays emerging from a denser medium to a rarer medium if it strikes the interface at an angle > critical angle it is reflected back which is called total internal reflection. • It occurs at the cornea:air interface, and prevents visualisation of the AC angle. 28
  • 29. • Near point: – Closest point at which an object can be placed and form a focused image on retina within eye’s accommodation range. • Far point: – Point in space that is conjugate with fovea when accommodation is relaxed. – Emmetropia: Far point is at infinity. – Myopia: Far Point is between infinity and eye – Hypermetropia :-Far point is beyond infinity 29
  • 30. 30
  • 31. Refractive States of the Eye  Emmetropia: When parallel light rays are focused sharply on the retina when accomodation is at rest.  Ametropia: When eye is unable to bring parallel light rays from a distant object into focus. • Myopia • Hyperopia • Astigmatism  Isometropia:- The optical state with equal refraction in the two eyes.  Anisometropia: When the total refraction of the two eyes is unequal.  Aniseikonia: an anomaly of binocular vision in which the ocular images are unequal in size or shape or both.  Antimetropia: rare refractive condition in which one eye is myopic and the fellow eye is hyperopic. 31
  • 32. Optical Defects Physiological Optical Defects: • Diffraction • Chromatic aberration • Spherical aberration • Decentering • Peripheral aberrations • Coma 32 Pathological Optical Defects: •Myopia •Hypermetropia •Astigmatism
  • 33. Chromatic Aberration • Light of different wavelength gets refracted differently. Shorter wavelengths (blue) are refracted more than longer wavelengths (red). • Eye focuses light of the greatest intensity i.e. yellow. So, images formed by light of longer and shorter wavelength are less intense and are neglected. • The eye is hyperopic for red and myopic for blue light. 33
  • 34. Spherical Aberration: • Periphery of a lens has more refracting power than the center. • A convex spherical lens refracts peripheral rays more strongly than paraxial rays, so peripheral rays are focused closer.(d/t the prismatic effect). • Seen only when the pupil is widely dilated. Small pupil cuts off the peripheral rays. 34
  • 35. Natural mechanisms to decrease aberrations • Cornea at periphery is flatter than centre. • Iris blocks the peripheral rays of light. • High refractive index of central nucleus of lens than cortex, • Low sensitivity of the peripheral retina, and • Stiles Crawford effect i.e. light passing near the edge of the pupil is less efficient at evoking sensation than through the center of the pupil. 35
  • 36. Myopia (Nearsightedness):- The light rays focus anterior to the retina when accommodation is at rest due to excessive convergent power. A concave lens(-) is used to correct myopia. Types: • Axial, • Curvatural, • Index, • Positional, • d/t excessive accomodation 36
  • 37. Hyperopia (Farsightedness):- the light rays focus posterior to the retina when accommodation is at rest due to insufficient convergence power .A convex lens(+) is used to correct hyperopia Types: • Axial, • Curvatural, • Index, • Positional, • Aphakic • Consecutive 37
  • 38. Astigmatism: type of refractive error wherein the refraction varies in different meridian. Corrected with cylindrical lens. a) Regular Astigmatism: when principal meridians are perpendicular & the refractive power changes uniformly. • against-the-rule astigmatism (0-30)°: Horizontal meridian steeper • oblique astigmatism (30-60)° • with-the-rule astigmatism (60-90)°: vertical meridian steeper b) Irregular Astigmatism: irregular change of refractive power in different meridia. it cannot be corrected by spectacles 38
  • 39. 39
  • 40. Presbyopia is a progressive loss of accommodative ability of the crystalline lens caused by the natural process of aging. It manifests after the age of 40years. It can be corrected optically with use of plus lenses for near work. 40
  • 41. Refraction • Phenomenon of bending of light as it passes from one transparent medium into another of different optical density. • Snell’s Law of Refraction: – The incident ray, refracted ray and the normal all lie in same plane. – Ratio of sine of angle of incident to sine of angle of refraction is constant in any two transparent mediums, which is termed as refractive index 41 n1 sin θ 1 = n2 sin θ 2
  • 42. • Definition:- Process of measuring patient’s refractive error & determining the optical correction needed to focus light rays from distant and near objects onto retina and providing the patient with clear and comfortable vision. 42
  • 43. Refraction Methods  Objective Refraction I. Retinoscopy II. Refractrometry III. Keratometry IV. Photorefraction  Subjective Refraction I. Subjective adjustment of refraction II. Subjective refinement of refraction III. Binocular balancing 43
  • 44. Retinoscopy aka ‘Skiascopy’ or ‘Shadow Test’ or ‘pupilloscopy’.  An objective method of assessing error of refraction.  Utilizes technique of neutralization.  Focault's Principle - when light is reflected from a mirror into the eye, the direction in which light will travel across the pupil will depend upon the refractive state of the eye.  aim of retinoscopy is to convert the far point of the subject's eye to the nodal point of the observer's eye.  It is performed with Retinoscope. 44
  • 45. Objective of retinoscopy To locate the far point of the eye conjugate to the retina. Myopia or hyperopia Bring far point to the infinity by using appropriate lenses. Determines amount of ametropia 45
  • 46. Retinoscopy - Optics  Part of fundus illuminated by light reflected into patient’s eye with retinoscope.  Illuminated area – serves as an object. The rays which emanate from this illuminate the pupillary area and forms its image at the far point of the eye  When the immediate source of light is moved across the eye, behavior of the luminous reflex in pupil will depend upon the refractive status of the eye. 46
  • 47. Retinoscopy - Stages 1.Illumination Stage:- Light is directed into the patients eye to illuminate the retina. 2.Reflex Stage:- An image of the illuminates retina is formed at the far point of the eye. 3.Projection Stage:- The image at the far point is located by moving the illumination across the eye(fundus) and noting the behaviour of the reflex in the patients pupil 47
  • 48. Types of Retinoscope • Reflecting mirror retinoscope – Plane mirror – Priestley-smith’s mirror • Self illuminated retinoscope – Spot retinoscope – Streak retinoscope 48
  • 49. Consists of:-  Projection System:- • light source (bulb) • condensing lens • mirror • focusing sleeve • current source  Observation system:- • peephole 49 Parts of Streak Retinoscope
  • 50. • By sliding the sleeve up or down, the distance between the bulb and lens is changed to converge or diverge the light streak. 1. Plane mirror effect: apparent light source behind retinoscope, creating parallel light rays. 2. Concave mirror effect: apparent light source in front of the retinoscope, creating concave light rays. 50
  • 51. Pre-requisites for Retinoscopy: 1. Dark room 2. A trial set consisting of: -32 Spherical lenses (plus & minus) from 0.12 upto 20 -20 Cylindrical lenses (plus & minus) from 0.12 upto 6 -Prism (1/2,1,2,3,4,5,6,8,10,12pd) -Accessories: Plano lens, Pinhole, Occluder, Stenopaeic slit, Maddox rod & Red and Green lenses. 3. A trial frame 51
  • 52. 4. Retinoscopes 5. Phoroptor (alternative to trial set & trial frame) 6.Distance vision chart: commonly Snellen’s self illuminated vision box. 7. Near Vision charts: Jaeger’s chart, Reduced Snellen’s test types & Times Roman type face. 52
  • 53. Trial Frame & Phoroptor: 53
  • 54. Refractive States of the Eye • Optics of reflex and projection stage when performed at 1 meter distance from subject with plane mirror effect depends upon the refractive status of eye as: Emmetropia or Hypermetropia or Myopia <1D: with movement Myopia of 1D: no movement Myopia >1D: against movement 54
  • 55. 55
  • 56. Optics of Movement of Reflex  Emmetropia: with movement. Light rays emerging out of eye from illuminated spot on fundus (F1) are parallel to each other. Hypermetropia: with movement. Light rays emerging out of eye from illuminated spot on fundus(f1) are divergent. 56
  • 57. Myopia (<1D): with movement. Light rays emerging out of fundus are convergent & meet behind observer sitting at 1 m from patient & examiner intercepts them before they meet. Myopia (1D): no movement. Light rays emerging out of fundus are convergent & meet at a point 1 m infront of patient i.e. at pupillary plane of observer. 57
  • 58. Myopia >1D: against movement. Light rays emerging out of the patient’s eye are convergent & meet in space between patient & observer. 58
  • 59. Procedure • Patient made to sit at an arm length distance (harmon’s distance) from examiner. • The eye level of the patient and examiner should match. • Sitting at the rightside of patient hold the retinoscope with right hand, using your right eye look for patients right eye & vice versa. • Patient is instructed to look at far point (to relax the accommodation) in dry retinoscopy or, directly into light (if cycloplegics is used). 59
  • 60. • Examiner observes red reflex in pupillary area of patient through a peephole in retinoscope. • Retinoscope is moved in horizontal and vertical meridians and characteristics of moving retinal reflex are noted. 60
  • 61. Finding Neutrality  "Against" movement - Myopic - neutralizes with minus lenses  "With" movement - Hyperopic - neutralizes with plus lenses 61
  • 62. 62
  • 63. Characteristics of Reflex  Speed: Large refractive errors have slow reflex whereas small refractive errors have faster reflex.  Brilliance: -Reflex is dull when far point is away from the observer and becomes clearer as it comes closer to neutrality. -Against reflex are usually dimmer than with reflex.  Width: Streak is narrow when the far point is away, becomes wider and at neutrality it fills the entire pupil. 63
  • 64. 64
  • 65. Finding Cylinder Axis • Before power the axis of meridians must be determined • Characteristics of streak reflex : Break : is seen when the streak is not oriented to the principle axes and it disappears when it is rotated correctly. Width : Appears narrowest when streak aligns with principle axis. Intensity: Brighter when streak is on correct axis. Skew : is the oblique motion of the reflex. The streak moves slightly differently than the reflex when the axes are not aligned. It can be used to refine axis in small cylinders. 65
  • 66. 66
  • 67. Confirmation of Axis • The axis may be confirmed with straddling technique • Streak is turned 45° off axis in both direction • If axis is correct, width of reflex should be equal in each of two positions • If axis is not correct, widths will be unequal in two positions 67
  • 68. Finding Cylinder Power +3.00 +2.00 68 With 2 Spheres :  Neutralize one axis with appropriate sphere  Keep on changing sphere till second axis is neutralized  Difference between two sphere =Cylindrical power  For Eg: If 90° is neutralized with +3.00 sphere and 180° is neutralized with +2.00 sphere Gross retinoscopy: +2.00 /+1.00× 180° Examiner's working distance +1.50ds(for 66cm) is subtracted from sphere Final refractive correction: +0.50 /+1.00× 180°
  • 69. With a Sphere and Cylinder – Neutralize one axis with spherical lens – Neutralize other axis with a cylindrical lens at appropriate orientation – Spherical cylindrical gross retinoscopy can be read directly from the trial lens apparatus 69
  • 70. End Point of Retinoscopy: • Neutralization point i.e. no apparent movement of the reflex in any meridian & pupil filled with red glow. • Verified by following maneuvers: - over correction by 0.25 D should cause reversal of movement. - on altering the working distance by forward or backward movement of the head ‘with’ or ‘against’ movement should be observed. 70
  • 71. Working Distance • Distance between the examiner and patient’s eye must be measured and converted into diopters. • Subtract dioptric equivalent of working distance from neutralization point reached in retinoscopy. • Eg: working distance of arm’s length (̴ 66cm) requires +1.50 D, 50cm distance requires +2D and 1 meter distance requires +1D • Working Distance must remain constant throughout the examination. 71
  • 72. Cycloplegic Refraction Causes: paralysis of accomodation & dilatation of pupil. When cycloplegics is used - Wet Retinoscopy. Atropine: < 5 yrs, 1% , TDS x 3days, Retinoscopy on 4th day, Effect 10-20 days, PMT after 3wks, Tonus allowance 1D Homatropine: 5–8 yrs,2% Gtt. every 10min x 6times, Retinoscopy after 90 min, Effect for 48-72hrs, PMT after 3days, Tonus allowance 0.5D Cyclopentolate:8–20 yrs,1% Gtt every 15min x 3times, Retinoscopy after 90 min, Effect for 6-18 hrs, PMT after 3days, Tonus allowance 0.75D 72
  • 73. Indications:- • Childrens (<15yr) • Patient with active accommodation • Hyperopic patients upto 35yr (if symptomatic) • Early Presbyopia (if using glasses) • Variable powers on dry refraction • Symptoms disproportionate to refractive error • Accommodative Esotropia • Bilateral refractive asymmetry • Mentally disabled or uncooperative patients • Suspected Malingering 73
  • 74. 74
  • 75. Postmydriatic Test • It refers to repeating the refraction after the effect of cycloplegia has worn off. • Ideally, it is done in all cases of cycloplegics refraction if there is a difference in the correction in dry and cycloplegics refraction. • The next visit depends on the cycloplegics agent used. Eg. For atropine it is done after 3 weeks, for homatropine and cyclopentolate it is done after 3 days. 75
  • 76. Rough Estimate of Refractive Error after Retinoscopy: • Gross Retinoscopy: retinoscopic value • Net Retinoscopy= Gross Retinoscopy - [Working Distance + Tonus Allowance(if used)] • If retinoscopic finding is +7 Ds at 1 meter with Atropine then, Refractive Error = +7 – (1+1) = +5 D 76
  • 77. Autorefractometry  Alternative method of detecting refractive error using optical equipment called Refractometer or Optometer.  Based on 2 principles: -Scheiner Principle -Optometer Principle  Advantage: -Faster -No training required  Disadvantages: -Expensive -Need electricity -Wrong reading on irregularities & opacities 77
  • 78. Lensometer  An instrument used to measure the power of a patient’s present spectacle lenses. Both manual and automated lensometers are available.  Measures 4 principal properties : i. spherical and cylindrical power ii. axis if cylinder is present iii. presence and orientation of prism iv. optical centration 78
  • 79. Subjective Refraction • Should proceed as: A. Subjective adjustment of refraction B. Subjective refinement of refraction C. Binocular balancing 79
  • 80. A. Subjective Adjustment of Refraction 1. Trial & Error technique 2. Fogging Technique 80
  • 81. 1. Trial & Error technique • This employs use of different spherical & cylindrical lenses. • SPHERICAL LENSES: -the strongest convex lens and the weakest concave lens providing best vision should be chosen for hypermetropia and myopia respectively. 81
  • 82. • CYLINDRICAL LENSES Need verification of axis and power. Verification of axis: - Best done by simply rotating the cylinder in steps of 5 or 10 ˚ in either direction and asking whether acuity improves. Verification of Power -by changing the cylindrical lens in the trial frame. 82
  • 83. 2. Fogging 83 Principal: to make eye artificially myopic by increasing plus power (or decreasing minus power). Fog eye by adding 1 to 1.50D power more than retinoscopy finding. Then unfog by reducing 0.25D till best Snellen’s visual acuity is attained
  • 84. B. Subjective refinement of refraction - First refine sphere than cylinder 1. Refining Sphere i. Fogging test ii. Duchrome test iii. Pinhole test 2. Refining cylinder i. Jackson’s cross cylinder technique ii. Astigmatic fan & block technique 84
  • 85. 1. Refining Sphere • Done by:- i. Fogging technique ii. Duchrome test iii. Pinhole test 85
  • 86. Duchrome Test • Determines too much minus or too much plus correction. • Due to chromatic aberration, the shorter green wavelength is focused in front longer red wavelength • Patient is asked to read letters with red and green background • Emmetropic: both colors equally sharp • Hypermetropic: green is sharper, indicates over minused , plus should be added. • Myopic: red is sharper, indicates over plused, minus should be added. 86
  • 87. Pinhole Test • Pinhole size varies 1 - 2mm (ideal size 1.2 mm). • Can correct ref. errors between +5.0D and -5.0D. • Pinhole admits only central rays of light which do not require refraction by the cornea/lens, thus cutoff the scattered rays increasing the depth of focus. So that the blur created by optical irregularities or refractive error becomes reduced. • It confirms whether the optical correction is correct or not. • An improvement in vision through a pinhole indicates that more refractive correction is still possible. 87
  • 88. 2. Refining Cylinder Done by:- i. Jackson’s cross cylinder technique ii. Astigmatic fan & block technique 88
  • 89. Jackson’s cross cylinder technique • Combination of 2 cylinders of equal strength, with opposite sign placed at right angles (90°) to each other. • Cross cylinder is placed with its axis 45° to axis of cylinder in trial frame. • If no change, axis of correcting cylinder is correct. • If improvement occurs correcting cylinder should be rotated. Should be repeated several times until neutral point reached. 89
  • 90. Refinement of Cylinder Power • Align the JCC axis with cylinder axis. • Flip the cross cylinder and ask the patient “which is better?” • Add or subtract cylinder power according to preferred position of JCC • For every change in -0.50 dcyl add +0.25dsph and viceversa. 90
  • 91. Astigmatic Fan & Block Test • Consists of series of radiating lines spaced at 10o interval & arranged in a manner of rays of a rising sun around a central panel carrying a V and two sets of mutually perpendicular lines ( the blocks). • ‘V’ & ‘block’ simultaneously can be rotated through 180o. 91
  • 92.  Patient is asked to see the fan after fogging by + 0.5 D • Normal patients can see all the lines clearly. • In astigmatism, some lines are seen more sharply defined. • Concave cylinder is then added with each axis at right angle to the clearest line until all the lines are equally sharp 92
  • 93. C. Binocular Balancing • Process sometimes known as equalizing accommodative effort or equalization of vision • It allows both eyes to have retinal image simultaneously in focus • Methods: – Alternate Occlusion – Prism dissociation 93
  • 94. Near Refraction • Usually done at age >40years or, pseudophakic eyes (irrespective of age). • Extra plus sphere is added to distant correction usually started with +1DS • Patient’s range of clear vision is determined by using near vision chart – If range is too close: power is reduced, add -0.25DS in steps – If range is too far: power is increased, add +0.25DS in steps 94
  • 95. 95
  • 96. Lens Notation • A written spectacle prescription follows a standard format • Power of sphere(Sph) is recorded first with sign(+/-), followed by power of cylinder(Cyl) with its sign(+/-) and axis(°) • Prescription is recorded for each eye using abbreviation OD (oculus dexter) for the RE, OS (oculus sinister) for the LE. 96
  • 97. -To change the lens to another optically equivalent form. -Usually spectacle are transposed & made as minus cylinders, even if the prescription was written in plus cylinder form. WHY?? • Minus cylinder (concave) lens is thinner, lighter and more cosmetic. • Minus cylinder reduces some amount of aberrations. • Plus cylinder produce more magnification than the equivalent minus-cylinder form. • Plus cylinder is often difficult to adjust for the wearer. 97 Lens Transposition
  • 99. Cylindrical power can be recorded in either plus or minus form . 3 Steps:- • Add cylinder power to sphere power • Sign of cylinder power is reversed • Rotate axis of cylinder through 90° For Eg: +2.50 ds/ +0.50dc x 180° +3 ds/ -0.50dc x 90 ° -1.00ds /+1.50dc× 90° +0.50ds/ -1.50dc ×180° 99
  • 100.  Optical Cross into Sphero-Cylinder: • Small power (sphere) • Big-small power difference (cylinder) • Axis of small power  Sphero-Cylinder into Optical Cross: • Sphere (given axis) • Sphere+Cylinder (opposite axis) 100 +3 +2.50 Eg:- +2.50 dsph/ +0.50dcyl x 180° +3 +2.50 Eg:- +2.50 dsph/ +0.50dcyl x 180°
  • 101. Spherical Equivalent  It is average power of a spherocylindrical lens. It represents the dioptric position of the circle of least confusion of the conoid of Sturm. -to compare or balance both eyes, and -to reduce an excessive cylindrical correction.  Calculated as: Spherical equivalent = power of the sphere + (cylinder power/2) 101
  • 102. References • Borish’s Clinical Refraction 2nd Edition • Duke-Elder’s Practice of Refraction 8th Edition • AAO-Clinical Optics 2022-2023 • A. R. Elkington, Clinical optics (1999) • Practical Ophthalmology AAO 8thEd 102

Editor's Notes

  1. Electromagnetic spectrum from short to long wavelength, gamma rays, x-rays, ultraviolet, visible light, infrared, microwaves, radio waves Visible light is usually defined as having wavelengths in the range of 400–700 nanometres
  2. constructive interference: resultant intensity is greater than the sum of the intensities due to separate waves destructive interference: resultant intensity is lesser than the sum of the separate intensities.
  3. Uses of prisms • In gonoiscopes,aplantation tonometer, keratometry • Relieve diplopia • To examine angle of deviation (squint)
  4. It can form both real and virtual images based on the object’s position. object beyond the focus (F1) of the lens forms a real image object between the focus (F1) and the optical centre of the lens forms a virtual image.
  5. Images in a Concave Lens -Causes light rays to diverge. -Always VIRTUAL, UPRIGHT, REDUCED. -The image is found on the same side of the lens as the object.
  6. To make simple optical calculations a Reduced Schematic Eye, based on Gullstrand’s model, was developed that approximately matches the ocular dimensions & simplifies the calculations by combining all the refracting surfaces into one power and location and all the refractive indices into one. Conceptualized by a Swedish Professor , Awarded Nobel Prize in 1911
  7. Near Point: The near point is defined as the closest point on which an unaided human eye can focus without straining. It is taken to be 25cm for a normal eye, but can change greatly with age. Far Point: The far point is defined as the largest distance an eye can focus without straining. a point in space that is conjugate to the fovea of the non accommodating eye. In emmetropic eye, far point is at infinity.
  8. Low myopia (<2 D) Moderate myopia (2-6 D) High myopia (>6 D)
  9. Low hyperopia (<2 D) Moderate hyperopia (2-5 D) High hyperopia (>5D)
  10. Eye has different powers across different meridians PRINCIPLE MERIDIANS: The steepest and flattest meridians of the eye are called principal meridians. WTR: more power in vertical meridian (V>H), corrected with axis of plus cyl close to 90 (vertical meridian is normally 0.25 D more convex than the horizontal meridian by the pressure of eyelids) ATR: more power in horizontal meridian (H>V), axis of plus cyl close to 180 - > 0 > +
  11. Illumination Stage: illumination of subject’s retina. Reflex Stage: reflex imagery formed by subject’s diopteric apparatus. Projection Stage: projection of image by observer.
  12. When a plane mirror is used to reflect the light, the immediate source of light moves against the movement of the mirror. When a concave mirror is used, the immediate source of light moves with the movement of the mirror.
  13. Red filter: convergence, color dissociation, diplopia charting,worth 4 dot test, suppression of eye Green : binocular vision Maddox rod: phoria measurement Prism: phoria and tropia measurement Stenopic slit: irregular astigmatism
  14. Emmetropia: no movement of reflex, i.e., Neutralization. Parallel rays so far point at optical infinity. Myopia: convergent rays, so far point in front of the eye. Against movement reflex. Hypermetropia: divergent rays , so far point behind the eye. With movement reflex.
  15. Emmetropia: Far point is at infinity. Myopia: Far Point is between infinity and eye Hypermetropia :-Far point is beyond infinity
  16. Accomodation relaxed is achieved by fixating at non accommodative target or fogging fellow eye placing +1.5 or 2 d spherical lens on top of presumed refraction With spot retinoscope whole of pupil glows as red reflex With streak retinoscope red reflez is seen on a band of light
  17. Checking the HORIZONTAL meridian by vertical streak and Checking the VERTICAL meridian by horizontal streaks
  18. Myopia: far point in front of the eye. Against movement reflex. Hypermetropia: far point behind the eye. With movement reflex.
  19. Speed: slow in large refractive error and vice versa Bright in low degrees of ametropia and dull in high ameteropia Streak narrow in high degree of ametroia and wide in low
  20. In a patient with astigmatism we want to neutralize 2 reflexes 1 from each principle meridian Break: reflex streak in pupil is not aligned with streak projected on the iris and surface of eye and line appears broken Skew: The reflex follows the direction of the correct meridian rather than the following the streak. It is a feature of astigmatism
  21. Break: appears when the streak is not parallel to one of the meridia. - orientation of reflex streak in pupil is not the same as that of streak. Skew: If streak is off axis, it moves slightly in different direction from pupillary reflex. - If reflex & streak move in same direction, it is aligned with one of the principal meridia.
  22. Straddling:- Done to refine small errors in the determined axis. Move closer to the patient till we get with movement in both axis• The streak is turned 45 degrees off axis in both directions. If the axis is correct the width of the reflex is EQUAL in each of two position
  23. Emmetropia: no movement of reflex, i.e., Neutralization. Parallel rays so far point at optical infinity.
  24. Examiner cannot seat at infinity
  25. An autorefractor helps detect the lens power required to accurately focus light on your retina. It takes a few seconds and requires you to put your chin on the chin rest and look towards the picture seen straight ahead in the machine without blinking for a few seconds to get an accurate reading. It is a time saving procedure compared to the manual refraction taking only a few seconds.
  26. refraction techniques, the examiner relies on the patient’s responses to determine the refractive correction.. Subjective acceptance to the retinoscopic power is assessed & to obtain most suitable power a/c to patients acceptance Performed with the referance to retinoscopic/ AR power
  27. the strongest convex lens and the weakest concave lens providing best vision should be chosen for hypermetropia and myopia respectively.
  28. Light of different wavelength gets refracted differently. Shorter wavelengths (green) are refracted more than longer wavelengths (red). The eye is hyperopic for red and myopic for green light
  29. Process of equalization of vision of both eyes
  30. Where there is loss of accommodative ability of crystalline lens d/t ageing or IOL
  31. TRANSPOSITION: When a lens prescription is changed from one lens form to another optically equivalent form Types: Simple & Toric transposition (manufacturing level) To compare the present refraction with a previous prescription Can be transposed to optically equivalent form a/c the commercially availability
  32. Reduce 90° to cylinder axis, if resulting axis > 90° or Add 90° to cylinder axis if resulting axis < 90.