Clinical optics and ophthalmic instruments

Basics of Clinical optics,
ophthalmic Instruments and
eye examination
Dr. Zia-Ul-Mazhry
FCPS(Pak),
FRCS(Edin),
FRCS(Glasgow),
CIC Ophth- (UK)
Associate Professor
Head of Eye Department
Central Park Medical College &
WAPDA Teaching Hospital Complex Lahore
Pakistan.

• For Education purpose Only
• Material Taken from Various Internet
Resources

Overview
• Part 1
– Pretest
• Part 2
– Clinical Optics and their ophthalmic
Significance
• Part 3
– Ophthalmic instruments and Dark Room Tests
in Ophthalmology
• Part 4
– Post Test

Objectives
• The students should be able to:
– Their learning needs and gains
– Understand Clinical Optics and their
ophthalmic Significance
– Use Ophthalmic instrument and perform
eye Examination
– Put it al together to Evaluate, diagnose and
investigate common ocular conditions

MCQ-1
• The Focal length of a lens is:
a) Inversely proportional to radius of the
lens.
b) Directly proportional to the power in
diopter.
c) Inversely proportional to power in
diopter.
d) Is not related to power to diopter.

MCQ-2
• Average dioptric power of adult cornea is:
a) 32 D
b) 42 D
c) 52 D
d) 62 D

MCQ-3
• Average axial length of the normal adult
eye is:
a. 18-20 mm
b. 12-14 mm
c. 16-18 mm
d. 22-24 mm

MCQ-4
• Clinical assessment of cataract
progression is done through
a. Snellen’s visual acuity test
b. Distant Direct ophthalmoscopy
c. Slit lamp Examination
d. all of above

MCQ-5
• A perimetry is used to measure:
a. Central and peripheral fields
b. Visual acuity
c. Intra ocular pressure
d. Central field only

MCQ-6
• Objective of distant direct ophthalmoscopy
is:
a. Scleral examination
b. Conjunctival examination
c. Examination of media opacities
d. Fundus examination

MCQ-7
• The image formed during direct
ophthalmoscopy is:
a. Inverted and real
b. Erect and real
c. Inverted and virtual
d. Erect and virtual

MCQ-8
• In the process of reflection Angle of
incidence is always:
a. Equal to angle of reflection
b. Grater than angle of reflection
c. Lesser than angle of reflection
d. None of above

MCQ-9
• Most of the refraction occurs at the site
of:
a. Retina
b. cornea
c. Lens
d. Aqueous humor

MCQ-10
• Test being
performed is used to
measure:
a. Corneal diameter
b. Lens Thickness
c. Anterior chamber Angle
d. All of above

Part 2
Clinical Optics and their

Clinical Optics and their
• Language of Optics
• Vergence
• Reflection and Refraction
• Lenses, Mirrors, prisms and fiber optics
• Focal Length and focal power

Language of Optics
• Light and Light rays
• Vergence

Language of Optics
• Object and Image
O
I

Language of Optics
• Lateral Reversal of Image

Real vs. Virtual Images
• Real images are formed by mirrors or lenses
when light rays actually converge and pass
through the image.
• A real image can be projected onto a piece of
paper or a screen. If photographic film were
placed here, a photo could be created.
• Virtual images occur where light rays only
appear to have originated.
• Virtual images can’t be projected on paper,
screens, or film since the light rays do not really
converge there.

Language of Optics
• Medium and interface
Interface

Language of Optics
• Focal Length and Dioptric Power
The degree of convergence or divergence of a lens is expressed in
terms of its power.
Thus power of a lens is defined as the reciprocal of its focal length.
D = 100
f(cm)

Lenses: Focal Length
• Like mirrors, lenses have a principal axis perpendicular to their
surface and passing through their midpoint.
• Lenses also have a vertical axis, or principal plane, through
their middle.
• They have a focal point, F, and the focal length is the distance
from the vertical axis to F.
• There is no real center of curvature, so 2F is used to denote
twice the focal length.

Language of Optics
• Erect vs. Inverted Image

Language of Optics
Simplified Ray Diagram

Reflection of Light
• A ray of light, the
incident ray, travels in
a medium
• When it encounters a
boundary with a
second medium, part
of the incident ray is
reflected back into the
first medium

Reflection
surface normal
same
angleincident ray exit ray
reflected ray

Diffuse reflection is reflection
from a rough surface
Specular reflection is reflection from
a smooth surface

Critical angle is the angle of
incidence for which the angle of
refraction becomes 90

Total internal reflection is an optical
phenomenon that happens when a ray of
light strikes a medium boundary at an angle
larger than the critical angle.

Mirages
Mirages are
caused by the
refracting
properties of a
non-uniform
atmosphere.

The incident angle of light reﬂected from the
anterior chamber angle is greater than the
critical angle at the cornea–air interface.

Dispersion of Light
Dispersion is the separation of light into a
spectrum by refraction.
This effect causes white light to split into it
spectrum of colors.

One of the most
common of these is the
rainbow, which is
caused by water
droplets dispersing
sunlight.
Dispersion-Rainbows

REFRACTION
• Refraction is the change in direction of a wave
due to a change in its optical medium.
Normal
Lighter Medium
Denser Medium

REFRACTION IN
OPHTHALMOLOGY
• The refraction test is an eye exam that
measures a person's prescription for eyeglasses
or contact lenses.
• This test is performed by an ophthalmologist
or optometrist

Lenses and Mirrors
Lenses
• A piece of glass
material, used to
converge or diverge
transmitted light and
form optical images.
Mirrors
• A surface that
reflects light without
diffusion.

Concave and Convex Mirrors
Concave mirrors reflect
light from their inner
surface, like the inside
of a spoon.
• Convex mirrors
reflect light from their
outer surface, like the
outside of a spoon.

Convex Mirrors
• Curves outward
• Reduces images
• Virtual images
– Use: Rear view mirrors, store
security…

Lens
• A transparent material that is has at least
one curved side
• Refracts light in a predictable and useful
way

Types of Lenses
Converging Lens
• makes parallel light rays
come together
• Also known as convex
lens
• Thicker at the center,
thinner at the edges

Types of Lenses
Diverging Lens
• makes parallel light rays
move apart
• Also known as concave
lens
• Thinner in the center,
thicker at the edges

Lens Function
• Change the appearance of
objects (image appears
larger, smaller, upside down
or misshapen)
• Magnify images
• Project images onto a
screen

Terminology
• Optical Centre (O) – centre of the lens; also the
point where the principal axis crosses
• Axis of Symmetry – an imaginary vertical line
drawn through the optical center of a lens;
perpendicular to principal axis
Optical
Centre (O)

Focal Point
• Since lenses have 2 sides, they have a focal point
on each side.
• The principal focus is where the light rays converge
• A converging and diverging mirror has the principal
focus on different sides of the lens

Focal Point – Converging Lens
• Principal focus is on the opposite side of the lens
as the incident rays
Principal FocusSecondary
Principal Focus

Focal Point – Diverging Lens
• Principal focus is on the same side of the lens
as the incident rays
• The focal point is virtual

Focal Length
• Focal length (f) is the distance between the
principal focus and optical centre
• Distance from F to O, F to 2F, F’ to O, F’ to 2F
are all equal.
• 2F and 2F’ are twice the distance away from the
optical centre than the focus points.
FF’ 2F2F’ O

Terminology
Principal Focus (F)
• A point on the principal axis where the light rays that are traveling
parallel to the principal axis appear to converge
• In a converging lens it is on the opposite side as the incident rays
• In a diverging lens it on the same side as the incident rays
Secondary Principal Focus (F’)
• Focus that is on the opposite side as the principal focus, located the
same distance from the lens as F
• In a converging lens it is on the same side as the incident rays
• In a diverging lens it is on the opposite side as the incident rays
Focal Length (f)
• distance from the principal focus to the axis of symmetry
• Focal length is the same distance with the secondary principal focus

Light Rays with a Converging Lens
1. A light ray parallel to the principal axis will
refract through the principal focus (F).
2. A light ray passing through the secondary
principal focus (F’) will refract parallel to
the principal axis.
3. A light ray through the optical centre (O)
will keep travelling in the same direction
without being refracted.

FF’

the principal axis.

the principal axis.
FF’

FF’

LOCATING AN IMAGE ON A CONVERGING LENS
• What happens when object is at F’?
2F’ 2F’

1. Pick a point on the object (usually the top of
the object)
2. Send any two light rays off the point on the
object.
3. Determine the refracted rays. Light rays only
refract when it hits the lens
4. Find the intersection of the light rays. If the
rays do not intersection, extend the refracted
ray until they do (exception: don’t do this for
parallel light rays)
5. Use the point of intersection to location off an
object is needed to locate its image.

Ray 1 – travels parallel to the principal
axis and is refracted through the principal
focus (F)

Ray 2 – travels through the optical centre
(O) and continues straight without being
refracted

Ray 3 (optional since you only need 2 lines to form an intersection) –
travels through the secondary principal
focus (F’) and refracts parallel to the
principal axis

Draw the image given the point of
intersection.

5 Scenarios for the Object
1. Beyond 2F’: Object is greater than 2 focal
lengths from the lens (do>2f)
2. At 2F’: Object is at 2 focal lengths (do=2f)
3. Between 2F’ and F’: Object is between 1
and 2 focal lengths from the lens (f<do<2f)
4. At F’: Object is at the focal point (do=f)
5. Between F’ and lens: Object is less than 1
focal length away from the lens (0<do<f)

Case 1 – Object beyond 2F’
Size Attitude Location Type
Reduced Inverted
Between F
and 2F
Real

Converging Lens
Object beyond 2F’

Case 2 – Object at 2F’
Same
size
Inverted At 2F Real

Case 3 – Object between 2F’ and F’
Enlarged Inverted
Beyond
2F
Real

Case 4 – Object at F’
No Image Formed!

Case 5 – Object between F’ and lens
Enlarged Upright
Same side
as object
Virtual

Converging Lens
Object between F’ and Lens

CONVERGING LENSES
The first lens known to be used was a converging lens called
a reading stone. Reading stones formed images that were
upright and larger than the object.
How can a converging
lens produce both
upright and inverted
images?

DIVERGING LENSES
• The focal point is virtual
• Thus the principal focus is on the same side
of the lens as the incident rays

Light Rays with a Diverging Lens
the principal axis.

F’F 2F’2F O

principal focus (F’) will refract parallel to the
principal axis.
F’F 2F’2F O

3. A light ray through the optical centre (O) will
keep travelling in the same direction without
being refracted.
F’F 2F’2F O

LOCATING AN IMAGE ON A DIVERGING LENS
1. Pick a point on the object (usually the top of
the object)
2. Send any two light rays off the point on the
object.
3. Determine the refracted rays. Light rays only
refract when it hits the lens
4. Find the intersection of the light rays. If the
rays do not intersection, extend the refracted
ray until they do (exception: don’t do this for
parallel light rays)
5. Use the point of intersection to location off an
object is needed to locate its image.

• Q. Draw the ray diagram for this pencil to
locate its image
F’F 2F’2F O

F’F 2F’2F O
Ray 1 – travels parallel to the principal axis and
is refracted through the principal focus (F)

F’F 2F’2F O
Ray 2 – travels through the secondary principal
focus (F’) and refracts parallel to the principal
axis

F’F 2F’2F O
Ray 3 (optional since you only need 2 lines to form an intersection)
travels through the optical centre (O) and
continues straight without being refracted

F’F 2F’2F O
Reduced Upright
Same side
as object
Virtual

Diverging Lens
The image formed is always virtual,
upright and smaller

Concave Lens Ray Diagrams
Ray 1 - travels from the tip of the object parallel to the
principal axis. When it emerges from the lens, it appears to
come from the principal focus (F)

Ray 2 - travels from the tip of the object through the optical
centre of the lens and is not refracted.

The image occurs where these rays appear to intersect.

Part 3
Ophthalmic instrument
and Examination
Methods

Dark room Tests
• Oblique Illumination/Diffuse light
examination/Inspection
• Distant Direct Ophthalmoscopy
• Direct Ophthalmoscopy
• Retinoscopy
• Indirect Ophthalmoscopy
– Indirect Ophthalmoscope
– Slit Lamp Biomicroscopy

Oblique Illumination/Diffuse
light examination
• Objective
– Examination of External and anterior
Segment structures
• Instruments
– Illumination Beam
– Magnification Aid/Microscope
• Method
– Illumination beam at 45 degrees

Slit Lamp
• The slit-lamp
is a low-power
microscope
combined with
a high-
intensity light
source that can
be focused to
shine in a thin
beam.

SLE
• Observation by optical
section
• Direct diffuse illumination
• Indirect illumination
• Retro-illumination
• Scattering sclero-corneal
illumination
• Fundus observation and
gonioscopy with the slit
lamp

Distant Direct
Ophthalmoscopy
• Objective
– To Examine and classify media
opacities against fundal glow
• Instrumentation
– Direct Ophthalmoscope
• Methods
– Throw the light with DO at half
meter 25-50 cm distance
– Parallax Method of deviation

Distant Direct
Ophthalmoscopy
• Parallax Method of deviation
– Ask the patient to move his/her eye
– Opacities Moving
• With
– Anterior to nodal point
• No movement
– At or very near to nodal point
• Against Movement
– Behind the nodal point

Distant Direct
Ophthalmoscopy
• Viewing ocular media
– Observe red reflex
– Look for media
opacities
• Cataracts
• Corneal scars
• Large floaters

Direct Ophthalmoscopy
• Objective
– To examine the retina/fundus
• Instrumentations
– Direct ophthalmoacope
• Principle

Principle of Direct
Ophthalmoscopy
• If patient and observer are both
emmetropic, rays emanating from a
point in the patient's fundus will emerge
as a parallel beam and will be focused
on the observer's retina.

Fundoscopy
• Fundoscopy is the
assessment of the
fundus using an
ophthalmoscope

Examination Technique
• dim the lights.
• ask the patient
to fixate on a
distant target.
• approach the
patient from
the side.
• examine the
optic nerve and
surrounding
retina.

Direct Ophthalmoscopy:
Basic skills
• Proper position for
central fundus viewing
• Right eye to right eye
• Left eye to left eye
• Don’t rub noses…

Basic skills
• Proper position for
peripheral fundus
viewing

Exam technique
• Be systematic
• Start at optic disc & work radially
• Observe:
– Optic disc: C/D ratio
– Vessels: course & caliber, AV ratio, light
reflex, crossings/banking
– Macula
– Peripheral fundus

Indirect Ophthalmoscopy
• Objective
– Fundus Examination
• Instrumentation
– Indirect Ophthalmoscope
– Condensing Lens
• Methods

Funduscopy
Techniques/instruments
• Direct Ophthalmoscopy
• Indirect Ophthalmoscopy
• Fundus Biomicroscopy
• Fundus Contact Lens

Direct Ophthalmoscopy
• Advantages
– Portable
– Easy to use
– Upright image
– Magnification  15x
– Can use w/o dilation
• Disadvantages
– Small field of view
– Lack of stereopsis
– Media opacities can degrade
image

Indirect Ophthalmoscopy
• Monocular or binocular
• Advantages:
– Wide field of view
– Binocular instruments
provide stereopsis
• Disadvantages:
– Requires more skill
– Decreased magnification
(3x)
– Requires dilation
– Inverted image

Indirect Ophthalmoscopy principle
• The principle of indirect
ophthalmoscopy is to
make
• the eye myopic by
placing a strong convex
lens in front
• of it. This forms a real
inverted image of the
fundus in
• the air between the lens
and the observer.

Fig. 5. Limited field of
view in the direct
method. Peripheral
pencils of light do not
reach the observer's
pupil.
Fig. 6. Extended field
of view in the indirect
method. The
ophthalmoscopy lens
redirects peripheral
pencils of light toward
the observer.

Fundus Biomicroscopy
• Field of View & Mag:
– FOV <indirect but
>direct
– varies w/lens & slit
lamp mag
• Inverted image
• Stereopsis
• Dilated pupil
• Requires skill

Fundus Contact Lens
• Requires physical
contact w/eye
• Viewed
w/Biomicroscope
• Advanced dx &
surgery
• Field of view & Mag
vary w/lens design

Viewing the Optic Nerve Head
• Observe:
– Size
– Shape
– Color
– Margins
– Cup to disc ratio (C/D) horiz & Vert

Blood Vessel Evaluation
• Observe:
– Vessel diameter
– Shape/tortuosity
– Color
– Crossings
– Light reflex
– Artery/Vein (A/V) ratio: after 2nd
bifurcation

Retinoscopy
• Objective
– To determine refractve status of the eye
• Instrumentation
– Retinoscope
– Trial Lens set
– Trial frame

Retinoscopy
• Methods
– Ask the patient to fixate on a distant target
– Half to 1 meter distance
– Project the streak on pupil
– Move the streak and observe the movement
of red reflex

Retinoscopy
•Moves with
–Emetropia
–Hyperopia
–Myopia of less than 1 diopter
•No Movement
–Myopia of exactly 1 diaopter
•Moves against
–Myopia of more than 1 diopter

Retinoscopy
• Insert +1 D Lens in front of the eye
while working at 1 meter
– No movement
• Emetropia
– With movement
• Hyperopia
– Against movement
• Myopia of < 1 D

RETINOSCOPE
BASIC CONCEPT
1) EMMETROPIA
• Light emerge Parallel
2) HYPEROPIA
• Light emerge Diverge
3) MYOPIA
• Light emerge Converge

• BREAK PHENOMENON
Iris pupil Streak light light reflex
No Break Break
RETINOSCOPY

• THICKNESS PHENOMENON
Same thickness Different thickness
RETINOSCOPY

• SKEW PHENOMENON
light reflex movement streak movement
No skewness Skew
RETINOSCOPY

• MOVEMENT PHENOMENON
With movement Against movement
RETINOSCOPY

• NEUTRALITY
- pupil fills
- no movement
RETINOSCOPY

• With movements
– Hyperope
– Neutralize with Plus lenses (convex lenses)
• Against movements
– Myope
– Neutralize with Negative lenses (concave
lenses)
RETINOSCOPY

• Working Distance Lenses
– Purpose : Light emerge from Pt eye
conjugate with examiner’s retina
• +1.50 D - @ 66.7cm
• +2.00 D - @ 50.0cm
RETINOSCOPY

• Thank you
• www.eyeacuity.com
• www.slideshare.net

Clinical optics and ophthalmic instruments

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