This lecture is based on post-graduate medical students of Ophthalmology Optics of Direct and Indirect Ophthalmoscope (DO. DCO. MS. FCPS)

2. BASIC PRINCIPLES OF DIRECT
OPHTHALMOSCOPY
The basic principle of direct ophthalmoscopy is simple. If
the patient’s eye is emmetropic, light rays emanating from
a point on the fundus emerge as a parallel beam.
If this beam enters the pupil of an emmetropic observer,
the rays are focused on the observer’s retina and form an
image of the patient’s retina on the observer’s retina. This
is called direct ophthalmoscopy.

3. Fig: 1. Imaging in 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

4. • However, the reality is not so easy coz if the
patient, light source and observer all in a same
plane there will be obstacle so how the observer
see the fundus.
• If the light source is away from the same plane,
what will happen look at the pic: 2

5. Fig: 2. The illumination problem in direct
ophthalmoscopy. If the light source and the observer
are not aligned optically, the observer views a part of
the fundus that is not illuminated.

6. So, what is the solution?
Optical principle
of direct
ophthalmoscope

7. Fig: 3. The solution, the entering and emerging
light will be different path and the light source
will not be middle of the patient and observer.
Look at the above picture

8. • The instrument consists of a system of
lenses which focus light from an electric
bulb on to a mirror where a real image of
the bulb filament is formed. The mirror
reflects the emitted light in a diverging
beam which is used to illuminate the
patient's eye.

9. • The mirror contains a hole through which
the observer views the illuminated eye.
The image of the bulb is formed just
below the hole so that its corneal
reflection does not lie in the visual axis of
the observer (Fig: 3). (EK: 165)

10. • There are several ways in which optical
alignment of the illuminating and observing
beams can be accomplished. (Fig: 4)
• Von Helmholtz solved the problem with a semi
reflecting mirror made up of several thin parallel
pieces of glass.

11. • Epkens and Ruete used a perforated concave mirror,
which places illuminating light rays all around the
observation beam.
• Most hand-held instruments now have a
small mirror or prism, which uses the lower
half of the patient’s pupil for illumination
and the upper half for observation.

15. Principle of direct ophthalmoscope: 𝑏𝑁𝐹𝑁𝑏 is the principle axis of
the two eyes; F is the point where the focus of both the eyes
coincide; N and 𝑁 are the two nodal points. A ray AF𝐴 is refracted to
reach the observer’s eye at 𝑎 forming an image 𝑎𝑏 of ab, of the
same size as ab aA is a ray parallel to optical axis bNF𝑁𝑏. A is the
point where a A meets the observer’s eye and refracted as 𝐴𝑎 . 𝐵𝑁𝑎
passes through the nodal point 𝑁 undeviated and meet the
observer’s eye at 𝑎. 𝑎𝑏 is the image of ab on the observer’s eye.

16. Field of view in direct Ophthalmoscope
The maximum field of view is determined by the
most oblique pencil of rays (shaded) that can still
pass from one pupil to the other.
Angle α, the field of view, is increased when the
patient's or the observer's pupil is dilated or when
the eyes are brought more closely together.
Most ophthalmoscopes project a beam of light of
about one disc diameter

17. Field of view in direct Ophthalmoscope
There are two ways to increase the field of view in
direct ophthalmoscope
a) when the patient's or the observer's pupil is
dilated or
b) when the eyes are brought more closely together

18.  Remember:
The area (of a circle)=Π x radius squared
So, if you double the pupil size with dilation, the
“window” (pupil) is open four times as wide (and
the view is much better)

19.  Angle α, the field of view, is increased when the
patient's or the observer's pupil is dilated or when
the eyes are brought more closely together.
 Most ophthalmoscopes project a beam of light of
about one disc diameter

20. Magnification by direct Ophthalmoscope
The formula for magnification achieved by a loupe
is M=F/4 where M is the magnification and F the
dioptric power of the loupe.
Dioptric power of an emmetropic eye is +60 D, the
magnification of the direct ophthalmoscope is 60/4
= 15. (EK: 173)

21. Practical application of Magnification
This degree of magnification makes the direct
ophthalmoscope particularly useful when
examining patients with retinopathy, for it allows
most micro-aneurysms to be seen.

22. How direct ophthalmoscope works?

23. R: Patient’s retina.
P: Principal plane.
N: Nodal point.
Fa: Anterior focal
point
R0: Observer’s retina.
P0: Principal plane.
N0: Nodal point.
Fa: Anterior focal point

24. • The position and size of the image formed in the
observer's eye can be constructed by first
constructing the image, xy, of the illuminated
retina XY which is formed at the patient's far
point.

25. A ray from the top of that image, passing through
the observer's nodal point, No, locates the
position of the top of the image, X'Y', on the
observer's retina Ro.

26. It can be seen from these figures that the image
formed in the observer's eye is inverted and is
therefore seen as erect. Also, the image size varies
with the refractive state of the patient's eye, the
image being smaller in hypermetropia, and larger in
myopia than in emmetropia.

27. BASIC PRINCIPLES OF INDIRECT
OPHTHALMOSCOPY
EXTENDING THE FIELD—INDIRECT
OPHTHALMOSCOPY

28. Look at the above picture, that among the four pencils
of rays only two can enter into the observer eye. And
two pass outside the eye.

29. So, what can we do to send all the rays into the
observer eye? If we placed a condensing lens
between the patient and observer all the rays
will enter into the observer eye. Therefore
illumination with indirect ophthalmoscope is
better than direct one

30. Properties of condensing lens
Dioptric power 30 D 20 D 14 D
Magnification 2 3 4
Field 50 deg 45 deg 40 deg
Stereopsis ½ normal ¾ normal 1 normal
Focal Length 3.3 cm 5 cm 7 cm

31. Features Direct Indirect
Magnification 15 x 2,3,4 X
Field of view Small Large
Illumination Low High
Stereopsis Absent Present
Orientation of image Direct Inverted
Reversed
View of periphery No Yes
Scleral indentation No Yes

34. 1) Adjusting the instrument
2) Positioning of the patient
3) The examination proper & the fundus
drawing
4) scleral indentation
Technique of Indirect ophthalmoscopy

35. a) Adjustment of Eye piece, by Head band
b) IPD: Adequately adjusted IPD – at arms
c) Aperture setting
d) Filter knob
e) Rheostat setting
1) Adjusting the instrument

36. What is Rheostat?
The rheostat is a device that varies the resistance in
a circuit without interrupting the flow of the
current in the circuit.
When the rheostat of the microscope increases the
resistance, less current flows, and a dimmer light
will strike the specimen. If the rheostat decreases
the resistance, more current flows, and more light
hits the specimen.

43. a) Axis perpendicular to the visual axis of examiner
b) The scope not resting on the nose of the examiner
c) The eyepiece as close to the examiners pupils as
possible
d) Adequately adjusted IPD
Ideal position of the ophthalmoscope

44. 2. Positioning of the patient
a) IDEAL POSITION: Head is not to flexed or
extended
b) Proprioception helps patient to look in cardinal
gazes
c) Patient should be urged to keep the other eye
open

49. a) Patient looks down, depressor on margin of
sup tarsal plate
b) Depressor advanced into the orbit as patient
looks up but no depression applied as yet
c) Scleral depression applied gently to area of
interest Scleral Depression technique
Scleral Depression Technique