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Delhi Journal of Ophthalmology
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Understanding your Direct Ophthalmoscope
Digvijay Singh, Rohit Saxena, Pradeep Sharma, Vimla Menon
Dr. R.P. Centre for Ophthalmic Sciences, All India Institute of Medical Sciences, New Delhi, India
The direct ophthalmoscope is an extremely important
examination tool not only for ophthalmologists but for
physicians as well. It is probably the only tool in ophthalmology
that can help perform a complete ocular examination. This
article highlights the development, functioning and use of
direct ophthalmoscopes.
A peak in the past
Scientists had tried to peer into the then unknown back of
the eye in the 18th and 19th century but were unsuccessful
in understanding and establishing a coaxial illumination
observation system. Then in the mid 19th century, several
scientists noticed that if they kept a light source (pointed at the
subject) very near their eye, then in some cases (emmetropes)
they could view the red reflex and retina. It was in 1849 that
Charles Babbage made what was probably the first practical
ophthalmoscope.[1] (Figure 1). It was a simple piece of
mirror with a silver patch rubbed off from the centre to
make it see-through. Shortly afterwards, in 1851, Hermann
Von Helmholtz published a monograph describing in detail
the optical working of an ophthalmoscope and a designed a
practical ophthalmoscope very similar to the ones used today.
(Figure 2) Helmholtz is recognized as the inventor of the
direct ophthalmoscope. An anecdote in this regard is that when
Helmholtz tried to interest the king’s physician in his newly
invented ophthalmoscope, he was told that it had no value as
every known disease of the eye could already be diagnosed
without it. Around the same time, the ophthalmoscope gave
Helmholtz an instant global fame in the field of optics.
Figure 1: Babbage and his ophthalmoscope
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Figure 2: Helmholtz and his ophthalmoscope
Instrument detailing
Currently, the ophthalmoscope comes in various sizes and
modifications though all follow the same optical principle.
Initially we describe in detail a standard direct ophthalmoscope
and later look at the variants available with their specific
advantages[2]. (Figure 3)
Figure 3: Detailed instrument scheme for the direct
ophthalmoscope
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Delhi Journal of Ophthalmology
Understanding your Direct Ophthalmoscope
The ophthalmoscope consists of a metallic optical tube,
usually made of a durable light weight metal such as chromeplated brass for proper alignment of the contents. Inside this
tube, glass condensing lens, objective lens, mirror/prism
aperture dial assembly, red-free/polarizer assembly and lamp
are sealed. The aperture dial is mounted such that it maintains
alignment despite a fall/accidental drop from a reasonable
height.
as a clinical science existed well before the development of
the ophthalmoscope as is exemplified by the founding of
Moorfields eye hospital in 1804 and many others[3].
Illumination system
• Incandescent lamp: This is usually a xenon halogen bright
white lamp powered by a 2.5V non-rechargeable or 3.5V
[NiMH (Nickel Metal Hydride) or LiION(Lithium ion)]
rechargable battery.
• Condensing lens: There are two condensing lens, one on
either side of the aperture dial which focus the light onto
the mirror/prism.
• Aperture dial: This has got various apertures such as
cobalt blue filter, fixation star, small spot, large spot,
pinhole, hemispot and alit. These have a specific function
each.
• Reflecting mirror/prism: This is a mirror angled at 45
degrees which is partially reflecting or has a central
peephole. It makes the light cone projected upon the
patients eye appear as if it has originated from the mirror
itself. Most modern ophthalmoscopes utilize a prism in
place of a mirror for this purpose.
Apertures
Viewing system
• Condensing Lens: These are aspheric lens with ranges
varying with every ophthalmoscope model. Eg. +1-10,
+15, +20,+40 and -1-10,-15,-20,-25,-35 in the Heine beta
200.
• Viewing window: Recessed, antireflective coated to
avoid glare.
• Polarizing/red free filter: This is mounted on a separate
dial and enables green, red free image viewing of the
fundus or a polarized view to detect nerve fibre layer.
The why, what and how of direct ophthalmoscopy?
Why?
Why direct? It is a direct ophthalmoscope as the image forms
directly on the retina and there is no intermediate image akin
to that seen in an indirect ophthalmoscope.
Why ophthalmoscope? It is indeed interesting to wonder
why this instrument came to be called as an ophthalmoscope
since the term ophthalmology was coined a good deal after
the invention of the ophthalmoscope by Helmholtz in 1851.
It is quite clearly argued in essays written in the nineteenth
century that instruments and developing technology such
as ophthalmoscope and laryngoscopes actually led to the
development of the respective specializations. Of course eye
Vol. 21, No. 3, January-March, 2011
What are all the knobs for?
Light intensity adjustment dial: This helps provide an
illumination of variable intensity for eliminating the corneal
reflex and patient comfort.
Small spot
This provides approximately a five degrees cone and is used
for a small pupil. It also helps decrease corneal reflexes and
increases patient comfort.
Large spot
This provides an approximately eight to ten degrees
illuminated circle (though highly dependent on the refractive
status and papillary diameter).
Macular spot/pinhole
This provides a small spot to observe only the fovea/macula
without any undue light thereby minimizing patient discomfort
and enabling viewing through a 1-2mm pupil.
Hemi-spot
Reduces corneal reflex and provides retinal depth perception.
Slit
Accurate assessment of retinal elevations and depressions.
Assessment of anterior chamber depth.
Cobalt blue spot
Examination of corneal abrasions and scarring
Fixation star (with polar coordinates)
Accurate eccentric fixation testing, disc assessment and retinal
mapping.
Red free filter
This may be combined with all filters. Contrasts features
by removing red colour and thus betters visualization of
blood vessels, hemorrhages and nerve fibre layer. Some
ophthalmoscopes may have a polarized filter to better evaluate
nerve fibre defects.
Condensing/focusing lens
They help focus the image onto the observers retina. Need to
be selected based on the subjects refractive status and distance
at which ophthalmoscopy is done.
What are the image properties?
The image formed by an ophthalmoscope is virtual, erect and
magnified. The area of retina imaged varies between 6.5 to 10
degrees. (Average area subtended by disc is 7 degrees vertical
and 5.5 degrees horizontal; thus an average sized disc should
just fit the 5 degree cone of ophthalmoscope)
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How does the ophthalmoscope function?
Fundamental Optics
The ray diagram of the direct ophthalmoscope is shown
below. It also depicts the usage of condensing lenses for eyes
with refractive error[2]. (Figure 4)
Understanding your Direct Ophthalmoscope
When the ophthalmoscopy is being done from very close to
the eye, the distance is less than f causing a virtual erect image
to be seen.
Magnification
To understand how the ophthalmoscope magnifies, we take
the example of an emmetropic eye. First we examine a small
segment of a retinal vessel from 25 cm (the comfortable near
vision distance). Let us suppose it subtended an angle of q0.
We now view the same vessel segment from very close to the
eye. Assuming the eye as a reduced lens of power 60D, we
now are seeing from within the focal length of this lens thus we
see a virtual erect image. On extrapolating this image to 25cm
distance, you can observe that it is much larger and subtends
an angle of q10. Thus we observe an angular magnification
and no linear magnification. M ang= q10/ q0 = distance(d) ×
power(D)= 0.25 × 60.This is equal to 15. (Figure 7)
Figure 4: Ray diagram depicting optics of ophthalmoscope
Image Properties
The image properties depend on the working distance used for
ophthalmoscopy. When done at a distance of 25 cm for distant
direct examination, we observe a real inverted unmagnified
image of the fundus as shown in Figure5.
1
A object (upright arrows) at 25 cm subtends angle q at unaided observer’s
eye. B Virtual Image (Large grey Arrow) of the same object subtends angle
Figure 5: Image properties during distant direct ophthalmoscopy
When the fundoscopy is done from a very near distance to the
subject’s eye, the image is a virtual erect one.(Figure 6)
Figure 6: Image properties while viewing fundus during close range
ophthalmoscopy.
This image changes from a real to virtual may be understood
with a basic knowledge of optics. The focal length of the
reduced eye model is 1.67 cm. The retina therefore lies between
f and 2f. When we do a distant direct ophthalmoscopy, we are
observing the image from distance between 2f and infinity as
also, the light source is originating from this point. This thus
gives a real inverted image forming on the observer’s eye.
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q1 when viewed.
Figure 7: Image depicting magnification property of
ophthalmoscope
Field of view
The field of view seen in a direct ophthalmoscope varies with
the distance at which the examination is carried out and the
pupil diameter. For example, if we observe the fundus from
a distance of 15 cm in a 2mm pupil, then we can only see
an area of about 200-300µ or a short segment of a vessel. In
contrast on observing from very close to the eye in a well
dilated pupil (8mm), we can see more than 10degrees of field.
Theoretically it may be possible to see up to the equator in
a fully dilated pupil in a cooperative patient on moving the
ophthalmoscope and patients eyes appropriately.
How to perform a complete ophthalmoscopic
examination?
Steps
Before proceeding, it is important to understand the instrument
well.
The first step in the use of an ophthalmoscope is to do
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Understanding your Direct Ophthalmoscope
examination at 1m distance. This sheds light on any
abnormalities of the eyelids, orbit and periorbita as well as
highlights any obvious ocular deviations.
This should be followed by a distant direct examination
at 22-25cm( a comfortable near vision distance). Some
ophthalmologists prefer to do this at a closer distance of 10cm
as it gives better details. If the examination is done at 10cm,
we should select a +10D condensing lens to view the best
glow. At 25 cm, a +4D lens may be used. This examination
shows a red reflex and highlights any opacities in the media
as black images. The patient may then be asked to look in the
four cardinal gazes and the movement of the opacity noted.
Movement against the ocular movement means the opacity is
behind the nodal point of the eye (i.e. in the lens or vitreous)
while a movement with would indicate corneal or anterior
segment opacity. The distant direct examination is also used
to examine the lens, iris, cornea and adnexa. Any squint in a
child may be picked up due to an unequal reflex (Bruckner’s
test). The presence of an RAPD can also come forth in this
step.
The third step involves moving closer to the patient and
correspondingly increasing the power in the condensing lens
to examine in detail the magnified anterior segment structures.
The fourth step entails reducing the condensing lens power
such that any part of the retina comes into focus. While
reducing the power, the vitreous cavity will come into focus
and any pathology in it may be seen. Once the retina is focused,
we may localize any blood vessel and follow it backwards
against the branching pattern to reach the optic disc. Then
move temporally from the disc to reach the macula. We can
ask the patient to look into the light and the fovea will come
into focus. The blood vessels can be traced into the periphery
from the disc to reach second and third order vessels. This
completes the posterior pole examination to examine the
periphery, we ask the patient to look in the four cardinal gazes
while continuing to focus the retina. The ophthalmoscope
illuminated cone may be moved to further our view into the
peripheral retina.
Practical tips
•
•
•
•
To eliminate the irritating corneal reflex, one can slightly
tilt the ophthalmoscope and view obliquely. One can
also decrease the illumination intensity and use a smaller
aperture.
Corneal reflex is also negated if the patient is approached
from 15 degrees rather than from straight ahead.
Use a small aperture for a smaller pupil as only an
illuminated cone equal to the size of the pupil can enter or
exit the eye while the rest will reflect off the iris creating
unnecessary glare and poor contrast.
The fovea lies 3 degrees temporal to the optical axis of
the eye, the disc lies 10 degrees nasal and the peripheral
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•
•
•
•
•
•
•
retina commences about 16 degrees temporal to it. Thus,
if the patient is seeing straight ahead, then we know how
much to tit the ophthalmoscope to view these landmarks.
The patient should be asked to look straight ahead into
the distance or preferably to a target on a far off wall.
The patient should be instructed to stay steady and
frequently blink during the examination.
The examiner should use his right eye to view the patient’s
right eye and vice versa. They should keep their opposite
hand on the patient’s forehead to support and steady it.
The examiner should keep both their eyes open during
examination and imagine as if the retina is at 6 meters to
prevent accommodation.
Normally, the examiner should continue to wear his
glasses while the patient has to remove his. The field of
view decreases if the examiner wears his glasses therefore
for low myopes or hyperopes (± 3Ds) and astigmats
(below 2.5Dc) may remove their glasses especially in a
small pupil.
A trick to decide the appropriate selection of the
condensing lens is described as follows. Observe the
light reflex on the retinal vessels. If a white line is seen
then either the patient is emmtropic or hyperopic. In that
case, add plus lens and the highest plus when the line
reflex disappears is the appropriate power. This would
also be the approximate refractive error of the patient if
the examiner had not accommodated. If there is no line of
light reflex on the vessels, then the patient is myopic. Add
minus power and the smallest minus lens when the reflex
appears is the refractive error of the patient.
The lenses of the ophthalmoscope can be used to focus
variously the apex and base of any intraocular mass and
thus helps determine its height in dioptres.
How to select an appropriate ophthalmoscope?
The ophthalmoscope selection should be guided by a number
of factors. Prime among these is the intended clinical role.
For a physician who needs to evaluate whether a fundus is
normal or abnormal, a basic design should suffice while an
ophthalmologist should look for one with highest quality
optics and maximum functionality for accurate diagnosis. The
ophthalmoscope should have apertures such as the small and
large spot, cobalt blue filter, slit and the fixation star. Presence
of a green filter is mandatory for diagnostic purposes. The
instrument should contain adequate number of focusing lenses
with a 1 dioptre minimum count for fine focusing. The battery
should preferably be rechargeable Li ion or NiMH which
provide extended power. Beyond this minimum specifications,
any of the advanced machines may be used.
How to Care for the direct ophthalmoscope?
The direct ophthalmoscope is a sturdy built instrument for
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heavy handling but requires proper care for its longevity. The
following precautions should be observed:
• The battery should be fully discharged and recharged
once every few months to maximize battery life.
• After fully charging the battery, the bulb should first be
switched on at sub-maximal illumination for 1-2 cases
before employing full illumination to maximize bulb life.
• Use only genuine bulbs and replace in accordance with
the owner’s manual instructions.
•
The condensing lenses are stuck to the dial using glue
which is soluble in acetone and related solvents. Therefore
never use these to clean your ophthalmoscope.
• The ophthalmoscope may be cleaned using mild alcohol
or detergent and a soft cloth.
• A cotton bud should be used to clean the viewing window
and aperture window in a circular sweeping manner.
• Ophthalmoscopes come with twist and fit as well as
automatic lock heads. Ascertain the head connector in the
scope and fix head accordingly.
• Always store the ophthalmoscope in ints case when not
in use. If it will be unused for a long time, remove the
battery and store.
• Dispose the NiMH or Li ion battery appropriately.
possible but not mutually interfering. This gives a larger view
in smaller pupils as well as eliminates corneal reflex artifacts.
Apart from this the incorporation of aspheric designs in lenses
has led to decreased aberrations and reflex artifacts.
Battery life has been greatly enhanced with the modern NiMH
or Li ion batteries while their size and weight has more than
halved. This has enabled manufacture of Pocket sized or mini
ophthalmoscopes which have an added convenience factor.
Further advancements are underway to add better and more
functionality to this principal tool of every clinician.
Conclusion
Innovations in direct ophthalmoscopes
The direct ophthalmoscope has an immense contribution in
furthering the development of ophthalmology as a specialty
science. Familiarity with the use of this instrument would go
a long way in aiding the diagnosis of diseases by physicians
and ophthalmologists alike. Although technology has brought
the ophthalmoscope a long way from its humble beginnings,
one should realize that like every instrument, it too has its
limitations. These include the limited field of view, poor
image visibility through hazy media, inability to appreciate
the full picture, a high need for patient cooperation and non
stereoscopic viewing among others. If we works bearing these
in mind, we are unlikely to get mislead by false signs and
would gain a lot more from this brilliant instrument.
The direct ophthalmoscope has come a long way from the
polished mirror made by Babbage or the more practical model
of Helmholtz.
Illumination technology has shifted from the use of a gas
flame as an external source of illumination to the first directly
illuminated ophthalmoscope made in 1915 to the current day
instruments using halogen and xenon bulbs.
The viewing systems have improved drastically over time.
The latest innovations include the panoptic ophthalmoscope
which uses axial point source optics whereby the light is
focused at a point on the cornea before moving onwards into
the eye. This enables a wider field of view (up to 5 times
wider) in smaller pupils. Another leap forward is the use of
advanced coaxial optics where the illumination and viewing
is done along the same path of light keeping them as close as
References
1. Keeler CR. Babbage the Unfortunate..Br J Ophthalmol
2004;88:730-732.
2. Timberlake
GT,
Kennedy
M.The
Direct
Ophthalmoscope:How it Works and How to Use It. 2005.
University of Kansas Press.
3. Silvester A. The emergence of medical specialties in the
nineteenth century: a discussion of the historiography.
History of Medicine online 2010 Priory.com publication
4. Instruction user manual of Heine Beta200 ophhalmoscope.
5. Ghosh S, Collier A, Varikarra M, Palmer S. Fundoscopy
made easy. 2010. Churchill livingstome Elsevier. 1-21.
6. Fisher WA. Ophthalmoscopy, Retinoscopy and Refracion.
1937:1-34.
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