direct ophthalmoscope
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Direct Ophthalmoscope
Dr.Mohammad Khabbaz
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Methods of Fundus Examination
Direct
ophthal
moscope
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Direct Ophthalmoscope
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System of Lens focusing light on a 45 dgree mirror with Hole.
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Mirror reflect light in diverging beam then illuminate patient,s eye.
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Light reflected from illuminated retina of the patient ,then passes back through the hole to observer,s eye.
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Image of blub is formed just below the hole,so the reflection does not interfere with viewing.
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1.Preliminary examination by a plane mirror at 1 meter give an idea about refrective error: Emmetrope only red reflex. Hypermetropia with movement. Myopia against movement.
Routine way of usingDirect Ophthalmoscope
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Red reflex1 meter
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2.Preliminary examination by a plane mirror at 22 cm Diagnosis of opacity eg: cataract. Recognition of RD or Tumour. Level of the opacity can be identified
by the parallactic displacement.
Routine way of usingDirect Ophthalmoscope
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22 cm
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Movement of the opacity in relation to the movement of the observer.
Parallactic displacement
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Parallactic displacement
Behind the pupil opacity moves with examiner,s motion.
At the pupil the opacity is
stationary with examiner,s motion.
In front of the pupilopacity moves
opposite to examiner,s motion.
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It is method by which an Erect magnified image of the fundus can be seen.
Direct OphthalmoscopeDefinition
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Maculadisk
vessels
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• We assume that anterior focal point of the subject (Fs) coincide with the anterior focal point of the observer (Fo).
• Image on the examiner,s retina is always inverted seen as erect.
Optical principle
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• 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.
Optical principle
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34mm
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Emmetropia
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Rays emerging from subject,s retina are parallel
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Hypermetropia
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Rays emerging from subject,s retina are divergent
Rays enter observer,s eye are divergent
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Inverted smallest image Behind the observer,s retina
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Inverted smallest image Behind the observer,s retina
The image will be blurred
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An emmetropic obsrever has to:To accommodate or
To use a correcting convex lens
In order to bring the light to focus in his retina
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When accommodate the rays enter his eye converging so focused on the retina
The anterior focal point of the observer,s eye Fo approaches his eye
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Myopia
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Rays emerging from subject,s retina are convergent
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Invertedlargest image In front of the observer,s retina
The image will be blurred
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To focus this image on the observer,s retina he should use a concave lens.
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The image formed on the observer,s retina is• Smaller when a hypermetropic eye is
viewed ,Larger when a myopic eye is viewed when an emmetropic eye is examined
• But the use of a correcting lens reduces the discrepancy in size.
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The formula for Magnification achieved by a loupe is
If ascribe dioptric power of 60+ D to the patient,s emmetropic eye, the magnification of the direct ophthalmoscope is 15x.
Magnification
Magnification Dioptric power of the loupe
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Magnification
In E----M=60/4=15x.
In H----M= <15x (F<60D).
In M----M= >15x (F>60D).
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Size of the pupil of subject,s eye. Size of the pupil of observer,s eye. Error of refraction: H----> greatest M----> smallest The distance between the observed
eye and observer eye (decrease distance----> increase field)
OphthalmoscopicField of vision
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Thank you for your attention