direct ophthalmoscope

Direct Ophthalmoscope

Dr.Mohammad Khabbaz

Methods of Fundus Examination

Direct

ophthal

moscope

Direct Ophthalmoscope

System of Lens focusing light on a 45 dgree mirror with Hole.

Mirror reflect light in diverging beam then illuminate patient,s eye.

Light reflected from illuminated retina of the patient ,then passes back through the hole to observer,s eye.

Image of blub is formed just below the hole,so the reflection does not interfere with viewing.

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

Red reflex1 meter

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

22 cm

Movement of the opacity in relation to the movement of the observer.

Parallactic displacement

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.

It is method by which an Erect magnified image of the fundus can be seen.

Direct OphthalmoscopeDefinition

Maculadisk

vessels

• 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

• 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

34mm

Emmetropia

Rays emerging from subject,s retina are parallel

Hypermetropia

Rays emerging from subject,s retina are divergent

Rays enter observer,s eye are divergent

Inverted smallest image Behind the observer,s retina

Inverted smallest image Behind the observer,s retina

The image will be blurred

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

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

Myopia

Rays emerging from subject,s retina are convergent

Invertedlargest image In front of the observer,s retina

The image will be blurred

To focus this image on the observer,s retina he should use a concave lens.

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.

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

Magnification

In E----M=60/4=15x.

In H----M= <15x (F<60D).

In M----M= >15x (F>60D).

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

Thank you for your attention