3. 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.
4. 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.
5. However, there is a problem with this method: Sufficient
light for visualization of the fundus emerges only if the
patient’s fundus is properly illuminated.
Because of the optics of the eye incident light reaches
only the part of the fundus onto which the image of the
light source falls.
6. The fundus can be seen only where the observed and the
illuminated areas overlap:
in the emmetropic eye this can happen only if the light
source and the observer’s pupil are aligned optically.
7. System of lenses :focus the light from an electric bulb.
Mirror: reflect the emitted light .
Accessories :
-Fixation star.
-Slit diaphragm
-Red free filter
-Blue filter
8. If the patient’s fundus is properly illuminated, the field
of view is limited by the most oblique pencil of light
that can still pass from the patient’s pupil to the
observer’s pupil.
In direct ophthalmoscopy the retinal point that
corresponds to this beam can be found by constructing
an auxiliary ray through the nodal point of the eye.
The point farthest from the centerline of view that can
still be seen is determined by the angle α, that is, the
angle between this oblique pencil and the common
optical axis of the eyes.
9. Angle α, and therefore 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.
The more peripheral pencils of light use ever-smaller
parts of each pupil. This means that, even if the
patient’s fundus is uniformly illuminated, the
luminosity of the fundus image gradually decreases
toward the periphery, so that there is no sharp
limitation to the field of vision. In practice, therefore,
the effective field of vision is determined by the
illuminating system not by the viewing system. Most
ophthalmoscopes project a beam of light of about one
disc diameter.
10.
11. Refractive state of the patient:Largest in H, smallest
inM.
Pupil size:enlarged with dilatation.
Distance between the patient &the examiner: decrease
the distance ,enlarge the field.
12. It is greater than the field of view because the size of
the light source is greater than the size of examiner
pupil.
Factors affecting:-
The type of mirror:Plane mirror, or Concave mirror.
Refractive state of the patient.
Intensity of light.
13. In direct ophthalmoscopy the image on the observer’s
retina is about as large as the fundus detail viewed and
is 15 times larger than it would be if the same fundus
detail were viewed from 25 cm.
An additional 2× magnification can be achieved by
placing a small Galilean telescope on the
ophthalmoscope, but fundus microscopy with the slit
lamp and a contact lens is a better way to achieve this
level of magnification.
14. If the patient and the observer are not both
emmetropic, the calculations are more complex.
Axial length of both eyes.
Refractive power of both eyes.
The position of the compensating lenses in the
ophthalmoscope must all be considered.
15. The eyes of myopic patients have extra plus power and
the ophthalmoscope must carry a negative lens. This
combination, in part, acts as a Galilean telescope for
the observer, and fundus details are seen larger.
In aphakia the reverse happens: fundus details are
seen smaller.
16. If we examine H fundus: small image size with wider
field of view.
If we examine M fundus:larger image size with
reduced field of view.
17. Portable.
Brilliant illumination.
Larger magnification .
Relaxation of doctor accommodation
Erect view.
Ability to examine ares rather than fundus.
