14. Instruments Flashcards
Principles of direct ophthalmoscope
o Lenses which focus light form a bulb onto a mirror real image formed (just below corneal reflection so it does not lie over the visual axis)
o Mirror reflects light in a diverging beam to illuminate the patients eye
o Light reflected back by the retina into observers eyeF
Field of view
o Governed by hole in mirror or the observer’s pupil
o Large when dilated (obviously)
o Becomes larger as the distance between patient and observer DECREASES
Image formed by a direct ophthalmoscope
ERECT
direct ophthalmoscope and astigmatism
- Unable to correct for astigmatism (only spherical lenses incorporated)
Direct ophthalmoscope and myopic eyes
Field of view: small
Size of image: large
Emmetropic observer: converging light –> concave lens
Direct ophthalmoscope and hypermetropic eyes
Field of view: large
Size of image: small
Emmetropic observer: diverging light –> convex lens
- Observer’s retina = L
- Patients retina = I
- Image formed in observer’s eye = A
- Principal plane = J
- Observer’s retina = E
- Patients retina = B
- Image formed in observer’s eye = AF
- Principal plane = C
- Observer’s retina = D
- Patients retina = A
- Image formed in observer’s eye = IL
- Principal plane = A
indirect ophthalmoscope
- Used with powerful CONVEX lens – must be aspheric to minimise aberrations
- Condensing lens held at arms length, image viewed at 40-50cm distance
- Binocular indirect has +2.0D in prismatic eye piece-viewer does not need to accommodate
indirect ophthalmoscope image formed
real, vertically and horizontally inverted, situated around 2nd principle focus
indirect ophthalmoscope field of illumination
is limited by: size of subjects pupil (dilated = larger), refractive status
indirect ophthalmoscope field of view
size of observer’s pupil, apparatus of condensing lens
indirect ophthalmoscope
* D: 2nd principle focus
* G: 1st principle focus
Indirect ophthalmoscope: myopia
field of illumination: largest
position of image: inside second prinipal focus
image size as lens moved away: increases
Indirect ophthalmoscope: normal
field of illumination: normal
position of image: at second principal focus
Indirect ophthalmoscope: hypermetropia
field of illumination: smallest
position of image: outside second principal focus
image size as lens moved away: decreasesl
linear magnification
= focal length of the condensing lens / distance between nodal point and the retina of subjects eye
linear magnification example 13D at 15mm
o if the distance is 15mm, the linear magnification is the focal length x15 mm
o For a 13D (f = 75mm) = x5 linear magnification (75/15=5)
angular magnification
o = power of subjects eye in D / power of condensing lens in D
direct vs indirect ophahloscope
retinoscopy
accurate objective measurements of the refractive state of an eye
3 stages of retinoscopy
o Illumination stage: light directed into the patients eye to illuminate the retina
o Reflex stage: image of the illuminated retina is formed at the patient’s far point
o Projection stage: noting the behaviour of the luminous reflex by the observer in the patients pupil
illumination stage
Light is reflected onto the patients fundus using a plane or concave lens
Plane = with movement
Concave = against movement
