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
reflex stage
An image of the illuminated retina is formed at the patient’s far point
projection stage
Observers views the image of the illuminated retina as a reflex in the patients pupil at a convenient distance
Point of reversal or neutral point of retinoscopy is reached when the patients far point coincides with the observers nodal point no movement is observed
Placido’s disc
- Used to check the general shape of the cornea
- Examiner looks through the disc with a convex lens can see the regularity or distortion of the cornea
- Shorter the radius of curvature steeper meridian smaller the reflected image
keratometer
- Measure the radius of curvature of a central zone of the cornea approximately 3 to 4mm in diameter
- Radius of curvature of the axial zone of the emmetropic eye is 7.8mm
what is constant in all keratometers
u (distance of object from mirror) is fixed in all
what moves in keratometers
o Von Helmholtz: fixed = object, adjusted = image size
o Javal Schiotz: fixed = image, adjusted = object size
corneal topography indications
o Corneal astigmatism
o Contact lens fitting
o Refractive surgery
o Keratoconus
most commonly used corneal topography
- Computerised videokeratorapghy (CVK)
produces a colour coded map
compound microscope
- Produces a magnified view of a near object
- Consists of two convex lenses – objective and eyepiece lens
image from a compound mircoscope
: virtual, inverted (horizontally and vertically), magnified
how are porro prisms used
incorporated into microscope so that the imaged are erect and non-inverted
slit lamp
low powered binocular compound mircoscope with bank of galilean telescopes to change magnification
diffuse ilimunation
directing full beam
for anterior capsule
direct focal illumination
obliquely
specular reflection
gaze in the middle, beams bisecting
for corneal endothelium
slceoritc scatter
off axis illumination
scattered around the cornea
retro-oillumiantion
co-axial
ir
lateral
cornea opacities
without lenses can only examine
anterior third of the vitreous
hruby lens
plano-concave lens
vitrual, erect and diminshed image
90D and 78D
inverted image
20D angular mag, field of view and laser spot
Mag: 2.97
Field: 46
Laser: 0.34
28D angular mag, field of view and laser spot
Mag: 2.16
Field: 55
Laser:0.46
78D angular mag, field of view and laser spot
Mag: 0.87
Field: 73
Laser: 1.15
90D angular mag, field of view and laser spot
Mag: 0.72
Field: 69
Laser: 1.39
SF angular mag, field of view and laser spot
Mag: 0.72
Field: 12
Laser: 1.39
standard area for goldman
3.06mm
overestimates
Excessive fluorescein
Thick cornea
ATR astigmatism
Cornea scar
underestimates
Inadequate fluorescein
Thin cornea
WTR astigmatism
Corneal oedema
pachymeter
uses iamge I and II
OCT
uses IR 843 and the time delay produces iamges
