Optics Flashcards
(55 cards)
A Scan:
1D IOL difference in axial length?
0.4mm
Normal torsional cyclo vergence fusional amplitude?
2-3 degrees
Point of reversal reach in retinoscopy?
When patient’s FAR point = observer’s NODAL point
How does pinhole increase acuity?
Improves irregular astigmatism
Decentring convex lens
Where P is the induced prismatic power in prism dioptres; F is the power of the
decentered lens in dioptres; and D is the decentration in centimetres.
The easiest way to determine the direction of prism power is to imagine convex
lenses as two prisms attached base to base and concave lenses as two prisms
attached apex to apex. Thus when a convex lens is decentred the base of the lens
is always towards the optical centre, i.e. in the same direction as the lens is moved,
whereas the opposite is true for concave lenses.
Pair/Peye = (nIOL - nair)/ (nIOL - neye)
Intraocular lens (IOL) powers are labelled according to their power immersed in aqueous humor
Stiles-Crawford effect no.2
In addition to changes in the efficiency with which photons are captured as a function of angle of incidence, there is also a shift in the hue and saturation of monochromatic lights depending on the angle of incidence
doublet
reduces spherical aberation
Principal lens + weaker lens cemented together to reduce power of periphery
Oblique astigmatism
line of light not parallel with principal axis of lens
worse in biconvex and biconcave lenses than in meniscus lenses
AC/A Ratio heterophoria method
IPD (cm) + (Ocular deviation for near (D) - for distance(D)/near fixation distance (D)
AC/A Ratio: Gradient method
Ocular deviation for distance - for near)/minus lens dioptre
Image formed: Concave
Real
Image formed: Convex
Virtual
Minified - always
PRK - below which diameter can cause haloes?
Diameter < 4mm
Spot factor magnification
size of spot at retina / size of setting at laser machine
Lens
Image Magnification Laser Spot
60D
1.15 0.87
78D
0.93 1.08 90D
0.76 1.32
Volk Superfield
0.76 1.32
Volk Super 66
1.00 1.00
Volk Digital 1.0
1.00 1.00
Volk Digital Clear Field
2.79 0.36
Volk Digital High Mag
1.30 0.77
Volk Digital ClearMag
3.89 0.26
Volk Digital Wide Field
0.72 1.39
Syntophere
Angle of anomaly
AOA = OA - SA
normal retinal correspondence (NRC), the SA is equal to OA and the AOA will be zero.
In unharmonious ARC, the SA will be less than the OA (but the SA will not be zero)
In harmonious ARC, the SA will be zero, so the AOA will be equal to the OA.
Effective power of lens formula
d is POSITIVE If moves towards eye
d is NEGATIVE if moves away from eye
RSM of aphakic spectacle
1.33
iseikonic lens
no power lens
but increases the magnification by increasing hte visual angle subtended to by the eye
pure magnification - front lens surface and thickness
Near visual point (bifocals)
Should be 2mm nasal and 8mm inferior to distance visual point
Where should the top of the near visual point be placed?
tangential to the inferior limbus
Children - lower margin of pupil
How can prismatic jump be reduced?
If the optical centres of the lens lies near or at the junction of the 2 portions
incorporating base-up prism in near segment
Effect of tear lens on steeper base curve
makes acial hight greater, and therefore more positive (convex)
What dioptre is a limitation to soft contact lenses in corneal astigmatism?
1.5-2D
