Optics Flashcards
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
decentring Myopic prismatic effect
temperoally: Base in prism
Nasally: Base out
Inferiorly: Base up
Superiorly: Base out
Increasing the negative power of the contact lens
increasing temperature (evaporation)
Assessing microtropia
4D bae OUT prism
Power of refractive surface
(n2 - n1)/r1
Diffraction and wavelength
INCREASES with wavelength
Minimum angle of RESOLUTION
Visual acuity is reciprocal
Image found anywhere in convex mirror
Found within centre of curvature, erect, laterally inverted
Total internal reflection
Light travels from higher refractive index to lower
Incident ray > critical angle
Axial magnification
magnification of the depth of the image along the optical axis
Correcting eso/exotropia with prisms
Usually correction split between the two eyes
To correct convergence: prism must be base OUT
To correct divergence: prism must be base IN
To correct vertical deviation: prism opposite for both eyes
Contact lens nomenclature
base curve/diameter/power
Reducing lens movment
- Increase contact lens diameter
- Decrease base curve of CL
- Increase thickness of CL
A change in the radius of corneal curvature of 0.1 mm produces a change in refraction of what in contact lens fitting?
0.5 dioptres.
Aspheric lens
radius changes from the center to the outside (becomes less curved usually)
Hyperope and contact lens
Will use less accomodation with contact lenses
Angular magnification
Power eyepiece/power objective
Eyepiece lens is more powerful > objective
object is not changed in position or size, but has an optical system interposed between the object and the eye to make the object appear larger.
Angular magnification takes into account not only the size of an image, but also its distance from the observer
Should hyperopes/myope use round or flat top bifocals?
Myope - flat top
Hyperope- round top
Mode lock pulse
30 pico seconds
x100 powerful than Q switch
Ionisation lasers
Ng-YAG
Argon-fluride excimer
Thermal lasers
CO2 lasers
Photochemical
Argon lasers for retinal photocoagulation
Back vertex power
1/posterior vertex focal length
Captoptric images
1- keratometry
1+2 - corneal thickness
2+3 - AC depth
3+4 - accomodation
Chromatic aberation
NOT affected by refractive index
affected by dispersive power
Spherical abberation reduced by what in the eye?
- aplantic curvature of cornea
- iris - stop light coming obliquely
- lens - nucleus higher refractive index
- Retina - cones sensitive to light paraxially
What do Best form lenses improve?
spherical and oblique astigmatism
What is vergence power affected by?
RI on each side of medium
thickness of lens
Vergence power of each surface
Angular magnification
Quotient of angle subtentded by image OVER object
How do fresnel prisms reduce VA?
Chromatic aberration
Absolute RI
velocity of light in vacuum/velocity of light in medium
Tear lens
K>BC = convex
K < BC = concave
K = BC = plano
