Opticalprescriptions2010 1938 1938
Opticalprescriptions2010 1938 1938
Opticalprescriptions2010 1938 1938
Spectacle Lenses
F2
Far point
F2
Far point
Lens Shape
In most cases, a meniscus lens is used
Both lenses concave towards the eye
Best off axis optical quality
Room for eyelashes
Trial lenses, phoroptors etc use other types where field of view is irrelevant
Lens Power
Back Vertex Power Fv=(F1/(1-(t/n2)F1))+F2
Spectacles and contact lenses
Fe=F/(1-dF) d=.015
Lens Transposition
Relates to base curve of the I want a +4.00 DS lens
lens
Any lens power is the
algebraic sum of the two pl +4.00
surfaces (approx power)
Base curve defined as
std curvature ground on
to the lens +1.00 +3.00
Lens aberrations (oblique
astig/curvature of image)
minimized with correct
selection of base curve
-10.00 +6.00
Lens Transposition- sphere
You need to make a -
3.00 lens.
The front base curve is
a +12.00. What is the
back surface curve
going to be?
Toric Transposition
Nearly all lenses are now made with minus
cylinder, a meniscus design where front
surface is sphere and the back is toric
-1.00 = -100x 180
This has been shown to have superior optics
Plus cyl designs have the toric surface on the
front and are now obsolete
Plano = +1.00 X 090
Toric transposition
Made with one sph surface and one toric surface.
The principal meridian of weakest power of the toric surface is
the BASE CURVE
pl +4
+4
1) Put into Rx form +3.00= +1.00 x 090 note that the cyl is not the same
Sign as the bc, so must transpose to – cyl form. +3
+4 -1
2) +4.00 = -1.00 x 180
Think about this in optical crosses: + =
+4 pl +4
90
120 60
150 30
0 180
Against the rule (-) cyl convention
With the rule (-) cyl convention
Oblique is in between
Geometry of Lens Design
Off axis ray must pass
Thru CR to be focused
RS
CR
SCD
27mm
Why is Base Curve Important?
Determines the shape of the lens
Determines off axis image quality
Power, index, thickness, vertex distance
cannot really be varied to a significant degree
This is actually a huge research field…. Lens
companies pay big money to have the
clearest lenses, etc. Especially progressives.
The Lens Clock
This device is useful for
determining base curve
Can tell you if lens is
front or back surface
toric
Can tell you if there is
warpage of lens
Good for patients in
whom you suspect BC
may be an issue- match
the old glasses.
Lens Materials
Glass
Heavy
Ophthalmic crown glass n=1.523
Scratch resistant
Plastic (CR-39)
Decreased weight (1/2 that of glass)
ease of tinting
Impact resistant but scratches
Lower index
POLYCARB- shatter resist/UV protect
What is Index of Refraction?
Amount of light bent per unit thickness
“High index”
Glass n = addition of titanium oxides.
Increase n, but also increases chromatic
dispersion (abbe #) & density (weight)
Too thin and will not pass the drop ball test
withstand impact of 5/8inch steel ball weighing
16g dropped from height of 50 inches.
High Power Spectacles
Aphakic (>+10D)
Aspheric design plus flattest BC possible
Poor cosmesis
Thick
Eyes look huge
Heavy
Lots of Prism
Magnification issues 25-30%
Aphakic spectacles
Dabezies OH Jr. Defects of vision through aphakic spectacles Ophthalmology 1979 Mar 86(3) 52-79
Pediatric Considerations
Lens implantation (PCIOL) is the tx of choice
Best VA results
Best binocular results
(protective against aphakic glaucoma)
AR
NO-AR
Effects of Radiation on the Eye
Electromagnetic Spectrum
UV Light and the eye
Cornea 200-320
Conj 270-310
Uvea 295-310
Lens 295-320 uva uvB
Retina 310-380
UVR
Between 10am-2pm over 50% of UVR
reaches the earth
Green grass reflects 3.5% of UVR
Sand reflects 35%
Snow reflects 85-95%
Skiiers have increase of 15% increase exposure
per each km of altitude above sea level
Ocular Effects of UVR
UVC exposure up to 290nm damages K epi
290-135nm K stroma and endo damaged and
metabolic changes occur
Acute UVB induced cataracts in rabbits,
primates
(Taylor 1989)
Retinal effects of UVR
Immediate large losses in absolute retinal threshold
from 350nm that did not return to normal for 6mo.
(Henton and Sykes 1983)
Role of UV in ARMD?
UVR protection
Crown glass not UVR protector
Reduced to 0.2% with UV absorbing plastic
Protection increases with lens area
13cm2 lens = 65% protection
20 cm2 lens= 96% protection
Ocular exposure increases with vertex
distance
Prescription of Absorptive Lenses
Transmittance- ratio of amount of radiant
energy transmitted through the lens to the
amount incident on the front surface. It is
expressed as a percentage.
Luminous transmittance- describes the visual
characteristics of tinted lens
Optical density- another descriptor, usually
for occupational tint (welders)
OD=-log10T
•
Transmittance Curve
Absorbance
A = 2 - log10 %T - allows you to easily
calculate absorbance from percentage
transmittance data.
So, if all the light passes through a solution
(or lens) without any absorption, then
absorbance is zero, and percent
transmittance is 100%. If all the light is
absorbed, then percent transmittance is zero,
and absorption is infinite.
Sunglasses- eliminating the radiation
not needed for vision
>+/- 3D
Tints- plastic
Plastics are tinted via immersion in a hot dye
bath
Color is uniform
Can be bleached and re-tinted
All plastic lenses contain a UV inhibitor that
protects against wavelengths below 330nm
Some tints offer additional UV protection
Photochromic lenses
Change optical density and hue in response
to ambient UV
Developed during Cold War to protect in
nuclear holocaust.
Glass made with silver halide crystal
impregnation, which dissociates on exposure
to UV or cold
Plastics are OK, but prone to scratching
Don’t turn dark inside the car
Polarization
TRANSVERSE WAVE
Polarization
Polarized light
Polarization
The most common method of polarization
involves the use of a Polaroid filter. Polaroid
filters have a chemical composition which can
block one of the two planes of vibration of an
electromagnetic wave.
Polarized light wave emerges with one-half the
intensity and with vibrations in a single plane
The general rule is that the electromagnetic
vibrations which are in a direction parallel to
the alignment of the molecules are absorbed.
Polarizing Lenses
Any vibrations which are perpendicular to the polarization axis are blocked by the filter.
Thus, a Polaroid filter with its long-chain molecules aligned horizontally will have a
polarization axis aligned vertically.
Polarization
Polarization
When light travels through a linear polarizing material, a
selected vibration plane is passed by the polarizer, while electric
field vectors vibrating in all other orientations are blocked.
So the filter selects one component from all of the different planes of
light and lets that one component get through!
Polarization by reflection
Unpolarized light can become polarized by
reflection off of a non-metallic surface and
can cause glare
Depends on the angle of incidence
Depends on the material
Asphalt, snow, water
Polarization cont’d
Unpolarized light ,when reflected, seems
white and can obscure vision
Put a polarizing filter perpendicular to the
reflected light and it will absorb light
Restores the balance of light intensities and
restores fidelity to surface color.
Fishing, skiing, driving (anytime you get a lot
of reflections)
http://www.colorado.edu/physics/2000/polarization/polarizationI.html
http://micro.magnet.fsu.edu/primer/java/scienceopticsu/polarizedlight/filters/
Polarization by refraction
Polarization from scatter
As light bounces off of atoms in the
atmosphere, it is scattered and becomes
partially polarized
Produces glare in the sky and washes out
photographs
This is why cameras have polarizing filters,
as you rotate it, glare can be reduced.