IOLs Flashcards
what’s something to be aware of in toric IOLs? When does it become ineffective?
must be aligned. ineffective once 30 degrees misaligned.
Theoretical formulae of IOLs?
F = n/(l-c) - nK/(n-Kc)
F = lens power (inside eye)
n = refractive index (eye)
K = corneal power (D)
c = distance (ant. cornea to lens)
types of IOLs?
- anterior chamber
- Iris fixation
- Iridocapsular fixation
- Posterior chamber
What materials are IOLs often made of?
PMMA - similar chromatic aberration to normal eyes.
for loops: nylons, polypropylene.
what are important aberrations of intraocular lenses?
- spherical aberration
- Coma and astigmatism
- aberrations due to tilt and displacement
- chromatic aberration
describe PRK.
photorefractive keratectomy. epithelium and some underlying stroma removed.
describe LASIK
laser assisted insitu keratomileusis.
flap of epithelium is removed and stroma ablated.
quicker healing, less uncomfortable for patient.
typical ablation size for PRK and LASIK?
can vary. typically of the order of 6mm.
disadvantage of PRK and LASIK?
high level of spherical aberration, problems with large pupils (e.g. glare at night).
increased aberration with peripheral?
how does aspherising a surface help with spectacle lenses?
reduce off-axis aberrations. aspherising one surface as conicoid makes third order aberrations cubic functions of shape. for any power, can always eliminate an aberration.
can also improve cosmetics and weight without sacrificing optical performance.
what are some problems with spectacle magnification?
false depth perception.
false projection perception.
what are the two parts of a mirror coating?
highly reflecting front surface (constructive interference)
strong absorption tint. (reduce reflection seen by wearer)
simply describe a relay system in optometers.
way to transfer light from one place to another. essentially transferring image from eye to measuring device. consists of two lenses, distance at twice their focal lengths.
magnification of this system is negative of the ratio of their powers.
How can Badal optometers be improved?
- increased power range
- eye clearance
can be improved through use of auxiliary lenses/lens systems.
what are the four systems for an optometer to measure refractive error of the eye accurately? what is used to achieve this?
Alignment system: This ensures that the device and the eye are properly aligned for accurate measurement.
Fixation system: This helps the patient’s eye to focus on a specific point or object during the measurement.
Stimulus system: This provides a visual stimulus for the eye to respond to.
Imaging system: This captures the response of the eye to the stimulus
Achieved through relay system.
