Visual systems optics Flashcards
What is refraction based on?
light is passing through one medium into another
How do you calculate index of refraction?
Speed of light in a vacuum divided by speed of light in medium
What happens to the light as it reaches a new medium?
Some of the light reflects off the boundary and some of the light refracts through the boundary.
Angle of incidence = Angle of Reflection
What are the two types of lens?
Convex
Concave
What is a convex lens?
A converging lens (convex) takes light rays and brings them to a point.
What is a concave lens?
A diverging lens (concave) takes light rays and spreads them outward.
What is emmetropia?
Perfect eye
Adequate correlation between axial length and refractive power
Parallel light rays fall on the retina (no accommodation)
What is ametropia?
Refractive error
Mismatch between axial length and refractive power
Parallel light rays don’t fall on the retina (no accommodation)
Near-sightedness (Myopia)
Farsightedness (Hyperopia)
Astigmatism
Presbyopia
What is myopia?
Parallel rays converge at a focal point anterior to the retina
What are the causes od myopia?
excessive long globe (axial myopia) : more common
excessive refractive power (refractive myopia)
What are the symptoms of myopia?
Blurred distance vision
Squint in an attempt to improve uncorrected visual acuity when gazing into the distance
Headache
How do you treat myopia?
Correction with diverging lenses (negative lenses)
Correction with contact lens
Correction by removing the lens to reduce refractive power of the eye
What is hyperopia?
Parallel rays converge at a focal point posterior to the retina
What are the causes of hyperopia?
excessive short globe (axial hyperopia) : more common
insufficient refractive power (refractive hyperopia)
What are the symptoms of hyperopia?
visual acuity at near tends to blur relatively early
nature of blur is vary from inability to read fine print to near vision is clear but suddenly and intermittently blur
blurred vision is more noticeable if person is tired , printing is weak or light inadequate
asthenopic symptoms : eyepain, headache in frontal region, burning sensation in the eyes, blepharoconjunctivitis
How do you treat hyperopia?
Correction with converging (positive lenses)
Correction with positive lens + cataract extraction
Correction with contact lens
Correction with intraocular lens
What is Astigmatism?
Parallel rays come to focus in 2 focal lines rather than a single focal point
What causes Astigmatism?
refractive media is not spherical
–>refract differently along one meridian than along meridian perpendicular to it
–>2 focal points (punctiform object is represent as 2 sharply defined lines)
What are the symptoms of astigmatism?
Asthenopic symptoms (headache , eyepain)
blurred vision
distortion of vision
head tilting and turning
What is the treatment of astigmatism?
Regular astigmatism : cylinder lenses with or without spherical lenses (convex or concave)
Irregular astigmatism : rigid cylinder lenses, surgery
What is the near response triad?
Adaptation for near vision
What comprises the near response triad?
Pupillary Miosis (Sphincter Pupillae) to increase depth of field
Convergence (medial recti from both eyes) to align both eyes towards a near object
Accommodation (Circular Ciliary Muscle) to increase the refractive power of lens for near vision
What is presbyopia?
Naturally occurring loss of accommodation (focus for near objects)
Onset from age 40 years
Distant vision intact
Corrected by reading glasses (convex lenses) to increase refractive power of the eye
What is the treatment for presbyopia?
convex lenses in near vision Reading glasses Bifocal glasses Trifocal glasses Progressive power glasses
What are the different types of spectacle lens?
Monofocal lenses : spherical lenses , cylindrical lenses
Multifocal lenses
What are the features of contact lenses?
higher quality of optical image and less influence on the size of retinal image than spectacle lenses
indication : cosmetic , athletic activities , occupational , irregular corneal astigmatism , high anisometropia , corneal disease
What are the disadvantages for contacts?
careful daily cleaning and disinfection , expense
complication : infectious keratitis , giant papillary conjunctivitis , corneal vascularization , severe chronic conjunctivitis
What are intraocular lenses?
replacement of cataract crystalline lens
give best optical correction for aphakia (absence of crystalline lens)
avoid significant magnification and distortion caused by spectacle lenses
What are the different surgical corrections?
Keratorefractive surgery :RK, AK, PRK, LASIK, ICR, thermokeratoplasty
Intraocular surgery : clear lens extraction (with or without IOL), phakic IOL
What are the process of laser surgical correction?
Initial cutting of corneal flap
Cutting of corneal flap
Flipping of corneal flap
Photorefractive treatment (laser)
Corneal stroma reshaped post laser
Corneal flap back in position
What is an ICL implant?
Add lens ontop of natural lens
Catarct inevitable
Intra-collamer lens
How is cataract surgery conducted?
Remove natural lens with ultrasound tip that breaks it and sucks it up
Insertion of artificial lens
