Optics of the eye Flashcards
What is accommodation?
What is the consequence when the ciliary muscles contract?
varying of the total power of the eye by changing the curvature of the lens in order to focus sharp for objects at different distances
ciliary muscles contract → increased curvature (smaller R) → larger D → focusing on close objects (smaller f)
(cf. lens-maker equation)
What is normal range of the power of the eye?
sum of the powers of all refractive surfaces
Deye = nair/o + nvitreous humor/i = 59 - 72 dpt
(nair, nvitreous humor don’t change)
What is the far and near point of vision?
How do are they related to the accommodation power of the eye?
Formula.
far (or) or near point of vision (op) is the distance from the eye to a farthest or closest point resp., whose image can be focused on the retina with an accommodated eye
→ accommodation power ΔD = Dp - Dr = 1/op - 1/or
= difference btw smallest and highest power
What is myopia?
What is the anatomical reason for such a disorder?
How can it be corrected?
nearsightedness
- only close objects are sharp
- elongated eyeball → light rays from far objects focused in front of the retina
- can be corrected by divergent lenses
What is hyperopia?
What is the anatomical reason for such a disorder?
How can it be corrected?
farsightedness
- only far objects are sharp
- shortend eyeball → light rays from close objects focused behind the retina
- can be corrected by convergent lenses
What is presbyopia?
What is the anatomical reason for such a disorder?
How can it be corrected?
effect of aging of the eye
- loss of elasticity of the lens → near point recedes farther and farther from the eye
- can be corrected by convergent lenses, using different lenses for different object distance range (varifocal = progressive power lenses)
What is the reduced eye model?
Give important numerical values.
simplified model of the eye that would be a homogenous body possessing only a single refracting surface
- nodal point K = 17 mm distance from the yellow spot (macula lutea)
- refractive index n = 1.34
- radius of surface r = 5.1 mm
What is the limiting angle of view?
.. of a normal eye?
smallest angular view of 2 seperated points that can be just distinguished
usually 1’
a’ = a/x (rad) * 360°/2π(rad) * 60 (‘/°)
What is visual acuity?
Other names?
What influences it?
= resolution of the eye, visus
ratio of normal limiting angle of view to the actual limiting angle of view in percent:
visual acuity (visus) = 1(‘)/α(‘) * 100%
influenced by:
- optical aberrations (e.g. irregular shape of lens)
- diffraction on the pupil (cf. Airy disks)
- anatomical density of the photoreceptors of the retina
What are Airy disks?
Explain their influence on the visual acuity w/r/t the theory of wave optics.
Airy disks = special diffraction pattern
theory of wave optics
- objects closer than smallest resolvable distance δ
- Airy disks merge, thus no distinguishable information
- objects not resolved
Explain the influence of anatomical receptor density on the visual acuity.
anatomical receptor density
- the smaller the diameter of the pupil
- the stronger the diffraction → larger Airy disks
- blur on retina
- some cones not stimulated → less sharp image
Differentiate btw types of photoreceptors and their function.
- cones: daylight (photopic) color vision
- rods: night (scotopic) vision
(REMEMBER: cones - color, daylight → CCD)
What is Landolt’s broken ring used for?
Formula.
test visual acuity
⇒ limiting angle of view and visual acuity can be calculated
a’ = 17a / x
in [mm]
- a = size of smallest recognizable break
- a’ = limiting angle of view
- x = distance from the chart
Formula to calculate receptor density.
= 1/(a’)2
in [1/mm2]
- a’ = limiting angle of view
What is the blind spot?
area at the point of exit of the n. opticus → no photoreceptors → detail is lost
