W16 Limits of vision - Image formation

what is diffraction

the bending of waves around a barrier or through an opening

when diffraction takes place what is created

diffuse image on the retina

aka Airy Pattern

what % of energy falls in the central airy disc

86%

what % of energy forms the concentric rings around the airy disc

14%

how are the ripples of alternating dark and light bands created around the airy disc

light waves interfere constructively and destructively

how does diffraction limit angular resolution

when separation distance between adjacent airy patterns is greater than central disc radius it creates 2 peaks on a graph

airy patterns are resolved

what is the relationship between diffraction and pupil size

more diffraction as pupil size dcreases

to what extent does diffraction limit angular resolution

minimum angle of resolution by diffraction decreases (imporves) with increasing pupil size

what is the modulation transfer function

how much contrast in the original input signal is successfully transferred through the optics of the eye

function of spatial frequency

what is the rayleigh criterion

The minimum separation between two light sources that may be resolved into distinct objects

why does MTF get worse with increasing pupil size

interaction between pupil size and aberrations

what are aberrations in the eye

small optical irregularities of the eye that prevent light being focused onto the retina effectively

what are low order aberrations

refractive errors

examples of LOA

myopia

hyperopia

astigmatism

can be corrected with lenses, gls, surgery

what are high order aberrations

complex irregularities

produce vision errors such as: difficulty seeing at night, glare, halos

examples of HOA

coma

spherical aberration

trefoil

what is a wavefront

surface of equal time

object rays that all reach the wavefront simultaneously

describe the ideal wavefront in a aberration free eye

spherical wavefront

describe the real wavefront

contains imperfections

what is the wavefront aberration W

difference between the deviations of the real and ideal wavefront

how to measure W

aberrometry

describe the Hartmann-Shack method

light source projected onto the eye

returning light passed through an array of multiple small lenses

displacement of each resultant light from the non-aberrated reference position used to determine the shape of the wavefront

how is the data from the Hartmann-Shack method analysed

zernike polynomials

what is the HOA coma

off axis points of light which appear comet shaped

describe positive coma

main oval of light farther from principle focal point of light

what is the HOA spherical distortion

light rays pass through the edges of a spherical lens or mirror and focus at a different point than rays passing through the centre

what can spherical aberrations cause

peripheral blurriness

what can substantial HOAs result from in pathological eyes

irregularities in tear film, cornea, lens, aq humour

relationship between aberrations and pupil size

increase

relationship between diffraction and pupil size

decreases

what can increase impact of HOA

pathologies

within individual components of the eye

age

eccentricity

what is intraocular light scatter

local irregularities of the RI within ocular media

straylight spreads out at larger angles over the retina

how does scattered light reduce image quality

reduces contrast

causes of intraocular scatter in healthy eyes

reflectance from cornea, lens, retina

causes of intraocular scatter in clinical instances

eg: cataract

build up of protein aggregates

define the Nyquist limit?

highest frequency that can be accurately sampled without introducing aliasing