limits of vision: image formation

Question 1

describe the process of image formation?

Answer 1

1. the eye has 2 positive lenses the cornea and crystalline lens
2. the cornea and lens combine to project images onto the photoreceptor layer of the retina
3. before light reaches the lens, it travels through the pupil which controls retinal illumination
4. after the light is refracted by the lens, it enters the posterior chamber which is filled with transparent vitreous humour and converges on the retina
5. at the retina , light is transduced into electrical signals by photoreceptors, then passed on to ganglion cells for encoding and transmission
6. the signals are then further processed throughout the visual cortex and beyond

Question 2

what is the pupil diameter?

Answer 2

. varies in diameter from less than 2 mm in bright light ( photopic) to more than 8mm in the dark ( scotopic)

Question 3

what is diffraction?

Answer 3

. when a wave ( microwave , sound wave ) passes an edge, it spreads out

Question 4

how does diffraction occur in the eye?

Answer 4

. when light passes through a circular aperture such as the pupil
. the edges of pupil cause the light to spread
. this means that even a point source such as a laser will create a diffuse image on the retina known as airy patten

Question 5

what is an airy patten?

Answer 5

. diffuse image on retina

Question 6

what does an airy pattern consist of?

Answer 6

. central airy disk where 86% of the energy falls

. 14% of the light will form concentric rings around the disk

Question 7

what is an airy pattern formed?

Answer 7

this is because
. a light source produces many different light waves
. some waves interfere constructively ( add together ) , and some will interfere destructively ( cancel out)
. this results in a bright region ( airy disk ) surrounded by ripples of alternating dark/light bands

Question 8

how does diffraction limit angular resolution ( the smallest thing you can see) ?

Answer 8

1. when the separation distance ( D) between adjacent airy patterns is greater than the central disk radius ( r) , the sum of the intensities yields two individual peaks and both airy patterns are said to be resolved
2. as the disks approach each other , the separation distance will reach a value equal to the central disk radius ( this Rayleigh criterion ) . the two airy patterns are not resolved
   but still distinguishable
   3 . if D is less than r , the sum of the two peaks merges into a single peak and the two objects become indistinguishable

Question 9

what is Rayleigh criterion ?

Answer 9

. this is where the two peaks over lap

. the separation distance reach a value equal to the central disk radius

Question 10

to what extent does diffraction limits angular resolution?

Answer 10

. when passing through a circular aperture, such as the pupil, the amount of diffraction is proportional to the diameter of the gap
. more diffraction as pupil size decreases
. the minimum angle of resolution predicted by diffraction decreases ( improves ) with increasing pupil size

Question 11

what is the relationship between aperture size and Rayleigh criterion?

Answer 11

. as the size of aperture increases
. the Rayleigh criterion decreases
. the minimum angle of resolution improves

Question 12

is human vision diffraction limited?

Answer 12

. human vision is not diffraction limited
. this is because the minimum angle of resolution is 1 arc
. any pupil bigger than 2-3 this is worse than what diffraction predicts
. our vision is worse than what diffraction predicts

Question 13

what is modulation transfer function?

Answer 13

. MTF expresses how much contrast in the original input signal is successfully transferred through the optics of the eye, as a function of spatial frequency
. MTF is an overall index of image quality ( diffraction + aberrations + scatter )

Question 14

what is the relationship between MTF and pupil size?

Answer 14

. MTF gets worse with increasing pupil size
. less contrast is transmitted through the optics
. this is because of increasing aberration

Question 15

what is the summary of diffraction?

Answer 15

1. the pupil causes incoming light to diffract
2. the light becomes spread out and in the absence of any other optical factors forms an airy pattern
3. the diameter of the airy disk places a constraint on resolvability of 2 points
4. diffraction deceases with increasing pupil size and Rayleigh criterion decreases
5. when a pupil size is small , diffraction may limit vision, but at larger diameters visual acuity is worse than diffraction alone would predict
6. diffraction predicts that MTF should be worse with increasing pupil size but this is not true

Question 16

what are aberrations in the eye?

Answer 16

. small optical irregularities : imperfections of the eye that prevents light being focused onto the retina effectively

Question 17

what are lower order aberrations?

Answer 17

. also known as refractive errors. they include myopia, hyperopia and astigmatism
. they are corrected with glasses, contact lenses or refractive surgery

Question 18

what are high order aberration?

Answer 18

. these are complex irregularities , with unfamiliar names such as coma, spherical aberration and trefoil
. these aberrations can produce vision error s such as difficulty seeing at night , glare, halos, blurring, starburst patterns or double vision

Question 19

what is the wavefront?

Answer 19

. surface of equal time where object rays all reach the wavefront simultaneously
. as light enters the eye from that air, it’s speed is retarded according to the refractive index of the material along its path to the retina
. in the ideal ( aberration free) eye , this results in the wavefront becoming spherical

Question 20

what is the aberrated wavefront?

Answer 20

. in the real eye, imperfections result in an aberrated wavefront

Question 21

what is wavefront aberration?

Answer 21

. the difference ( deviation) between the real wavefront as measured , and the ideal wavefront is known as wavefront aberration ( W )

Question 22

what is aberrometry?

Answer 22

. this is the measurement of wavefront aberration (W)

Question 23

what is the Hartmann-shack method?

Answer 23

. this is a method of aberrometry

1. a light source is projected into the eye
2. the outgoing ( returning) light is passed through an array of multiple small lenses ( lenslets), somewhat like the compound eye of an insect
3. the displacement of each resultant light spot from the corresponding nonaberrated reference position is used to determine the shape of the wavefront
4. a summary measure of total wavefront error can be computed as root-mean-square ( RMS ) microns
5. output typically colour coded

Question 24

how are aberrations analysed?

Answer 24

. to make sense of the data, we can it break down into simple/independent ‘’ basis function’’
. 2nd order aberration termed lower order aberration
. all higher aberrations termed higher order aberration
. there are many high order aberration but 15 is sufficient

Question 25

what is coma ( cromatic aberration ) ?

Answer 25

. higher order aberration
. off-axis points of light appear comet-shaped. Parts of the lens vary in magnification , creating a series of asymmetrical circular shapes
. can be positive ( main oval of light farther from principle focal point) , or negative ( opposite )

Question 26

what is spherical distortion?

Answer 26

. higher order aberration
. light rays striking then lens off-centre are refracted more/less than those striking close to the centre
. causes blurriness at the edges of an image
. ‘‘positive’’ spherical aberration means peripheral rays are bent too much
. ‘‘negative’’ spherical aberration means peripheral rays are not bent enough

Question 27

how important are higher order aberration to image formation?

Answer 27

. in normal eyes, higher order aberration are relatively small component, comprising about 10% of eye’s overall aberration
. higher order aberration tend to be 0 except spherical aberration
. correcting higher order aberrations in normals is equivalent to 0.25 DS

Question 28

what happens when you remove higher order aberration?

Answer 28

. in normal eyes, correcting astigmatism and HOAs can have a measurable impact
. removing higher order aberration increases visual acuity

Question 29

what are higher order aberration a result of?

Answer 29

. substantial higher order aberration can result from irregularities in any of the refractive components ( tear film , cornea, aqueous humour , crystalline lens , vitreous humour ) for example

• thickened proteins in cataracts
• insufficient/irregular tear film in people with chronically dry eyes
• corneal scarring from disease, injury or eye surgery
• irregularities in the lens or vitreous humour

Question 30

what is the treatment of higher order aberration?

Answer 30

. treatment involves remedying the underlying cause ( e.g. dry eyes)

Question 31

when are higher order aberrations a big deal?

Answer 31

. higher order aberrations are a big deal in individual components of normal eye
. this is done be measuring individual components , this causes higher order aberration to increase

Question 32

what is complete wavefront aberration ( W) ?

Answer 32

. is a measure of the whole system ( the cornea + lens aberrations)

Question 33

what is the relationship between higher order aberration and age?

Answer 33

. corneal spherical aberration doesn’t change with age
. but whole eye spherical aberration does increase with age
. internal compensation by lens reduces with ages

Question 34

how is higher order aberration a big deal in more eccentric regions of the eye?

Answer 34

. just like astigmatism : higher order aberration increase with eccentricity

Question 35

how are higher order aberrations and lower order aberration a big deal with increasing pupil size?

Answer 35

. all aberrations increase with pupil size

. with increasing pupil size . there is a reduced optical performance , even though diffraction decreased

Question 36

what is the ideal pupil size for VA?

Answer 36

. aberration increases with pupil size
. diffraction decreases with pupil size
. under lab lighting , the ideal pupil size for optimum VA is around 3mm but this varies with light level

Question 37

what is the Campbell and Gregory experiment finding?

Answer 37

. natural pupil size optimises VA at different light levels

Question 38

what is an example of natural compensatory mechanism?

Answer 38

. aberration of eye increases with age and the average pupil diameter decreases, reducing their impact
. the eye is able to exploit this relationship to optimise vision

Question 39

what is summary of aberration?

Answer 39

1. abnormalities in any refractive components of eye such as the lens can prevent the light from being focused correctly
2. these errors can be quantified in terms of wavefront aberration ( W ) : which is the deviation between the observed ( aberrant ) and ideal ( spherical ) wavefront
3. these errors can be classified as lower order aberration and more complex higher order aberration
4. in normal eyes HOAs are minor ( <0.25 D) , but their impact can increase with pathology , age and eccentricity
5. LOAs and HOAs increase as a function of pupil size

Question 40

what is intraocular light scatter?

Answer 40

. local irregularities of the refractive index within the ocular media, leading to stray light ( inbound light being spread out at large angles over the retina)

Question 41

how does scattered light reduce retinal image quality?

Answer 41

. scattered light reduces retinal image quality by reducing contrast particularly in scenes containing a bright source , such as night driving

Question 42

what is the main cause of intraocular scatter in healthy eyes?

Answer 42

. in healthy eyes, the main source for intraocular light scattering are light reflectance from cornea, lens , retina and light penetrating through sclera and iris

Question 43

how does the visual system minimise light scatter?

Answer 43

. there are few mitochondria and other large intracellular organelles in the lens , and those present exist in lens fibres tucked behind the iris so not to scatter light
. the choroid contains dark melanin pigment to absorb stray light

Question 44

what is the most common cause of scatter?

Answer 44

. scatter most common due to imperfections in the lens

. in particular : cataract which is a build up of protein in the lens

Question 45

what are other causes of intraocular scatter include ?

Answer 45

. disease of cornea ( Fuchs's dystrophy)
. lack of retinal pigment ( albinism)
. laser refractive surgery 
. opacities in vitreous humour ( floaters )
. intraocular inflammation ( uveitis )
. contact lenses

Question 46

what are the effects of intraocular light scatter?

Answer 46

. loss of contrast
. strongest impact in mesopic and scotopic conditions , and in the presence of a strong light source away from fixation
. stray light primarily affects contrast sensitivity , it can also affect a range of visual functions such as visual acuity

Question 47

what is the interaction between intraocular scatter and aberration ?

Answer 47

. it has been shown that contrast sensitivity is reduced slightly less by spherical aberration when scatter is present

Question 48

what is retinal sampling resolution?

Answer 48

. photoreceptors mosaic has finite density
. this places another potential limit on resolution ( the Nyquist limit )
. most ganglion cells pool across multiple photoreceptors, with more peripheral ganglia having increasingly large receptive fields
. in healthy eye, neural sampling is more of a limiting factor than optics everywhere outside the fovea