Image formation and the eye Flashcards by harmeet randhawa

what is the definition of visual perception

it is the mental image interpretation of the optical image of our surroundings

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list the three series of processes that make up a visual perception

image is formed at the back of the eye
neural processing: starting at photoreceptor level, then behind to the v1 cortex
percept: a perception of whats around

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what 2 things work hand in hand to make a visual perception

the visual side and the neural side, work hand in hand

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list some pathological things that can cause a less than optical image at the back of the eye, and what problem dies this result in

irregular corneas
dystrophies
keratoconous
cataracts
can all cause light scatter and produce a less than optimal image at the back of our eyes
so this cannot produce a perfect visual percept

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list the 7 ways of investigating image formation in vivo

retinoscopy: gets rid of problem of poor image quality by correcting the sph and cyl
aberrometry
modulation transfer function (MTF)
neural contrast threshold (NCT)
phakometry (lens)
keratoscopy (cornea)
OCT (anterior chamber)

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name a way of investigating image formation in vitro

refractive index profile of cornea and lens (as it is not uniformal)

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list the three models of investigating image formation

reduced eye model - 1 surface
paraxial schematic eyes with 3-6 surfaces
wide angle eye models with aspheric surfaces and refractive index gradients

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list the 5 things that affects optical performance and therefore limits visual perception

aberrations
MTF
diffraction
light levels - pupil size
light scatter

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what are the 2 main performances that can have limits on visual perception

optical performance

- detector performance

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list the 3 things that affects detector performance and therefore limits visual perception

‘pixels’ (photoreceptors)
retinal connections - receptive fields
variation with light levels

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what type of optical system is the eye

a general purpose, very wide angles optical system

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what can the eye resolve at high contrast

can resolve 36cpd = 6/5 snellen or 20/17

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what is 36cpd = 6/5 snellen or 20/17 that the eye can resolve in high contrast equivalent to at the retina

equivalent to 120 cycles per mm at the retina

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what are we better at resolving over letters

better at resolving gratings

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what design of the eye gives us wide viewing angles

the eye’s concentric design

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what does the wavefront aberration W =

the optical difference between real and ideal wavefront

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what is the ideal wavefront

a plane wavefront, where the rays are parallel

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what does wavefront aberration measure

measures optical performance/quality of the optics of the eye

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how can wavefront aberration quantify the measurement of the optical performance/quality of the optics of the eye, and what can it create as a result from this

can quantify by adding different amounts/types of aberrations
and can mix them in different amounts to create e.g. an intraocular lens specifically to correct this aberration

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what are colour coded maps of wavefront aberrations W, composed of and what do they show

composed of/uses different colours, which shows that the optical surface is not flat
the stronger colour changes = stronger changes to the surface = higher aberration

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what do stronger colour changes i.e. stronger changes to the optical surface indicate in wavefront aberration colour coded maps

higher aberrations

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what 2 things does wavefront aberrations W, represent

refractive error and higher order aberrations

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list the 6 different types of aberrometry

hartmann-shack
laser ray tracing
tscherning
skiascopy
spatially resolved refractometry
double pass methods

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what is the principles of the hartmann-shack method of aberrometry

wavefronts that are coming out of the eye is interpreted by small grids of lenses
each lens then splits up and produces small foci produced by the lens
if get a flat wavefront, it will be a regular grid pattern
if wavefront has aberrations, then the spots get displaced
and can measure the displacements
these wavefronts can be turned into a colour coded map

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what type of grid pattern will a flat wavefront produce in the hartmann-shack method of aberrometry

a regular grid pattern of spots

what type of grid pattern will a wavefront with aberrations produce in the hartmann-shack method of aberrometry and how can this be quantified

the spots on the grid get displaced, and can measure these displacements in order to quantify the wavefront aberrations

list the 4 properties of aberrations in normals

- a lot of inter subject variability with spherical aberration - we do not have much monochromatic aberration - we do have chromatic aberrations - in normals they are good and realistic - but in pathology these findings can change a lot

what is a complete wavefront aberration a combination of

the cornea and lens (both contribute to a complete wavefront aberration)

in which types of people is wavefront aberration much lower in and why

in young eyes as the lens compensates for the corneal spherical aberration, but in older patients the compensation reduces = spherical aberrations in higher

which type of spherical aberration NOT change with age

corneal spherical aberration

what does the whole eye spherical aberration increase with, and what trend does this show

increases with age, shows a positive trend

what causes spherical aberration to increase with age

the reduction in compensation by the lens

what is still variable in spherical aberration with age

a lot of individual variability

what causes off axis aberrations

astigmatism = significant off axis aberrations

what do we get when we go to increasing eccentricity i.e. away from the fovea

oblique astigmatism/aberrations as we go from +/- eccentricity from the fovea = U shape is very common in our optical systems

what is peripheral aberrations and what does it affect

is a peripheral image quality and partially affects myopia progression

what are produced that can be used to prevent off axis aberrations to control myopia progression

contact lenses

what does the pupil form

and approx. circular entrance pupil

what do all apertures/edges do with light and what does this cause

diffract light | this causes an imperfect image which is present all the time and we can't do anything about it

what will the image of a point source be with no aberrations present (and only diffraction present)

point source = an airy pattern/disc

what is diffraction a potential limiter of

of what we can see

what is point resolution

how close two points can be seen as two separate points | the shapes i.e. bright centres with rings around them cause diffraction

what does working out the minimum angle of resolution according to the rayleigh criterion state

diffraction predicts we will get 6/6 vision acuity with a 2mm pupil and as it gets larger, we will get aberrations

which pupil diameter does the rayleigh criterion state will start producing aberrations

> 2mm

what does diffraction have interactions with

photoreceptors

what is always present, even if our optical system is perfect

diffraction

what can aberrations have an effect on with large pupil sizes

contrast, the MTF decreases

what is MTF

the measure of the quality of the optical system

what does MTF not account for

changes in the photoreceptor sensitivity/neural response or ambient light levels only purely about the quality of the optics of the eye

which diameter pupil pupil does not have too much of an effect on image quality

3 and 4 mm pupil

which diameter pupil does a decline in image quality start to show

5mm pupil (the decline in image quality jumps from 3/4mm to 5mm i.e. is non linear)

what is evident in a 6mm pupil

more aberrations are present = poorer image performance shows a non-linear change, which is shown in the difference in the curves in the graph

between what diameters is the pupil in a lot of common lighting conditions

2-4mm

what is the ~ pupil size in moonlight

~5mm

how much of luminance is a pupil diameter of 6mm equivalent to

-2.50 log10 luminance

what does light scatter produce more of in comparison to diffraction

the spreading of light on a wider scale than diffraction

what are the major components of light scatter and which one is the main cause

light diffracted/refracted by local changes in refractive index e.g. the cornea and lens crystalline lens is the main cause of light scatter

when is light scatter mainly caused by the crystalline lens

in cataracts, the light scatter is caused by the build up of protein aggregates in the lens

why does cataract cause adverse affects on vision and reduced contrast

because the cataract causes light to flow more widely

what is the reason if you can't see anything in slit lamp due to a cataract

due to light scattered out of the eye

what does stray light have an effect on

vision

what is stray light

a visually perceived effect of light scatter

what is the cause of light scatter

light coming from a source e.g. a peripheral source that can hit the lens and the lens scatters the light in lots of different directions, even where you don't want

what can more glare/light scatter cause with contrast and what is the effect on vision

can cause reduced/lower contrast so cannot see e.g. a person as well, so theres a threshold you need to exceed in order to see well enough, but light scatter can push you below that threshold

what type of changes with age causes increased light scatter/straylight

mostly due to lens changes, not only cataract, but general changes, so can be pre cataract

what can be done in order to abolish light scatter/straylight

replace the affected media e.g. intraocular lens for cataract surgery or conceal graft

when is light scatter/straylight significantly increased

in certain pathologies that affect clarity of media e.g. cataract and corneal disease e.g. corneal endothelial dystrophy or scarring involving keratoconous

how large is a cone diameter at the fovea

~2 microns (varies)

what megapixel array is a ~2 micron cone diameter equivalent to

72 megapixel array

what can reduce a ~2 micron cone diameter from being equivalent to a 72 megapixel array

limting factors in the eye e.g. the lens

how much higher than a very high resolution camera sensor, is a 72 megapixel array caused from the ~2 micron cone diameters at the fovea

3x higher

what does our media pixel value change with

eccentricity (goes down)

list 4 variations with photoreceptors that occur with eccentricity

- increase in cone size at the parafovea - cone density falls away from fovea quickly - rapid changes in cone and rod density - pooling of photoreceptor outputs in the peripheral retina - receptive fields

what is a spatial receptive field

an area in space that results in a response from a neuron if a stimulus is presented within that area i.e. if you have a stimulus in that same area, the receptive field will generate the same response

how are the receptive fields organised in the centre of the fovea, what effect does this have if you move a stimulus

there are lots of receptive fields in the centre of the fovea, so if you move a stimulus slightly, you will notice the difference

how are the receptive fields organised in the periphery of the fovea, what effect does this have if you move a stimulus

the receptive fields are larger at the periphery, so need larger movements of the stimulus to notice a change

what is peripheral vision NOT limited by, and what is it limited by instead

NOT limited by optics | it is limited by retinal sampling (RF's getting larger = being the limiting factor)

what 2 things is foveal vision optically limited by

diffraction and aberration (at lower light levels)

how is peripheral vision limited by photoreceptor pooling

photo receptor pooling gives receptive fields that exceed the optical point spread function

what 2 things increase with peripheral vision/eccentricity

aberrations and receptive field sizes

what does neural contrast threshold reduce with and why

retinal illuminance (our sensitivity goes down with higher light levels) which is expected as the sensitivity of photoreceptors changes

what does neural contrast threshold combine with, to give our visual performance

combines with the optics of our eyes to give our visual performance