Image formation and the eye Flashcards
what is the definition of visual perception
it is the mental image interpretation of the optical image of our surroundings
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
what 2 things work hand in hand to make a visual perception
the visual side and the neural side, work hand in hand
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
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)
name a way of investigating image formation in vitro
refractive index profile of cornea and lens (as it is not uniformal)
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
list the 5 things that affects optical performance and therefore limits visual perception
- aberrations
- MTF
- diffraction
- light levels - pupil size
- light scatter
what are the 2 main performances that can have limits on visual perception
- optical performance
- detector performance
list the 3 things that affects detector performance and therefore limits visual perception
- ‘pixels’ (photoreceptors)
- retinal connections - receptive fields
- variation with light levels
what type of optical system is the eye
a general purpose, very wide angles optical system
what can the eye resolve at high contrast
can resolve 36cpd = 6/5 snellen or 20/17
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
what are we better at resolving over letters
better at resolving gratings
what design of the eye gives us wide viewing angles
the eye’s concentric design
what does the wavefront aberration W =
the optical difference between real and ideal wavefront
what is the ideal wavefront
a plane wavefront, where the rays are parallel
what does wavefront aberration measure
measures optical performance/quality of the optics of the eye
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
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
what do stronger colour changes i.e. stronger changes to the optical surface indicate in wavefront aberration colour coded maps
higher aberrations
what 2 things does wavefront aberrations W, represent
refractive error and higher order aberrations
list the 6 different types of aberrometry
- hartmann-shack
- laser ray tracing
- tscherning
- skiascopy
- spatially resolved refractometry
- double pass methods
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
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
