limits of vision: image formation Flashcards
describe the process of image formation?
- the eye has 2 positive lenses the cornea and crystalline lens
- the cornea and lens combine to project images onto the photoreceptor layer of the retina
- before light reaches the lens, it travels through the pupil which controls retinal illumination
- 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
- at the retina , light is transduced into electrical signals by photoreceptors, then passed on to ganglion cells for encoding and transmission
- the signals are then further processed throughout the visual cortex and beyond
what is the pupil diameter?
. varies in diameter from less than 2 mm in bright light ( photopic) to more than 8mm in the dark ( scotopic)
what is diffraction?
. when a wave ( microwave , sound wave ) passes an edge, it spreads out
how does diffraction occur in the eye?
. 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
what is an airy patten?
. diffuse image on retina
what does an airy pattern consist of?
. central airy disk where 86% of the energy falls
. 14% of the light will form concentric rings around the disk
what is an airy pattern formed?
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
how does diffraction limit angular resolution ( the smallest thing you can see) ?
- 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
- 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
what is Rayleigh criterion ?
. this is where the two peaks over lap
. the separation distance reach a value equal to the central disk radius
to what extent does diffraction limits angular resolution?
. 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
what is the relationship between aperture size and Rayleigh criterion?
. as the size of aperture increases
. the Rayleigh criterion decreases
. the minimum angle of resolution improves
is human vision diffraction limited?
. 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
what is modulation transfer function?
. 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 )
what is the relationship between MTF and pupil size?
. MTF gets worse with increasing pupil size
. less contrast is transmitted through the optics
. this is because of increasing aberration
what is the summary of diffraction?
- the pupil causes incoming light to diffract
- the light becomes spread out and in the absence of any other optical factors forms an airy pattern
- the diameter of the airy disk places a constraint on resolvability of 2 points
- diffraction deceases with increasing pupil size and Rayleigh criterion decreases
- when a pupil size is small , diffraction may limit vision, but at larger diameters visual acuity is worse than diffraction alone would predict
- diffraction predicts that MTF should be worse with increasing pupil size but this is not true
what are aberrations in the eye?
. small optical irregularities : imperfections of the eye that prevents light being focused onto the retina effectively
what are lower order aberrations?
. also known as refractive errors. they include myopia, hyperopia and astigmatism
. they are corrected with glasses, contact lenses or refractive surgery
what are high order aberration?
. 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
what is the wavefront?
. 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
what is the aberrated wavefront?
. in the real eye, imperfections result in an aberrated wavefront
what is wavefront aberration?
. the difference ( deviation) between the real wavefront as measured , and the ideal wavefront is known as wavefront aberration ( W )
what is aberrometry?
. this is the measurement of wavefront aberration (W)
what is the Hartmann-shack method?
. this is a method of aberrometry
- a light source is projected into the eye
- the outgoing ( returning) light is passed through an array of multiple small lenses ( lenslets), somewhat like the compound eye of an insect
- the displacement of each resultant light spot from the corresponding nonaberrated reference position is used to determine the shape of the wavefront
- a summary measure of total wavefront error can be computed as root-mean-square ( RMS ) microns
- output typically colour coded
how are aberrations analysed?
. 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
what is coma ( cromatic aberration ) ?
. 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 )
what is spherical distortion?
. 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
how important are higher order aberration to image formation?
. 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
what happens when you remove higher order aberration?
. in normal eyes, correcting astigmatism and HOAs can have a measurable impact
. removing higher order aberration increases visual acuity
what are higher order aberration a result of?
. 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
what is the treatment of higher order aberration?
. treatment involves remedying the underlying cause ( e.g. dry eyes)
when are higher order aberrations a big deal?
. 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
what is complete wavefront aberration ( W) ?
. is a measure of the whole system ( the cornea + lens aberrations)
what is the relationship between higher order aberration and age?
. corneal spherical aberration doesn’t change with age
. but whole eye spherical aberration does increase with age
. internal compensation by lens reduces with ages
how is higher order aberration a big deal in more eccentric regions of the eye?
. just like astigmatism : higher order aberration increase with eccentricity
how are higher order aberrations and lower order aberration a big deal with increasing pupil size?
. all aberrations increase with pupil size
. with increasing pupil size . there is a reduced optical performance , even though diffraction decreased
what is the ideal pupil size for VA?
. 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
what is the Campbell and Gregory experiment finding?
. natural pupil size optimises VA at different light levels
what is an example of natural compensatory mechanism?
. 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
what is summary of aberration?
- abnormalities in any refractive components of eye such as the lens can prevent the light from being focused correctly
- these errors can be quantified in terms of wavefront aberration ( W ) : which is the deviation between the observed ( aberrant ) and ideal ( spherical ) wavefront
- these errors can be classified as lower order aberration and more complex higher order aberration
- in normal eyes HOAs are minor ( <0.25 D) , but their impact can increase with pathology , age and eccentricity
- LOAs and HOAs increase as a function of pupil size
what is intraocular light scatter?
. 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)
how does scattered light reduce retinal image quality?
. scattered light reduces retinal image quality by reducing contrast particularly in scenes containing a bright source , such as night driving
what is the main cause of intraocular scatter in healthy eyes?
. in healthy eyes, the main source for intraocular light scattering are light reflectance from cornea, lens , retina and light penetrating through sclera and iris
how does the visual system minimise light scatter?
. 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
what is the most common cause of scatter?
. scatter most common due to imperfections in the lens
. in particular : cataract which is a build up of protein in the lens
what are other causes of intraocular scatter include ?
. disease of cornea ( Fuchs's dystrophy) . lack of retinal pigment ( albinism) . laser refractive surgery . opacities in vitreous humour ( floaters ) . intraocular inflammation ( uveitis ) . contact lenses
what are the effects of intraocular light scatter?
. 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
what is the interaction between intraocular scatter and aberration ?
. it has been shown that contrast sensitivity is reduced slightly less by spherical aberration when scatter is present
what is retinal sampling resolution?
. 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
