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0p312 > Aberrations > Flashcards

Aberrations Flashcards

1
Q

What is an abberation

A

optical system causing light to to spread over than be focused at one point

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2
Q

what do aberrations do to an image

A

abberations cause image formed by the lens to be blurred (image formation imperfections)

refracting system of the eye is subject to aberrations - correcting mechanisms are built into the eye

adaptive optics built into the instrument allows greater resolution

white light Is dispersed

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3
Q

define chromatic abberations

A

when white light is refracted at an optical interface , it is dispersed into is component wavelengths/ colours

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4
Q

white wavelength is most deviated

A

shorter wavelength e.g. blue is more deviated on refraction

series of coloured images formed when white light is incident on spherical lens

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5
Q

what is refractive index dependent on

A

refractive index is dependent on wavelength - - short wavelength is deviated the most

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6
Q

how to good material eliminate chromatic abberations

A

good material = little difference tp each wavelength

bad = wide spread and difference between short and long wavelength focusing

when lenses are used in instruments desirable to eliminate chromatic abberations

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7
Q

what wavelength is deviated the most

A

refractive index is wavelength dependent - not the same for short , medium , long

any optical systems has a range of focal points depending on wavelength

short wavelength is deviated the most

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8
Q

how many diopters of longnitudinal chromatic aberration does the eye have

A

2 diopters

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9
Q

what end of the spectrum do macular pigments absorb

A

macular has specific pigments and they strongly absorb the short end of the spectrum

short wavelengths contribute to the image formation at the macular

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10
Q

what is the ideal material for a spec lens

A

ideal material for a spectacle lens = high refractive index and a high abbe value - thinner lens

polycarbonate = a poor spec material

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11
Q

what is duochrome

A

the principle that short wavelengths co tribute to the image formation at the macular is used in duo chrome - it is a subjective refraction test used to find the spherical end part in the prescription

trying to find a Lense where their is no difference between red and green

the position of the green and red focus dioptriclally lie alongside the wavelength of best focus

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12
Q

what colour preference on duchrome indicates to much +

A

red

too much +

the defocus of the rings of the green will be greater than the rings of red

if too much - green focus = closer to the retina - the patient will say the green letters are clearer

patient who have too much - patient will just not accommodate

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13
Q

in what refractive error will duo chrome not work

A

in high levels of ametropia duo chrome won’t work

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14
Q

refractive index is

A

wavelength dependent

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15
Q

in what pts can you not be sure you are giving maximum +

A

you can’t be sure your are giving maximum + to accommodating patients

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16
Q

what is duchrome used to find

A

used to find the spherical end of the prescription -

trying to find the Lense where there is no difference between red ad green

the position of the green and red focus dioptriclally lie alongside the wavelength of best focus

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17
Q

what does a fluctuating endpoint suggest

A

fluctuating endpoint suggests that they are accommodating , +1 maxes accomodation

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18
Q

what is accomodative lag

A

normal human when they are accommodating accommodate insufficiently this= accomodative lag - human eye downt supply enough + at near= accommodative lag - average 20yr old person would day green letters are clearer

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19
Q

what is near ducohrome used for

A

near duo chrome =subjective test for accomodative lag - - accommodative lead= atypical - they will report ‘‘red clear’’

impossible to get red and green clear at the same time

theoretical endpoint = when green is clear

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20
Q

what is dispersive power(nd)

A

need to quantify how much dispersion a optical medium does - splitting of white light

different optical mediums do it different amount - this is dispersive power

diamonds and spectacles have high dispersive powers

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21
Q

how are dispersive power and refractive index related

A

thinner lenses have higher dispersive power as you increase refractive Index - you increase dispersive power

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22
Q

what are frawnhoffer lines

A

this is for spectrometry - we use reference wavelengths of light , we use refractive index of medium width , short , medium and long wavelength

23
Q

what is the equation for dispersive power

A

dispersive power = wd = nf-nc/nd-1

f = short wavelength

c = long wavelength

refractive index = shorter for greater wavelengths

nf , c or d refers to the refractive index of material using medium , short or long wavelength

nf - nc refers to mean dispersion

nd-1 refers to refractivity of a medium

24
Q

how to use abbe value to work out dispersive power

A

instead of omega (w)
nd-1/ nf-nc

25
Q

how is refractive idex and abbe value related

A

in nature - high refractive index gives low abbe value for spec material

wear low dispersive power with high abbe value

26
Q

what is longitudinal chromatic aberration equation

A

fany = n’-n / r

you insert the wavelength you want after f and after the ‘ i..e

ff (nf-n)/ r for short wavelengths

and fc= nc/n / r for for longwavelengths

27
Q

what is tca

A

difference in prismatic effect of short and long wavelenghths

transverse chromatic abberation - measured perpendicular to the axis - tca= can cause chromastereposis

the equation is short wavelength - long wavelength
= Total prism / w

28
Q

what is chromosteropsis

A

long and short wavelengths stimulate different retinal points - stimuli are on different horopters

colours do not appear to be in the same plan - red appears to be closer - this has some effecting the light or dark depending on pupil position

29
Q

what are achromatic abberations

A

relates to the geometry of lens form

spherical abberations arises when optical surfaces are truly spherical

marginal rays get deviated more than corneal and paraxial rays
human eye confuses spherical aberration from geometry in the refractive surfaces in the human eye = asymmetrical

if we are flattening the curve at the periphery we are reducing the effects of abberent rays on the periphery

30
Q

in the human eye what type of aberration can be present

A

in the lens of human eye cautious for spherical aberration in the middle of eye= embryonic. ulcers - if refractive index is greater than the surrounding area

human eye controls effects of spherical aberration - pupil mitosis happens at near - we get a smaller pupil which reduces the abberent rays

31
Q

what are longitudinal chromatic abberations

A

error along the axis

fd x w

human eye has 2 diopters of lca

short wavelength focus 2d in front of long wavelength focus

duo chrome appears yellow for uncorrected myope

32
Q

what stereopsis affect arises from am abberation

A

stereopsis affects have been known to arise from transverse chromatic abberations- blue is seen further away due to

chromosteropsis - long and short wavelengths - stimulate disparate retinal points - stimulate on different horopters - pupils decentered nasally , optics decentred tempoarally

colours do not appear in the same plane

red appears closer

some see this effect in the light or dark depending on pupil position

33
Q

how does the human eye control the effects of spherical abberation

A

human eye controls spherical abberations from geometry in the refractive surfaces - in human eye- asymmetrical , if we are flattening the cone at periphery , we are reducing the effects of abberent rays on periphery

in the lens of human eye controls for spherical aberration in the middle of eye - embryonic nucleus - its refractive index is greater than surrounding area

human eye controls the effects of spherical aberration , pupil mitosis happens at near - we get a smaller pupil which reduces the abberent rays

34
Q

what are achromatic abberations

A

achromatic abberations - relating to geometry of lens form

spherical abberations - arises when optical surfaces are truly spherical - marginal rays get deviated more than the central paraxial rays

35
Q

what is ideal curvture

A

for any optic, system there is a curvture of field

petzval= ideal curvature - curvture of human eye -

curvature of field

increasing distances increases mergence

36
Q

what is oblique astigmatism

A

off axis object point

spherical wavefront

lens at an angle - this results in a non spherical wavefront of the lens which results in formation of two line images perpendicular to one and other

37
Q

what is an example of spherical aberration

A

coma - spherical aberration of an off axis point

distortion - wider appetrue and stronger lenses - a point has been decentred more

pin cushion distortion with a - lens

+ lens = banned distortion - images are pulled out

38
Q

how are achromatic abberations corrected

A

correction of achromatic lens system

dispersive power of material independent of its refractive index

materials of high dispersive power but Low refractive index or vice versa

achromatic lens systems composed of lenses of varying materials combined - dispersion neutralised whilst overall refractive power is preserved

39
Q

what is a ocular chromatic abberation

A

refraction of human eye subject to chromatic aberration - total dispersion is from red to blue image - 2 diopters

emmetropic eye focuses for yellow- green (55nm) - peak wavelength of photopic relative illuminosity centre

40
Q

what is the equation for transverse chromatic abberations

A

difference in prismatic effect of short and long wavelengths

41
Q

duo chrome - biochromatic test -

A

allows sensitive identification of spherical end point in subjective refraction (gives max plus)

tests consists of 2 ranks of Snellen letters silluhated against coloured glass

upper mark mounted on red glass and Lower rank on green glass

red and green used because their wavelengths wavelength foci straddle yellow and green by equal amounts (4 diopters either side)

patient views the letters by means of red and green light - which appears clearer

test sensitive to an alteration in refraction 0.25d

myopic eye sees red letters more clearly than green

test used in refraction pf myopic patients - expect eye strain if over corrected which forces them to use their accomodation for distance vision

colour blindness doesn’t invalidate test - depends on position of image respect to retina

42
Q

what are achromatic abberations

A

designed to limit effects of chromatic aberration and spherical aberration (aka monochromatic abberations) due to form of lens

bring 2 wavelengths - e.g. blue and red into focus (third order)

increase order theory , increase order aberration

paraxial theory-helps describe poor performance of optical surfaces when light travels through periphery

wavefront abberation (lask)

43
Q

what is spherical abberation

A

prismatic effect of spherical lens least in the paraxial zone and increases towards the periphery of the lens

rays passing through periphery of lens deviated more than rays passing through paraxial zone of the lens

spread of rays depends on distance of images

44
Q

how to correct spherical abberation

A

decrease spherical abberatiion by occluding the periphery of the lens - only paraxial zone used

adjust lens to decrease spherical aberration

plano and convex proffered to biconvex

45
Q

what is the effect of spherical aberration in eyes

A

anterior corneal surface flatter peripherally at centre - acts as aplanatic surface

nucleus of lens has greater refractive index than lens cortex

axial zone of the lens has a greater refractive power than at the periphery

iris acts as stop to decrease in spherical aberration - visual acuity impairment due to pupil dilation is due to spherical; aberration

retinal cones are more sensitive to light entering our eye paradoxically than to light entering obliquely through the peripheral cornea (stiles - Crawford effect)

directional sensitivity of cones limits visual effects of residual spherical abberatrion in the eye (no short cones at the fovea)

due to this we don’t notice 2 diopters of chromatic abberations

when looking near all achromatic abberations get worse

abberent rays eliminated to m minimise their effects

there are no short wavelength cones in the fovea - all humans have a small field of trtianopia due to this

46
Q

what wavelengths do macular pigements absorb

A

macular pigments absorb the shorter wavelengths

47
Q

what is oblique astigmatism

A

spherical wavefront strikes optical surface obliquely refracted - wavefront is not spherical anymore

considered to form 2 focal lines seperated by 90 degrees

this happens in all spherical surfaces used off axis

48
Q

what range of light does the cornea absorb and the crystalline lens

A

the cornea absorbs all the ultraviolet light less than 295nm

the cyrstaline lens absorbs between 300- 400 nm

crystalline lens absorption changes with age - you get yellowing (gets shorter With age)

can lead to inflammatory conea

vriteuous and aqueous transmission characteristics are similar to water

inferred above 760

cornea dosnt absorb infrared. ut viretons only absorbs wavelengths greater than 1400 nm

retinal hazard region - wavelengths from 400-1400 take it through the retina

49
Q

why don’t humans notice chromatic abberations

A

humans don’t notice chromatic abberations because of the macular pigments , macular pigments absorb short wavelengths

maximum absorbtion = 460nm - blue end of the spectrum 460nm

filters 40% of the visible light reaching the retina

another important role - absorbs free radicals and other scavenging oxygen species associated with normal photoreceptor activity

process of changing light energy requires lots of energy

results = too produce free radicals - high energy can cause damage to the biological tissue

50
Q

what is the role of macular pigments

A

normal process of vision is toxic to the structures

cells in retina - = in a toxic enviroment - due to the processing of photoreceptors

role of macular pigments = too absorb free radicals and reduce toxicity

51
Q

using longitudinal chromatic aberration explain the principles of a subjective refraction test

A

lca is when a lens cannot focus different colours in the same focal plane due to the foci of the different colour being at different points in the longitudinal direction

ducohrome= subjective refraction test

  • test used lea to determine sensitive identification of spherical entropic end point in subjective refraction

emmtrope or corrected antelope should view red and green as equally clear

same targets should be used to reduce bias

uncorrected myopes will indicate increase clarity for targets on the red backgrounds , here additional minus spheres should be added until the targets on both sides are equally clear or may appear yellow because their is an overlap between green and red

under correction recommended to rest accomodation system ]]

52
Q

what will uncorrected hypermetropes report on the duo chrome test

A

uncorrected hypermetropes may indicate a preference for the targets on the green background , thus additional plus spheres should be added until the position of equality is achieved this means the hypermetropic subject is overcorrected - needs to do less accomodation

the ducohrome test only used to confirm the spherical power so if the test indicates a patient requires additional lens power to achieve equality of clarity or the red is clearer this should be added - the dterminination of the final result should be the maximum plus that provides optimum va

53
Q

how does chromostereposis arise form transerse chromatic abberatons

A

tca is the angle between refracted chief rays - ray that goes through the optical centre for different wavelengths

tca depends on the object axis in the visual field for and pupil positon in the eye
chrompsteropsis is a visual illusion \

it arises due to the difference in position of the fovea relative too the optical axis because different wavelengths of light are displaced in non corresponding retinal positions of the eye during binocular viewing

fovea = located temporally to the optical axis thus the visual axis passes through cornea with nasal horizontal eccentricity

pupils are decanted nasally and optics decanted temporally

the average ray bound for the fovea undergoes prismatic deviation thus subject to chromatic dispersion

the prismatic deviation is in opposite directions in each eye , the opposite colour shifts created which leads ti a shift in steropitc depth between red and blue objects

as the pupillary distance from the foveal achromatic axis is increased perceived depth also increases due to tca , colours do not appear to be in the same place , positive chromosteropissi is exhibited when red bars are perceived in front of blue negative chrometeropsis when red bars are perceived behind the blue

0p312 (11 decks)

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