Aberrations Flashcards
What is an abberation
optical system causing light to to spread over than be focused at one point
what do aberrations do to an image
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
define chromatic abberations
when white light is refracted at an optical interface , it is dispersed into is component wavelengths/ colours
white wavelength is most deviated
shorter wavelength e.g. blue is more deviated on refraction
series of coloured images formed when white light is incident on spherical lens
what is refractive index dependent on
refractive index is dependent on wavelength - - short wavelength is deviated the most
how to good material eliminate chromatic abberations
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
what wavelength is deviated the most
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
how many diopters of longnitudinal chromatic aberration does the eye have
2 diopters
what end of the spectrum do macular pigments absorb
macular has specific pigments and they strongly absorb the short end of the spectrum
short wavelengths contribute to the image formation at the macular
what is the ideal material for a spec lens
ideal material for a spectacle lens = high refractive index and a high abbe value - thinner lens
polycarbonate = a poor spec material
what is duochrome
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
what colour preference on duchrome indicates to much +
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
in what refractive error will duo chrome not work
in high levels of ametropia duo chrome won’t work
refractive index is
wavelength dependent
in what pts can you not be sure you are giving maximum +
you can’t be sure your are giving maximum + to accommodating patients
what is duchrome used to find
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
what does a fluctuating endpoint suggest
fluctuating endpoint suggests that they are accommodating , +1 maxes accomodation
what is accomodative lag
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
what is near ducohrome used for
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
what is dispersive power(nd)
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
how are dispersive power and refractive index related
thinner lenses have higher dispersive power as you increase refractive Index - you increase dispersive power
what are frawnhoffer lines
this is for spectrometry - we use reference wavelengths of light , we use refractive index of medium width , short , medium and long wavelength
what is the equation for dispersive power
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
how to use abbe value to work out dispersive power
instead of omega (w)
nd-1/ nf-nc
how is refractive idex and abbe value related
in nature - high refractive index gives low abbe value for spec material
wear low dispersive power with high abbe value
what is longitudinal chromatic aberration equation
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
what is tca
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
what is chromosteropsis
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
what are achromatic abberations
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
in the human eye what type of aberration can be present
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
what are longitudinal chromatic abberations
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
what stereopsis affect arises from am abberation
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
how does the human eye control the effects of spherical abberation
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
what are achromatic abberations
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
what is ideal curvture
for any optic, system there is a curvture of field
petzval= ideal curvature - curvture of human eye -
curvature of field
increasing distances increases mergence
what is oblique astigmatism
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
what is an example of spherical aberration
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
how are achromatic abberations corrected
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
what is a ocular chromatic abberation
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
what is the equation for transverse chromatic abberations
difference in prismatic effect of short and long wavelengths
duo chrome - biochromatic test -
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
what are achromatic abberations
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)
what is spherical abberation
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
how to correct spherical abberation
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
what is the effect of spherical aberration in eyes
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
what wavelengths do macular pigements absorb
macular pigments absorb the shorter wavelengths
what is oblique astigmatism
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
what range of light does the cornea absorb and the crystalline lens
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
why don’t humans notice chromatic abberations
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
what is the role of macular pigments
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
using longitudinal chromatic aberration explain the principles of a subjective refraction test
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 ]]
what will uncorrected hypermetropes report on the duo chrome test
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
how does chromostereposis arise form transerse chromatic abberatons
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
