Aberrations Flashcards

Question

how is refractive idex and abbe value related

Answer 1

in nature - high refractive index gives low abbe value for spec material wear low dispersive power with high abbe value

Answer 2

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

Answer 3

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

Answer 4

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

Answer 5

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

Answer 6

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

Answer 7

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

Answer 8

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

Answer 9

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

Answer 10

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

Answer 11

for any optic, system there is a curvture of field petzval= ideal curvature - curvture of human eye - curvature of field increasing distances increases mergence

Answer 12

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

Answer 13

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

Answer 14

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

Answer 15

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

Answer 16

difference in prismatic effect of short and long wavelengths

Answer 17

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

Answer 18

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)

Answer 19

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

Answer 20

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

Answer 21

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

Answer 22

macular pigments absorb the shorter wavelengths

Answer 23

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

Answer 24

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

Answer 25

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

Answer 26

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

Answer 27

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 ]]

Answer 28

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

Answer 29

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