omi term 2 wavefront aberrations

Question 1

what is an abberation

Answer 1

failure of rays to converge at one point

light rays from object that dont image in image plane

Question 2

are abberations greater in periphery or middle

Answer 2

periphery

Question 3

prismatic effect in spherical lens

Answer 3

as light moves away from optical centre more prismatic effect and theres no prism power at optical

Question 4

what can abberations impact

Answer 4

• vision of person

- resolution of image on retina

Question 5

types of aberrations from high to low

Answer 5

chromatic aberration, spherical aberration, coma, prismatic error, astigmatism, defocus

Question 6

how to classify abberations

Answer 6

use zernike polynomials

Question 7

what is zernike polynomials

Answer 7

used to classify aberrations

aberrations positive or negative

Question 8

what is superscript m in zernike polynomials

Answer 8

its angular freq and it denotes number of times the wavefront pattern repeats itself

Question 9

lower order aberrations

Answer 9

elements of eye dint line up w optical axis
lens may be tilted
fovea often affect by coma and spherical aberration

for off axis light dominant aberration affect monochromatic rentinal imaging is oblique astigmatism

retina curved surface so circles of least confusion in astigmatism lie on curve and this approaches retinal curvature, field of curvature not problematic

Question 10

what is retinal imaging affected by

Answer 10

distortion and when distortion pattern changed by spectacle correction a patient will experience distorted lines and curvature but this will disappear after brain adapts to new distortion pattern.

Question 11

higher focus abberations

Answer 11

over central 2-3mm pupil most eyes only exhibit a small amount of wavefront aberration
under photopic conditions when pupil constricts the optical performance is diffraction limited
under lower illumination an increased pupil diameter leads to increased wavefront abberration and decreased optical performance.

defocus,spherical and astimatism can be determined using lenses in practice

derived from corneal or retinal shape

other higher order aberrations can be measured along shciner hartmann aberrometer or shack hartmann wavefront sensor

Question 12

sheiner optometer

Answer 12

sheiner found that point where eye focuses can be determined by placing occluder in front of eye with 2 pinholes in front of pupil(sheiner disc)

Question 13

how does sheiner optometer work in emmetropic eye

Answer 13

single focus on retina when paralell rays of light from distant object are reduced to small beams by disc.

Question 14

sheiner optometer on refractive error eyes

Answer 14

it will cause 2 spots to fall on retina and if light from diff sources one remains in fixed position but other can be repositioned.

adjusting light source will cause isolated ray of light to redirect until it intersects with fixed ray at retina.
the patient will see single spot of light
displacement distances change in x and change in y are measures of ray aberration of eye at given point.

Question 15

principle of wavefront aberrations

Answer 15

for eye w out aberrations the reflected plane would be focused in a perfect lattice of point images and each image would fall on optical axis of its corresponding lenslet

however eye with optical aberration reflects distorted wavefront and analysis of displacement of each spot from its corresponding lenlet axis enables determination of shape of aberrated wavefront.

Question 16

clinical application of measuring ocular aberrations

Answer 16

can manage refractive surgery patients-degree of aberrations influences clinical outcome of surgery

contrast sensitivity threshold large values of coma and trefoil reduce visual performance in terms of high and low contrast visual acuity

spectacle lens design, some aberrations are important to spectacle lens wearer as distortion, curvature of field, oblique astimatism and transverse chromatic aberration