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OD2-Optics 3 > Optical Quality of the Eye > Flashcards

Optical Quality of the Eye Flashcards

1
Q

what is spurious resolution

A

image phase shift at high spatial frequencies

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2
Q

In a diffraction limited system, what does defocus cause in the MTF

A

ringing or spurious resolution

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3
Q

when is the magnitude of spurious resoltion (amt of modulation beyond the first 0 in the MTF) larger when the type of SA is…..

A

when the type of spherical abberation is opposite in sign to the defocus lens

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4
Q

what is monocular dipolopia

A

when SA and defocus are of opposite sign
subj sees double images of lines
marginal and paraxial rays cross each other at the retinal plane => 2 hotspots in the LSF on the retina

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5
Q

how are pupil size and spatial frequency related to the best focus of the eye

A

small pupil=best focus indiependent of spatial freq

large pupil=best focus varies w/ spatial freq

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6
Q

low freq require more ______correction for optimal contrast sensitivity

A

minus correction

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7
Q

what is the measurement of wavefront aberration based on

A

measurement of ray deviation

-the angle btwn the actual ray path and the path expected if the optical system were perfect

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8
Q

what are the 3 methods to measure wavefront aberration of the human eye
what is the most common

A
  1. hartmann-shack: image a point on the retina and use a micro-lens array to sample the wavefront reflected from the eye (MOST COMMON)
  2. ray tracing: determine ray aberration for diff beam entry locatios in the pupil measured sequentially
  3. aberroscope aka tscherning method: send light in through matrix of pinholes in pupil plan and record ray positions on retina
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9
Q

how are the hartmann-shack method and the aberroscope method different

A

hartmann shack images the outgoing beam

aberroscope divide up the ingoing beam

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10
Q

what is wavefront error

A

difference btwn the eye’s actual wavefront and the reference wavefront
-represented as an elevation map over the pupil area

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11
Q

when is wavefront error considered positive

A

if the wavefronts are converging when leaving the eye

=the rim of the wavefront is toward the recordering camera

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12
Q

what kind of wavefront defocus error would a hyperopic eye have

A

negative bc the rays outside the eye would be digerging from the virtual far point of the eye
-wavefront rim facing the virtual far point, away from the recording camera

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13
Q

what is root mean square error

A

square root of the sum of the squares of all the indiv aberration coefficients

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14
Q

why are wavefront MTFs higher than double pass and interferometric meausrements

A

do not take into account retinal scatter, which affects both the double-pass and interferometric meausrements

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15
Q

what is the overall wavefront shape commonly specified in terms of

A

zernike polynomials

-which are a set of functions that are useful in describing intensity variation over a circular aperature

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16
Q

how to calculate zernike coefficient?

A

root mean square wave error that that particular term
total RMS=2nd order and higher
higher order RMS=3rd order and higher

-higher the order is smaller RMS values

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17
Q

what are the main aberrations of the eye according to zernike’s

A

lower-order aberrations

18
Q

who may benefit from correction of higher order aberrations

A

pts w/ abnormal amounts of higher order ab

-pts w/ keratoconus and post refractive surgery

19
Q

what are the 2 types of measurements on the human eye that lower optical quality

A
  1. retinal image quality

2. wavefront aberration

20
Q

what is the point spread function

A

the intensity of distribution of the image for a point object

21
Q

how is LSF and PSF related

A

LSF essentially a series of PSFs centered on each point on the line

22
Q

what is a diffraction limited system

A

system free of aberrations

-blur only due to diffraction

23
Q

what controls the size of the airy disc

A

pupil diameter

24
Q

when may the airy disk not be ciruclar

A

aberratiosn such as coma or oblique astig present

25
Q

what is convolution

A

when the PSF can be applied to every point in the letter chart and the resultant of the image is shown

26
Q

what is the modulation transfer function

A

the fourier transform of the PSF

  • represents how much the optical system attenuates (decreases) the contrast of different spatial freq
  • plots the ratio of the image contrast/obj contrast as a function of the spatial freq of the object
27
Q

what is the cut off sptial freq

A

the spatial freq at which the MTF reaches zero

28
Q

what is the strehl ratio

A

metric used to indicate how good a system’s optical quality is compared to that of a perfect optical system
-ratio of the area under the MTF of the system to the are aunder the MTF of a diffraction-limited system

29
Q

what is a good strehl ratio

A

0.8

30
Q

what is spurioius resolution

A

image phase shift at high spatial frequencies

31
Q

what is the optical transfer function

A

MTF + PTF
mtf=contrast of each freq in the image
ptf=describes phase of each freq in degrees of phase shift, the position of image spatial freq relative to their position in the object

32
Q

what are the 2 ways to measure retinal image quality

A
  1. double-pass method: light travels through the eyes optics twice and record light reflected back from the retina, measued LSF of eyes
  2. interferometric method: aka contrast sensitivity, either by viewing sinusoidal grating targets through the optics of the eye OR gratings imaged directly on the retina using a laser
33
Q

what is the aeiral image

A

not exactly the same as retinal PSF

PSF=square root of aerial image intensity

34
Q

what happens to the LSF as pupil size increases in a diffraction limited system

A

LSF becomes narrower

35
Q

what is the neural contrast sensitivity

A

laser fringe contrast sensitivyt bc it yeilds the contrast sensitivity of the visual system from the retina to the brain

36
Q

neural CSF/optical CSF yields what

A

the optical MTF of the eye

-neural CSF better than the CSF through optics

37
Q

the PTF mainly affects retinal image quality when the pupil diamter is greater than…

A

4mm

38
Q

what could improve peripheral visibility in peripheral vision ezperiemtns

A

additional off-axis refraction

subject to optical degradation also

39
Q

spurious resolution is larger when the type of SA is______ to the defocus lens

A

larger when th etype of SA is opposite in sign to the defocus lens
+ SA more spur res with a - defocus
-SA more w/ a +defocus

40
Q

humans tend to have a ____ SA for distance vision

A

positive

41
Q

hyperopic defocus (-) should produce _______ in the eye’s MTF than myopic defocus

A

larger notches

42
Q

does potiive or negative defocus have a greater effect on the eye’s CSF

A

positive or myopic defocus

-causes a greater overall loss in contrast sensitivity

OD2-Optics 3 (6 decks)

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