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Optics Block 8 > Retinal Image Quality > Flashcards

Retinal Image Quality Flashcards

1
Q

Extent to which the copy matches the original

A

Fidelity

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2
Q

What is fidelity always less than

A

1, the original object is gold standard

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3
Q

Filters and fidelity

A

Can make the image look better despite infidelity

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4
Q

Smaller pupil and image quality

A

Large DF

Quality cant be gerenalized, depends on what you want. There will be a clear background and image

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5
Q

Large pupil and image quality

A

Small DOF

Blurry background and clear up close object

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6
Q

The illuminance or luminance in the image of a point source of light

A

PSF

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7
Q

What does PSF depend upon

A

Diffraction, defocus, aberrations, and scattered light

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8
Q

In the absence of defocus, aberrations, and scatter, the PSF is called

A

Diffraction limited PSF

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9
Q

What do defocus, aberrations, and scattered light do to PSF

A

Shorten and broaden it

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10
Q

Aberrated PSF

A

For pupil diameter greater than 2mm, or in polychromatic light, aberrations tend to ‘spread out more’ relative to the diffraction limited PSF

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11
Q

What does the FORM of the PSF depend on

A

Shape and diameter of the aperture stop

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12
Q

Pupil size and best retinal image quality

A

3mm

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13
Q

PSF metrics to quantify image quality

A

The rayleighs criterion
The half width
The strehl ratio

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14
Q

This states the two point resolution limit occurs when the first minimum (the boundary of Airy’s disk) of one diffraction pattern coincides with the center maximum of the other diffraction limited pattern

A

Rayleighs criterion

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15
Q

The diffraction pattern formed on a distant screen resulting from a uniformly illuminated circular aperture has a bright region in the center known as

A

Airys disk

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16
Q

The central disk plus the surrounding series of concentric bright rings is called the

A

Airy pattern

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17
Q

The width of PSF at half the peak height is called

A

The half width, or full width and half maximum

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18
Q

What is the half width of the diffraction limited PSF

A

3.23

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19
Q

The half width of the PSF and aberrations

A

The half width of the PSF increases with the introduction and increase of aberrations

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20
Q

Half width and astigmatism/coma

A

When astigmatism and coma are Preston, the half width should be considered in two mutually perpendicular directions, where the width is minimum and maximum. These can be reduced to a single number by taking the arithmetic or geometric mean of the two widths

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21
Q

This is a measure of the effect of aberrations on reducing the maximum or peak value of the PSF

A

Strehl ratio

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22
Q

What is the equation for strehl ratio

A

PSF of image/PSF of diffraction limited

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23
Q

What is the strehl ratio always equal to and why

A

Since the effect of aberrations is too spread out, the PSF decrease the maximum peak height, the strehl intensity ratio is ways less than or equal to one

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24
Q

What happens to the strehl ratio when the aberrations are greater

A

The greater the aberrations, the lower the value of the strehl intensity ratio and the poorer the image quality

25
Q

What is considered a good, near to diffraction limited system strehl ratio

A

0.8

26
Q

The luminance or illuminance distribution in the image of a line source of light

A

Line spread function (LSF)

Used for multifocal lenses

27
Q

Measuring the PSF (LSF) of the human eye

A

The light distribution of a point source at the retina cannot be measured directly, but the light passing back out of the eye (the Aerial or external image) is measureabel. Because the light is passed twice through the eyes optical system, this method is known as double pass method

28
Q

What are two important factors in understanding optical transfer function (OTF)

A

Resolution and contrast

29
Q

Systems ability to distinguish object detail

A

Resolution

30
Q

Defined as how faithfully the minimum and maximum intensity values are transferred from object to image plane

A

Contrast or modulation

31
Q

Do abberations affect resolution or contrast

A

Both

32
Q

How well intensity of image is transferred to image

A

Modulation or contrast

33
Q

Frequency and OTF (MTF)

A

The higher the frequency, the worse the resolution and at some point no resolution, decreased MTF (OTF)

34
Q

What does the optical transfer function include

A

The MTF and PTF

35
Q

What does OTF equal

A

MTF + PTF

36
Q

The amplitude A’ of the image divided by the amplitude A of the object, and it is a function of spatial frequency

A

MTF

MTF=A’/A=1

37
Q

What is MTF normalized to

A

Due to light aberrations, MTF is normalized to 1

38
Q

What is MTF a function of

A

Spatial resolution, which refers to the smallest line-park the system can resolve

39
Q

What is the anzi standard for IOL lens

A

.43

40
Q

For a lens, this is the measurement of its ability to transfer contrast at a particular resolution from the object to the image

A

MTF, this is way to incorporate resolution and contrast into a single specification

41
Q

What happens to MTF as line spacing decreases

A

As line spacing decreases on the target test, it becomes increasingly difficult for the lens to efficiently transfer this decrease in contrast, as a result, MTF decreases

42
Q

What is the spatial domain

A

PSF

43
Q

What is the object domain

A

Optical transfer function

44
Q

Defocus effect

A

Decrease in amplitude

45
Q

Astigmatism effect

A

Decrease in amplitude (defocus) on both meridians

46
Q

Spherical aberrations effect

A

Decrease in amplitude

47
Q

Coma and its effect

A

Decrease in amplitude no cause transverse phase shift on the image position (PTF)

48
Q

Distortion and its effect

A

Transverse shift of the image position (PTF)

49
Q

Field curvature and its effect

A

Decrease in amplitude (defocus) independent of meridian

50
Q

Those aberrations that produce a transverse shift produce what?

A

Coma and distortion: an effective phase shift in the image

51
Q

The phase shift across a range of spatial frequencies is called

A

The phase transfer function

52
Q

Phase change in on-axis

A

In an on-axis in a rotationally symmetrical optical system, there is no change in phase with spatial frequency
-in these situations, OTF is identical to MTF and we can use the terms interchangeably

OTF=MTF+PTF
OTF=MTF

53
Q

Where in the eye is there no change in phase and the PTF=0

A

Central vision

54
Q

When does PTF become significant

A

If off axis

  • periphery
  • coma, produces TCA, phase shift increase
55
Q

MTF for higher levels of defocus

A

Decreases and eventually becomes negative

56
Q

What happens when the MTF is negative in defocus on the curve

A

The image pattern has reversed contrast compared with that of the object, this means that the brighter parts of the object become the darker parts of the image and vice versa

57
Q

The spatial frequency at which the modulation transfer function first goes to zero

A

Resolution limit

-grating not very visible here. No difference of black and white as it goes below zero, contrast reversed now

58
Q

Any resolution of higher frequency patterns than resolution limit is called

A

Spurious resolution

59
Q

Phenomenon in whihc the contrast of the image of a periodic grating first goes to zero or essentially zero as defocus levels is increases and then rises again at higher levels of defocus. Thus the grating may be visible at higher levels of defocus than that at which it first disappeared (went to zero)

A

Spurious resolution

Optics Block 8 (9 decks)

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