Optics: Retinal Image Quality

Question 1

What is fidelity?

Answer 1

The extent to which a copy matches the original

Question 2

Why is fidelity less than or equal to one?

Answer 2

Because the original is the “gold standard” -> a copy will never exceed the original

Question 3

Can you get a better quality image from something with infidelity?

Answer 3

Yes!

Question 4

Can the image ever be better than the object?

Answer 4

No

Question 5

Fidelity is not ______________ with perceived quality

Answer 5

Monotonic

Question 6

Large aperture has a __________ depth of field

Answer 6

Small depth of field

Question 7

What do you need to know in order to determine the quality of an image?

Answer 7

You need to know the purpose

Question 8

Method for quantifying the image quality:

Answer 8

1. Specify the purpose of image
2. Specify a relevant visual task that demonstrates accomplishment of the purpose
3. Task performance/achievement

Question 9

Three factors that affect the quality of the visual system:

Answer 9

-optical factors
-neural factors
-psychological facotrs

Question 10

Two metrics of image quality:

Answer 10

1. Point spread function (PSF)
2. Optical transfer function (OTF)

Question 11

What is point spread function (PSF)?

Answer 11

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

Question 12

What does point spread function (PSF) depend on?

Answer 12

-diffraction
-defocus
-aberrations
-scattered light

Question 13

What is PSF called in the absence of defocus, aberrations, and scatter?

Answer 13

Diffraction limited PSF

Question 14

What does defocus, aberrations, and scattered light do to the PSF?

Answer 14

Shorten and broaden the PSF

Question 15

As you increase the aberration more and more, what happens?

Answer 15

Light shortens and broadens and shifts laterally

Question 16

What other characteristics does PSF depend on (more simple than the others)?

Answer 16

The shape and the diameter of the aperture stop

Question 17

As pupil size increases in a perfect eye, what happened to the image?

Answer 17

Image gets better and better

Question 18

In a typical eye with diffraction effects, what happens as the aperture gets larger?

Answer 18

The image quality degrades significantly

Question 19

About what size pupil produces the best retinal image quality?

Answer 19

~3mm

Question 20

3 ways to get a single value to the PSF:

Answer 20

-rayleigh’s criterion
-the half-width
-the strehl ratio

Question 21

What is rayleighs criterion?

Answer 21

When the first minimum of one diffraction pattern coincides with the center maxima of the other diffraction pattern

Question 22

What is the air’s disk?

Answer 22

A uniformly-illuminated circular aperture that has a bright center region

Question 23

What is the half-width?

Answer 23

The width of the PSF at half the peak height

Question 24

What is the half-width of the diffraction limited PSF?

Answer 24

3.23

Question 25

When does half-width of the PSF increase?

Answer 25

With increased aberrations

Question 26

What is the strehl ratio?

Answer 26

A measure of the effect of aberrations in reducing the maximum value of the PSF

Question 27

Possible values to the strehl intensity ratio?

Answer 27

Always less than to equal to 1

Question 28

What happens to the value of strehl with increased aberrations?

Answer 28

The value is lower and the image quality is poor

Question 29

Value of good strehl intensity ratio?

Answer 29

0.8

Question 30

What is a method to measure the PSF of the human eye?

Answer 30

The double pass method -> light passed twice through eye optical system

Question 31

Which higher order aberration can be corrected with spectacles?

Answer 31

Spherical aberrations -> somewhat

Question 32

What is optical transfer function (OTF)?

Answer 32

A complex quantity that includes the modulation transfer function (MTF) and the phase transfer function (PTF)

Question 33

What is resolution?

Answer 33

An imaging systems ability to distinguish object detail

Question 34

What is contrast or modulation?

Answer 34

How faithfully the minimum and maximum intensity values are transferred from object plane to image plane

Question 35

optical transfer function is used to look at the ___________ and _________ information

Answer 35

Resolution and contrast

Question 36

What is Modulation transfer function (MTF)?

Answer 36

The measurement of the len’s ability to transfer contrast at a particular resolution from the object to the image

Question 37

Value for MTF

Answer 37

~1

Question 38

MTF if a function of _____________

Answer 38

Spatial resolution

Question 39

What is phase transfer function (PTF)?

Answer 39

The shift across a range of spatial frequencies —>ONLY in off axis aberrations

Question 40

What does defocus cause?

Answer 40

Decrease in amplitude

Question 41

What does astigmatism cause?

Answer 41

Decrease in amplitude on both meridians

Question 42

What does spherical aberration cause?

Answer 42

Decrease in amplitude

Question 43

What does coma cause?

Answer 43

Decrease in amplitude and transverse shift of image position

Question 44

What does distortion casue?

Answer 44

Transverse shift of the image position

Question 45

What does field curvature cause?

Answer 45

Decrease in amplitude independent of the meridian

Question 46

In higher levels of defocus, the MTF ___________ and eventually becomes __________

Answer 46

Decreases, negative

Question 47

What happens when the MTF is negative?

Answer 47

The image pattern has reversed contrast compared with the object

Question 48

What is the resolution limit?

Answer 48

The spatial frequency where the MTF first reaches zero

Question 49

What is spurious resolution?

Answer 49

Resolution of higher frequency patterns