Optics: Retinal Image Quality Flashcards
What is fidelity?
The extent to which a copy matches the original
Why is fidelity less than or equal to one?
Because the original is the “gold standard” -> a copy will never exceed the original
Can you get a better quality image from something with infidelity?
Yes!
Can the image ever be better than the object?
No
Fidelity is not ______________ with perceived quality
Monotonic
Large aperture has a __________ depth of field
Small depth of field
What do you need to know in order to determine the quality of an image?
You need to know the purpose
Method for quantifying the image quality:
- Specify the purpose of image
- Specify a relevant visual task that demonstrates accomplishment of the purpose
- Task performance/achievement
Three factors that affect the quality of the visual system:
-optical factors
-neural factors
-psychological facotrs
Two metrics of image quality:
- Point spread function (PSF)
- Optical transfer function (OTF)
What is point spread function (PSF)?
The illuminance or luminance distribution in the image of a point source of light
What does point spread function (PSF) depend on?
-diffraction
-defocus
-aberrations
-scattered light
What is PSF called in the absence of defocus, aberrations, and scatter?
Diffraction limited PSF
What does defocus, aberrations, and scattered light do to the PSF?
Shorten and broaden the PSF
As you increase the aberration more and more, what happens?
Light shortens and broadens and shifts laterally
What other characteristics does PSF depend on (more simple than the others)?
The shape and the diameter of the aperture stop
As pupil size increases in a perfect eye, what happened to the image?
Image gets better and better
In a typical eye with diffraction effects, what happens as the aperture gets larger?
The image quality degrades significantly
About what size pupil produces the best retinal image quality?
~3mm
3 ways to get a single value to the PSF:
-rayleigh’s criterion
-the half-width
-the strehl ratio
What is rayleighs criterion?
When the first minimum of one diffraction pattern coincides with the center maxima of the other diffraction pattern
What is the air’s disk?
A uniformly-illuminated circular aperture that has a bright center region
What is the half-width?
The width of the PSF at half the peak height
What is the half-width of the diffraction limited PSF?
3.23
When does half-width of the PSF increase?
With increased aberrations
What is the strehl ratio?
A measure of the effect of aberrations in reducing the maximum value of the PSF
Possible values to the strehl intensity ratio?
Always less than to equal to 1
What happens to the value of strehl with increased aberrations?
The value is lower and the image quality is poor
Value of good strehl intensity ratio?
0.8
What is a method to measure the PSF of the human eye?
The double pass method -> light passed twice through eye optical system
Which higher order aberration can be corrected with spectacles?
Spherical aberrations -> somewhat
What is optical transfer function (OTF)?
A complex quantity that includes the modulation transfer function (MTF) and the phase transfer function (PTF)
What is resolution?
An imaging systems ability to distinguish object detail
What is contrast or modulation?
How faithfully the minimum and maximum intensity values are transferred from object plane to image plane
optical transfer function is used to look at the ___________ and _________ information
Resolution and contrast
What is Modulation transfer function (MTF)?
The measurement of the len’s ability to transfer contrast at a particular resolution from the object to the image
Value for MTF
~1
MTF if a function of _____________
Spatial resolution
What is phase transfer function (PTF)?
The shift across a range of spatial frequencies —>ONLY in off axis aberrations
What does defocus cause?
Decrease in amplitude
What does astigmatism cause?
Decrease in amplitude on both meridians
What does spherical aberration cause?
Decrease in amplitude
What does coma cause?
Decrease in amplitude and transverse shift of image position
What does distortion casue?
Transverse shift of the image position
What does field curvature cause?
Decrease in amplitude independent of the meridian
In higher levels of defocus, the MTF ___________ and eventually becomes __________
Decreases, negative
What happens when the MTF is negative?
The image pattern has reversed contrast compared with the object
What is the resolution limit?
The spatial frequency where the MTF first reaches zero
What is spurious resolution?
Resolution of higher frequency patterns
