Retinal Image Quality Flashcards
Extent to which the copy matches the original
Fidelity
What is fidelity always less than
1, the original object is gold standard
Filters and fidelity
Can make the image look better despite infidelity
Smaller pupil and image quality
Large DF
Quality cant be gerenalized, depends on what you want. There will be a clear background and image
Large pupil and image quality
Small DOF
Blurry background and clear up close object
The illuminance or luminance in the image of a point source of light
PSF
What does PSF depend upon
Diffraction, defocus, aberrations, and scattered light
In the absence of defocus, aberrations, and scatter, the PSF is called
Diffraction limited PSF
What do defocus, aberrations, and scattered light do to PSF
Shorten and broaden it
Aberrated PSF
For pupil diameter greater than 2mm, or in polychromatic light, aberrations tend to ‘spread out more’ relative to the diffraction limited PSF
What does the FORM of the PSF depend on
Shape and diameter of the aperture stop
Pupil size and best retinal image quality
3mm
PSF metrics to quantify image quality
The rayleighs criterion
The half width
The strehl ratio
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
Rayleighs criterion
The diffraction pattern formed on a distant screen resulting from a uniformly illuminated circular aperture has a bright region in the center known as
Airys disk
The central disk plus the surrounding series of concentric bright rings is called the
Airy pattern
The width of PSF at half the peak height is called
The half width, or full width and half maximum
What is the half width of the diffraction limited PSF
3.23
The half width of the PSF and aberrations
The half width of the PSF increases with the introduction and increase of aberrations
Half width and astigmatism/coma
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
This is a measure of the effect of aberrations on reducing the maximum or peak value of the PSF
Strehl ratio
What is the equation for strehl ratio
PSF of image/PSF of diffraction limited
What is the strehl ratio always equal to and why
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
What happens to the strehl ratio when the aberrations are greater
The greater the aberrations, the lower the value of the strehl intensity ratio and the poorer the image quality
What is considered a good, near to diffraction limited system strehl ratio
0.8
The luminance or illuminance distribution in the image of a line source of light
Line spread function (LSF)
Used for multifocal lenses
Measuring the PSF (LSF) of the human eye
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
What are two important factors in understanding optical transfer function (OTF)
Resolution and contrast
Systems ability to distinguish object detail
Resolution
Defined as how faithfully the minimum and maximum intensity values are transferred from object to image plane
Contrast or modulation
Do abberations affect resolution or contrast
Both
How well intensity of image is transferred to image
Modulation or contrast
Frequency and OTF (MTF)
The higher the frequency, the worse the resolution and at some point no resolution, decreased MTF (OTF)
What does the optical transfer function include
The MTF and PTF
What does OTF equal
MTF + PTF
The amplitude A’ of the image divided by the amplitude A of the object, and it is a function of spatial frequency
MTF
MTF=A’/A=1
What is MTF normalized to
Due to light aberrations, MTF is normalized to 1
What is MTF a function of
Spatial resolution, which refers to the smallest line-park the system can resolve
What is the anzi standard for IOL lens
.43
For a lens, this is the measurement of its ability to transfer contrast at a particular resolution from the object to the image
MTF, this is way to incorporate resolution and contrast into a single specification
What happens to MTF as line spacing decreases
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
What is the spatial domain
PSF
What is the object domain
Optical transfer function
Defocus effect
Decrease in amplitude
Astigmatism effect
Decrease in amplitude (defocus) on both meridians
Spherical aberrations effect
Decrease in amplitude
Coma and its effect
Decrease in amplitude no cause transverse phase shift on the image position (PTF)
Distortion and its effect
Transverse shift of the image position (PTF)
Field curvature and its effect
Decrease in amplitude (defocus) independent of meridian
Those aberrations that produce a transverse shift produce what?
Coma and distortion: an effective phase shift in the image
The phase shift across a range of spatial frequencies is called
The phase transfer function
Phase change in on-axis
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
Where in the eye is there no change in phase and the PTF=0
Central vision
When does PTF become significant
If off axis
- periphery
- coma, produces TCA, phase shift increase
MTF for higher levels of defocus
Decreases and eventually becomes negative
What happens when the MTF is negative in defocus on the curve
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
The spatial frequency at which the modulation transfer function first goes to zero
Resolution limit
-grating not very visible here. No difference of black and white as it goes below zero, contrast reversed now
Any resolution of higher frequency patterns than resolution limit is called
Spurious resolution
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)
Spurious resolution
