Retinal Image Quality II Flashcards
In order to resolve fine details of the pattern imaged on the retina
The adjacent ‘receptor units’ must be sufficiently close together to correctly interpret the pattern
Pattern and the receptors are close enough to correctly interpret the pattern
Veridical
The visual system is just bale to resolve the pattern
Nyquist limit
The sampling rate of the receptor units are inadequate to correctly interpret the pattern. The visual system may perceive the under sampled pattern as much lower spatial frequency and different orientation
Aliasing
When the visual system correctly interprets the spatial frequency and orientation of pattern
Veridical system
The spatial frequency at which finest light pattern that can be correctly resolved by the retina
Nyquist limit
What is the nyquist limit in humans
60c/deg
The spatial frequency at whihc the visual system cannot distinguish between light patterns
Aliasing
Photoreceptor spacing limits resolution, true or false
False, loses resolution due to there being less ganglion cells for each photoreceptor
What is visual resolution limited by
The spacing of (beta) ganglion cells of the retina
Why is it the ganglion cells that limit the visual resolution
At fovea, there is a one to one correspondence between cones and ganglion cells.
Why is it that cone spacing is not a the limiting factor for visual resolution
Peripheral cones are spaced too closely to be a limiting factor for visual resolution
What is the correspondeance of ganglion to PR cells at 0 degrees (fovea)
1:3
Where is the correspondence 1:1
5-10 degrees outside the retina
Why does the resolution get poorer out in the periphery of the retina
Because fewer ganglion cells
Resolution/contrast cannot be better than 60cyl/degree, true or false
True-if optics + neural blur is taken into consideration
False-if only considering neural blur
What is the difference in contrast sensitivity between normal eye (optics + neural blur) and an eye that has no optics
10x difference
What could the nyquist limit be if we could bypass the optics
With laser interometry, the contrast sensitivity of the human eye could be increased from spatial frequency of 60cycl/deg to 120 c/deg
What limits the ability to resolve higher spatial frequency ?
Optics
When frequency increases, what happens to grating spacing
Decreases
What happens to MTF with increased frequency
Decreased
What is the difference between monofocal and multifocal lenses in terms of MTF
Monofocal is close to ideal at 0.75 and multifocal is about 0.3, so multifocal have poorer image quality
Resolves both SPATIAL FREQUCNY and ORIENTATION
Veridical
How do we get to 120 nyquist limit
Get rid of optics
What are the two major spatial tasks performed by human system
Resolution and detection
What do cones limit
Detection
Ability of the patient to detect that a spot of light is present. Details not impiotant
Detection
Half period of the finest detectable grating
Minimum angle of detection (MAD)
The detection in the central and peripheral vision is limited by the size of the
Cone photoreceptors
What is resolution limited by
Ganglion cells
Resolution and detection at the fovea
Patterns may be reliable resolved anad detected
Resolution and detection in the periphery
Can detect, cannot resolve
Difference between ability to detect and resolve in the periphery
Aliasing
Where is detection and resolution pretty much matched
Fovea
What are the 3 components of the recognition pyramid
- Detection
- Resolve elements
- Identification
Discriminating optical and neural blur
May help in distinguishing between diseases and in establishing a course of treatment
Central vision retinal image
Considerable variation
When is there an over estimation of retinal image quality
When scattering effect being ignored
The most important optical defect affecting retinal image quality is
Defocus
What are the main things that affect optic blur
Diffraction
Aberration
Scatter
Defocus
Why is it important to correct even the smallest refractive error
To prevent incorrect attribution of losses to retinal/neural pathological causes
TCA causes _____ much loss of resolution for 3mm displacement of small aperture
3 times
What is the anchoring role
Stiles crawford effect
What si the stiles crawford effect
Max intensity of light in the center of the pupil compared to the periphery, this helps reduce aberrations entering the eye
Pupil decentration on retinal IQ
Decentration of the eyes pupil induces additional optical aberrations such as transverse chromatic aberration and coma, which decrease spatial visual performance
This is either temporally or nasally.
What is peripheral vision important for
Detection, not resolution
Improving peripheral vision
Has little improvement in resolution but marked improvement in detection occurs
