Optical Abberations Flashcards
Higher order aberrations
What’s left after you take away the defocus and astigmatism
What can reduce higher order aberrations?
Reducing pupil size
Plane wavefront
Parallel beam
Spherical wavefront
Converging beam
Aberrated beam
Irregular wavefront
Aberrations
Failure for the light rays passing through an optical system to converge at one point. Possibly due to defects or limitations of the system
Lower order aberrations
Piston
Title
Astigmatism
defocus
Usually can always be corrected with glasses
Higher order aberrations
Trefoil Coma Spherical Tetrafoil Pentafoil Hexafoil
Zernike polynomials
- set of basic shapes that are sued to fit the wavefront
- analogous to the parabolic X^2 shape that can be use to fit 2D data
Orthogonal
Terms are not similar in any way, so the weighting of one term does not depend on whether or not other terms are being fit also
Normalized
The RMS wave aberration can be simply calculated as the vector of all or a subset of coefficients
Efficient
Zernike shapes are very similar to typical aberrations found in the eye
Picturing astigmatism
- 2nd order
- 2 different focal planes
Conoid of sturm
Shows the two planes where there is perfect focus and the half way point of those as well
Sturms interval
The distance between the two planes of perfect focus in the conoid of sturm
Circle of least confusion
The area right between the two planes where there is perfect focus
Aberrations in imaging media
Piston Tilt Defocus Spherical aberration Coma Astigmatism Field curvature Image distortion
What is the only aberration that occurs for both on-axis and off-axis object points?
Spherical aberration
-the thickens of the lens is different between the top and bottom and center, there will be spherical aberration
Positive and negative spherical aberration
Shows peripheral rays intersecting closer to the lens
What kind of eclipse does LASIK create?
Oblate
Coma aberration
Comet shaped pattern caused by off-axis point source passing through a spherical system
What eye disorder often has a lot of coma aberration?
Keratoconcus
What kind of aberration do they often find in pilots?
Vertical coma
Oblique, marginal, or radial astigmatism
- light coming from off axis (aberration, not talking about Rx)
- passing through cylindrical correction
- bending sunglasses around face induces cylinder, as does pantoscopic tilt
Field curvature/distortion
Focused on a curved surface
-petzval surface
Distortion shapes
Barrel and pincushion
-look exactly like what you would think
Resolution
The point where you can resolve two points
Unresolved
Two dots that look like one
Resolved
Two dots that can be differentiated as two dots
Rayleigh criterion
The point between resolved and unresolved.
-its the very beginning of being able to see two different dots, a threshold sorta
What is considered to be about the perfect pupil size?
2mm
Does resolution get worse to better when pupil gets smaller?
It gets better to a certain point and then it gets worse
What kind of limit is there when the pupil gets smaller?
Diffraction
How many arc minutes in a 20/20 E?
5 minutes
How many cycles in a 20/20 E?
30 cycles/degree
Airy disk and airy pattern
Descriptions of the best focused spot of light that a perfect lens with a circular aperture can make, limited by the diffraction of light
-as pupil gets smaller, you get this
What is one limitation of the image quality of an optical system?
Airy disk
If you want better resolution what type of lens should you use?
Sometimes, the bigger the lens, the better the resolution
What does diffraction do to light?
Causes it to bend perpendicular to the direction of the diffracting edge
What does interference do to light?
Causes the diffracted light to have peaks and valleys
What does a modulation transfer function compare?
Contrast and resolution
High resolution
How small you can see
High contrast
Color
With 20/20 vision the contrast
Will be higher (tighter), high frequency
Modulation transfer function
Indicates the ability of an optical system to reproduce (transfer) various levels of detail (spatial frequencies) from the object to the image
What are the units of a modulation transfer function?
Ratio of image contrast over the object contrast as a function of spatial frequency
What is the optical contribution to the contrast sensitivity function?
Modulation transfer function
How many cylces per degree are in a 20/20 E?
30
How many cylces per degree are in a 20/10 E?
60 c/d
More than 60 c/d
Not really necessary
cut off frequency
Increases by ~30c/d for each mm increase in pupil size
What does adding even a little bit of defocus to a “perfect eye” do?
Drastically changes vision, but helps with depth of field
What does more cycles/degree mean?
Better vision
1 cycle =
2 degrees
1 degree equals
60 minutes
And
30 cycles
the PSF is the
Fourier transform (FT) of the pupil function
The MTF is the
Amplitude of component of the FT of the PSF
The PTF is the phase component of what
Of the FT of the PSF
What does coma wave front aberration and point spread function look like?
Look on life 70
Shack-Hartmann wavefront sensor
Can tell the shape of the wavefront
Fitting the wavefront in the shark hartmann wavefront sensor
- the local slope of the wavefront I determined at each lenslet location
- corresponding wavefront is determined by a least squares fitting of the slopes to the derivative of a polynomial selected to fit the wavefront
- zernike polynomial is the most commonly used
What is the most commonly used polynomial for shack hartman wavefront sensor?
Zernike polynomial
What is determined at each lenslet location in the shake hartman wavefront sensor?
Local slope
What is the corresponding wavefront determined by in the shark hartman wavefront sensor?
By a least squares fitting of the slopes to the derivative of a polynomial selected to fit a wavefront
What kind of pattern do people see at night after RK?
Star
What kind of pattern does a LASIK patient see at night?
Ring
What can cause glares, haloes, scattering?
Dry eye, cataracts
iris tears can cause what?
Scattering
Age and glare
Increases with age (cataracts)
What are the metrics to define image quality
- wavefronts
- zernike terms
- root mean square
Root mean square wave aberration
Square each number, add up the squared number, and then take the square root of that number
Strehl ratio if pupil is small
Would be close to 1, could be 0.5 for large
What is the normal strehl ratio?
5%
What is strehl ratio
Point spread function
What will the area under the MTF if you have worse vision?
Smaller
MTF
Spatial frequency on X
Contrast on Y
Convolution
Adding a PSF on every corner of the E to see what someone with that aberration would be seeing
Strehl ratios for a 5mm pupil
Are about 5% for a 5mm pupil that has been corrected for defocus and astigmatism
Strehl ratios for a small pupil
Approach 1, but the image quality is poor due to diffraction
When do aberrations diminish?
Like most optical systems, they diminish as the aperture is reduced
Overall the eyes high order abberations ____with pupil size.
Reduce
Dynamic changes in the wave abberations are caused by
- accommodation
- eye movement
- eye translation
- tear film
Change in abberations with age
Increase
What can increase the depth of field?
Negative spherical aberration
Custom IOL for abberations
Go in after cataract surgery and use light to adjust wavefront correction after the surgery
Presbyopia relief
Bifocal lenses do not always provide bifocal vision
Emmetropization
Born hyperopic and shift towards emmetropization
What kind of abberations decrease with age?
Higher order
Adaptive optics
Deformable mirror takes shape of wavefront so that it refracts straight
-measure the wavefront
What does adaptive optics do to the wave aberration?
Flattens it
Real time adaptive optics
Tear film can change it, needs to be real time
How well can someone see with adaptive optics?
Can get someone to see 20/7
Adaptive optics and retinal image quality
It improves retinal image quality
Basic science imaging applications of adaptive optics
- reveal properties of single cells in living eyes
- correlate properties of cellular structure in living eyes with visual performance
- nonlinear imaging of structure and function
Pre-clinical applications of adaptive optics
- facilitate longitudinal tests on animal models
- test outcomes of drugs and treatments for eye disease
- correlate phenotype with genotype in animal models of eye disease
Clinical applications of AO
- provide early dx for retinal or other systemic disease
- better understand the etiology of retinal disease for which little is known
- discover more sensitive biomarkers for retinal disease
- track progression of eye disease
- measure response at a cellular levels to therapies that treat disease
- preselect patietns or diseases that may benefit best from therapies or treatments
Tracking cones
Use AO to track a cone over time to see how well treatment is working
-can also monitor cone density
Functional imaging applications of AO
- facilitate better relationships between structure and function
- reveal properties of cell networks in living eyes
Vision applications of AO
- pre-test the benefits of aberration correction on vision
- develop optical aberration profiles for long depth of focus
- test possible signals that drive accommodation and/or eye growth
- reveal the optical retinal and neural limits of human vision
What is Dr. H’s study?
Optical aberration profiles for long depth of focus
Light delivery applications of AO
- track and stimulate single cells or networks of cells for electrophysiology expts
- microperimetry
- targeted laser treatment
- track eye movement responses to stimulation
- study role of eye movements for vision
