OP: Wavefront Aberrometry + Image Quality - Week 4 Flashcards
Use a formula to describe the relationship between vergence and wavefront. When does this formula apply?
[applies in air:]
Vergence [diopters] = 1/distance from the wavefront to (its) point origin [metres]
(V = 1/dwavefront)
(alternatively: vergence = 1/distance between eye and object)
How would you describe negative and positive vergence?
Negative vergence: diverging light
Positive vergence: converging light
Mathematically describe the relationship between vergence and refractive error
Refractive error is -1X the vergence of the wavefront that emerges from a point source on your retina
e.g. if vergence of at eye is +4D, refractive error is -4D (since a -4D lens is required to correct the error)
What proportion of light entering the light comes back out?
Less than 1% (about 0.05%)
How does pupil size affect blur in an emmetropic patient?
No change at all
How does pupil size affect brightness?
Bigger pupil = more bright
How does pupil size affect blur in a myopic or hyperopic patient?
Size of blur circle (de-focus) increases linearly with pupil size
How does our level of blur compare between night and day?
If we have blur already, blur gets worse at night when the pupil dilates, causing a larger blur circle
When looking at a cross (+) at very close (4cm), which line should appear clearer?
Horizontal line. Because of the pupil size. The vertical extent of your pupil has been shrunk down
Describe Fraunhofer Diffraction.
a. k.a. “Far-Field Diffraction”
- a fraunhofer diffraction pattern is formed when a parallel beam passes through an aperture far way from a screen
How does the size of the aperture affect diffraction?
Diffraction gets worse with smaller apertures
How does a square aperture compare to a rectangular aperture in terms of diffraction effects?
Square apertures has same level of vertical and horizontal diffraction. Rectangular apertures have extended diffraction in the orientation of shortest length
What type of aperture can lead to the formation of an Airy Disc?
Circular aperture
What is the formula for calculating an Airy Disc?
theta = 1.22lambda/alpha
- theta: angle subtended at the nodal point of the eye
lambda: wavelength of the light
alpha: pupil diameter
Is the airy disc formula angle in radians or degrees?
Radians. Can convert to degrees by 180/pi
How does the PSF (point-spread function) compare between an ideal and aberrated eye?
PSF in aberrated eye is wider
How does pupil size affect the ability of measured diffraction to predict our line-spread function?
As pupil size increases, ability for diffraction to predict our line-spread function gets worse
Describe strehl ratio
Compares the PSF amplitude of a diffraction-limited eye to the PSF amplitude of a real eye.
Strehl Ratio = Heye/Hdl
dl stands for diffraction limited
What is the ideal strehl ratio? Does it happen?
0.8 or 0.9. It just doesn’t happen
How does a laser inferometer interact with the retina?
Projects sinusoidal patterns onto the retina
What’s the benefit in deciding to use a line-spread function over a point-spread function?
Lines are easier for subjects to see
True or False: Laser inferometers can be used to find what the PSF looks like
True
What is the strehl ratio for a 5mm pupil that has been corrected for de-focus and astigmatism?
about 5%
That is the strehl ratio for a 1mm pupil?
Approaches 1, but the image quality is poor due to diffraction
