OP: Wavefront Aberrometry + Image Quality - Week 4

Question 1

Use a formula to describe the relationship between vergence and wavefront. When does this formula apply?

Answer 1

[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)

Question 2

How would you describe negative and positive vergence?

Answer 2

Negative vergence: diverging light

Positive vergence: converging light

Question 3

Mathematically describe the relationship between vergence and refractive error

Answer 3

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)

Question 4

What proportion of light entering the light comes back out?

Answer 4

Less than 1% (about 0.05%)

Question 5

How does pupil size affect blur in an emmetropic patient?

Answer 5

No change at all

Question 6

How does pupil size affect brightness?

Answer 6

Bigger pupil = more bright

Question 7

How does pupil size affect blur in a myopic or hyperopic patient?

Answer 7

Size of blur circle (de-focus) increases linearly with pupil size

Question 8

How does our level of blur compare between night and day?

Answer 8

If we have blur already, blur gets worse at night when the pupil dilates, causing a larger blur circle

Question 9

When looking at a cross (+) at very close (4cm), which line should appear clearer?

Answer 9

Horizontal line. Because of the pupil size. The vertical extent of your pupil has been shrunk down

Question 10

Describe Fraunhofer Diffraction.

Answer 10

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

Question 11

How does the size of the aperture affect diffraction?

Answer 11

Diffraction gets worse with smaller apertures

Question 12

How does a square aperture compare to a rectangular aperture in terms of diffraction effects?

Answer 12

Square apertures has same level of vertical and horizontal diffraction. Rectangular apertures have extended diffraction in the orientation of shortest length

Question 13

What type of aperture can lead to the formation of an Airy Disc?

Answer 13

Circular aperture

Question 14

What is the formula for calculating an Airy Disc?

Answer 14

theta = 1.22lambda/alpha

• theta: angle subtended at the nodal point of the eye
  lambda: wavelength of the light
  alpha: pupil diameter

Question 15

Is the airy disc formula angle in radians or degrees?

Answer 15

Radians. Can convert to degrees by 180/pi

Question 16

How does the PSF (point-spread function) compare between an ideal and aberrated eye?

Answer 16

PSF in aberrated eye is wider

Question 17

How does pupil size affect the ability of measured diffraction to predict our line-spread function?

Answer 17

As pupil size increases, ability for diffraction to predict our line-spread function gets worse

Question 18

Describe strehl ratio

Answer 18

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

Question 19

What is the ideal strehl ratio? Does it happen?

Answer 19

0.8 or 0.9. It just doesn’t happen

Question 20

How does a laser inferometer interact with the retina?

Answer 20

Projects sinusoidal patterns onto the retina

Question 21

What’s the benefit in deciding to use a line-spread function over a point-spread function?

Answer 21

Lines are easier for subjects to see

Question 22

True or False: Laser inferometers can be used to find what the PSF looks like

Answer 22

True

Question 23

What is the strehl ratio for a 5mm pupil that has been corrected for de-focus and astigmatism?

Answer 23

about 5%

Question 24

That is the strehl ratio for a 1mm pupil?

Answer 24

Approaches 1, but the image quality is poor due to diffraction