Midterm 2: Visual Acuity

Question 1

What diffraction pattern does a point source make after passing through a lens? What does this tell you?

Answer 1

Airy Disk.

Tells us that diffraction degrades the image quality in our eye.

Question 2

Describe the point spread function

Answer 2

The point-spread function of any optical system is a mathematical description of the distribution of light across the image of a “point” of light.

Question 3

The point-spread function of a perfect optical system WITHOUT diffraction is the “_____________”. What is its shape?

Answer 3

Impulse function. A line straight up

Question 4

What are the x and y components on the point spread function

Answer 4

X: illuminance
Y: position on retina

Question 5

Equation for angle of airy disk

Answer 5

Angle = 1.22 x wavelength / pupil diameter

Question 6

As wavelength increases, will the airy disk angle increase or decrease?

Answer 6

It will increase since wavelength is a numerator in the equation

Question 7

What is the Rayleigh resolution limit? What acuity test is this associated with?

Answer 7

Allows person to see if there are two points or not when doing resolution (separation) acuity test.

the diffraction maximum of one image coincides with the first minimum of the adjacent image. The person will then be able to tell there are two points.

Question 8

What does good recognition equity require? What is the normal threshold?

Answer 8

• luminance differentiation (low contrast threshold)
• relative spatial localization (hyperacuity)
• prior experience of the objects

Normally, 1 arc minute threshold

Question 9

dimensions of sloan letter for acuity

Answer 9

5x5

Question 10

dimensions of British letter for acuity

Answer 10

4x5

Question 11

Visual acuity is the _________ of the smallest angle (denoted in minutes of arc) that a person can correctly identify.

Answer 11

reciprocal

Question 12

LARGER LOG10MAR = _________

Answer 12

worse acuity

Question 13

Describe amblyopic eye acuity

Answer 13

flatter psychometric function because other letters crown the letter of interest.

Question 14

Forced choice paradigm examples for acuity

Answer 14

tumbling E, landolt C’s, Konig bars

Question 15

True or False:

A steeper slope for acuity gives a more narrow range of error

Answer 15

True

Question 16

Describe the Bailey-Lovie chart

Answer 16

an example of charts that maintain proportional spacing between adjacent letters and lines.

• same #letters/row
• -1 letter width space
 between letters
• -Between row spacing = height
 of letters in smaller row
• -size progression between rows
 = 0.1 log unit.

Question 17

Advantages of logMAR scale

Answer 17

• Measurement error ­ constant for all acuities and testing conditions
• Equal logMAR steps represent approximately equal functional changes in acuity
• LogMAR charts “scale” with viewing distance
• Reduces likelihood of “inappropriate” acuity comparisons (e.g “twice as good”, “changed by an equal number of Snellen points,” etc.)