WEEK 1: SPATIAL VISION

Question 1

Why does a pinhole improve vision?

Answer 1

Because refractive error from beams of light that is not refracted correctly onto the retina is eliminated.

Question 2

What is the equation for Rayleigh’s Criteria?

Answer 2

A(radians) = 1.22 λ /d

Question 3

How do you convert radians to degrees?

Answer 3

x (180/π)

Question 4

Optics of the eye spread, point of light forms ________ distribution on the retina

Answer 4

Gaussian (Normal)

Question 5

Define minimum resolvable distance

Answer 5

To be able to resolve two stars, images on the retina have to be a certain distance apart

Question 6

Limits of Visual Acuity

Answer 6

1. Undersampling and aliasing

2. Need enough receptors to fully reconstruct Fourier wave form

Question 7

What are the four types of Visual Acuity? and what do they measure

Answer 7

1. Localisation - asking the patient to detect differences in location of features.
2. Recognition - Ability to recognise letters or shapes e.g. Snellen Chart, LogMAR chart
3. Resolution Acuity - ability to discriminate against two or more stimulus. e.g. 30” to 1’ checkerboards, falling C
4. Detection - ability to determine if the stimulus is present or not. e.g. VFT and children

Question 8

Grating Acuity

Answer 8

One cycle of grating = two detail widths of Snellen width

Question 9

High Spatial Frequency vs Luminance Contrast

Answer 9

1. At high spatial frequencies the loss of contrast in the retinal image is greater that at low frequencies reducing recognition.
2. Contrast sensitivity reaches a maximum at 4-6 cycles per degree

Question 10

What do you do if the patient can see the biggest letter on the chart?

Answer 10

1. Move the chart closer till they can see it and record new test distance
2. Move to counting fingers
3. Hand movement
4. Light perception
5. No light percerption