WEEK 4: VISUAL ACUITY

Question 1

When does diffraction occur?

Answer 1

Diffraction occurs when the pupil is less than 1 -2mm

Question 2

What is the rule for the angle of diffraction?

Answer 2

Angle of diffraction is directly proportional to the size of the wavelength.

Question 3

Formula for Min Angle of Resolution

Answer 3

tan θ = O/A

Question 4

Decimal Acuity to MAR

Answer 4

Dec = 1/MAR

Question 5

What unit is MAR in?

Answer 5

Degrees (remember to convert MAR to Mins of Arc)

Question 6

Disadvantages of Snellen Acuity

Answer 6

1. Different number of letters on each row
2. Uneven Increase in letter size from one line to the next
3. Ends at 6/5

Question 7

What is the Visual Acuity Rating Formula?

Answer 7

VAR = 100 - (50 x logMAR)

Question 8

Advantages and disadvantages of Snellen Chart?

Answer 8

A - Familiarity, incorporates test distance, fraction

D- Variables

Question 9

Advantages and disadvantages of LogMAR Chart?

Answer 9

A - Statiscally easy to work with, easy to score

D- not consistently used clinically

Question 10

What is a change in VA?

Answer 10

a difference of at least 0.15 logMAR

Question 11

Why is near vision important?

Answer 11

Detection and monitoring of ocular disease, presbyopia, functional and quality of life

Question 12

Types of Near Chart?

Answer 12

Snellen (40cm)
LogMAR
Jaeger Number
M point
N-point

Question 13

Quick n-point conversions

Answer 13

N1 = 0.353mm
N8 = 1M
1M letter ‘x’ = 1.45mm high
N3 = 6/6 = 0.4M

Question 14

How many minutes of arc does 1M subtend?

Answer 14

One M-Unit subtends a visual angle of 5 min of arc at 1 metre

Question 15

Difference between NVA and DVA letters?

Answer 15

DVA each letter subtends 5 mins of arc, it have 5 limbs

Question 16

What are the different types of Near VA charts?

Answer 16

Reduced snellen near charts
Bailey Lovie Word Chart (logMAR)
The Colenbrander english continous text near chards - contextual word charts

Question 17

Bailey Love NVA Chart - Advantages, Disadvantages and Range

Answer 17

A - logMAR, Words
D - 25cm only, unrelated words
Range - LogMAR 0 to 1.6 (6/240)

Question 18

Snellen NVA Chart - Advantages, Disadvantages and Range

Answer 18

A - can use matching tests with children
D - 40cm only, letters only
Range - 6/6 to 6/60

Question 19

EDTRS NVA Chart - Advantages, Disadvantages and Range

Answer 19

A - LogMAR
D - 40cm, letters only
Range: -0.3 to 1.3

Question 20

Continous text NVA Chart - Advantages, Disadvantages and Range

Answer 20

A - replicates reading, can convert N and distance to M and snellen
D - May not reflect acuity (cognitive, paramacular pathology) crowding

Question 21

Thompson Test Unit NVA Chart - Advantages, Disadvantages and Range

Answer 21

A - Smaller letters at the bottom
D - smaller letters at the bottom may be hard to see
Range 6/9.5 to 6/300