1.5 LOGMAR chart

Question 1

What are the advantages of the snellen chart?

Answer 1

• Quick
• Reliable
• Sensitive to ametropia
• Narrow (physical width)
• Less affected by guessing compared to grating acuity, Landolt C or illeterate E
• letters familiar to most
• enshrined in legislation- occupation requirements ( at least certain lines)

Question 2

What are the limitations of the snellen chart?

Answer 2

• unequal no.of letters per line- task is not equivalent for differing levels of VA ( task is different at different levels on the chart which isnt fair?)
• non- uniform change in letter size for successive lines (not in even steps)
• Line by line scoring system
• Fixed row spacing( letters look closer together at the top and further away at end - unequal)
• Unequal letter spacing - crowding effects change (e.g ambleyopia can be affected by crowding )

Question 3

What is the Bailey and Lovie chart?

Answer 3

Type of LogMAR chart (1976)

a new design pronciples for VA

Question 4

What had Bailey and Lovie included in the LogMAR chart?

Answer 4

• 5 letters per line( same number of letters per line)
• Letters of similar legibility chosen from BS 4274:2003- improve reliability test
• approximately equivalent task for each level of VA

Question 5

What are the advantages if the LogMAR chart?

Answer 5

• equal letter spacing
• same letters per line
• geometric progression: fixed ratio between lines
• equal line spacing

Question 7

What are the limitations of the snellen letter size progression?

Answer 7

• Not a fixed ratio between rows on a BS snellen chart

- geometric progression is the best

Question 8

What is the fixed ratio that the logMAR chart uses?

Answer 8

1.25 fixed ratio

Question 9

What is the LogMAR scoring like?

Answer 9

• 0.1 log unit per line
• Log(base 10) (MAR)

W.g 6/60=0.1 1/0.1= 10 MAR’
Log MAR = 1

• even progression
• therefore 0.02 logMAR units per letter since 5 letters per line
• size doubles of halves every 3 lines

Question 10

What is the letter spacing like in the LogMAR chart?

Answer 10

• letter spacing on each line = letter width
• spacing on the letters of the line are the same as the width of the letters
• Equivalent to 4 limb (units) wide due to the 5x4 grid letters on a bailey-lovie chart

Question 11

What is the line spacing like on Logmar

Answer 11

• Spacing between lines are the same of the height of the letters on the lower row.
• The height of the letters in the smaller row are equal to the width of the letters in the larger row.

Question 12

Where are the most obvious differences and why may the additional lines in a logMAR chart be useful?

Answer 12

The lines marked fill important gaps in the Snellen progression. This provides better sensitivity for low VA (less than 6/24) and at close to normal levels. Some practitioners argued for and used BS Snellen charts with a 6/7.5 line since the gap from 6/6 to 6/9 is large when trying to optimize corrections.