LVA - Acuity Charts

Snellen

Advantages

• Well known, commonly used standard
• Very sensitive to blur and ref. error
• Good size, easy to move closer to px
• Has O and H for x-cyl

Disadvantages

• Unequal numbers of letters on each line - crowding not constant
• No relationship betw line size and number of letters
• Poor control of contour interaction
• Designed to measure normal acuity
• Scale intervals change at non-standard distances

Snellen @ Reduced Distance

• 6m letter @ 6m has MAR of 1'
• Px will be able to see a letter half the size when half the distance away

Bailey-Lovie LogMAR Chart

Adv

• Size/letter spacing equivalent throughout chart (5/row)
• 1.25x progression (0.1 log) each line
• All letters equally legible
• Constant crowding for all VA levels - easy to use at different test distances
• Final score takes into account all letter that have been read successfully (see later)

Disad

• not used as routine measure of VA
• Scoring/conversion not as easy
• No O for x-cyl
• Bit big and therefore hard to illuminate

Scoring

• The size diff between 2 lines on a logMAR chart is 0.1 log unit so 10 to the 0.1 = 1.25 so the size diff is equal to factor 1.25. 3 lines = approx doubling in VA
  
• Log score decreases with improving VA
• Each line = 0.1 log units and each letter is therefore 0.02
• Doesn't use viewing dist as part of notation. What you do is add 0.3 to the VA score every time the viewing distance is halved.

EG Px reads 0.8 line and 2 letters on line below so logMAR VA = 0.76
then Px reads 0.5 line and 2 letter on line below so logMAR = 0.5-0.04+0.3 = 0.76

• Reducing viewing dist by 1.25 requires a correction factor of 0.1
• so 6.0m to 4.8m = 0.1, 6.0m to 3.84m = 0.2 etc

Other charts include sheridan gardener, glasgow acuity and Kay's pics.