University of Edinburgh

Functional Assessment of Vision

Presented on Friday 5 May 2006

Lesley Reid
Teacher of the Visually Impaired, Inverclyde

Assessment of Visual Acuity: Tests and Procedures

Visual Acuity Definition

"the ability to discriminate high contrast fine detail at a distance; the sharpness and clarity of vision"

(Mason, 1997)

Snellen Test Type

  • The most commonly used test to assess distance VA.
  • Dutch ophthalmologist Hermann Snellen (1834-1908)
  • One single letter (optotype) at top of chart.
  • Rows of letters (optotypes) decreasing in size towards bottom of chart.

Example of a Snellen Test Type

snellen chart

How to use the Snellen chart

  • Know your testing distance.
  • Note if spectacle correction worn.
  • If appropriate, occlude one eye.
  • Ask child to read down chart as far as they can.
  • Check child is not leaning forward to see better (reducing testing distance) or 'peeking'.

If necessary, point to each letter. Ensure you finger is straight and directly below the letter you are asking them to read.

When child is reaching their limit encourage them to attempt letters on the end of the next row. These are often easier to see than letters in the centre of a row due to the crowding phenomenon.

Recording Snellen Results

  • Results are recorded as a fraction.
  • Top number equates to the distance (in metres) at which the test chart was presented (usually 6m),
  • Bottom number identifies the position on the chart of the smallest line read by the 'patient'.
  • Eg; 6/60 means the subject can only see the top letter when viewed at 6m.

Can record parts of a line read by using e.g. "6/12 part" or "6/18+1" or "6/6-3"

(Remember to record whether or not glasses worn during test.)

Snellen recordings

6/60 Top line (1 letter)
6/36 2 letters
6/24 3 letters
6/6 7th line (approx7 letters)
6/5 (not included on all test charts)

Development of Visual Acuity

  • Newborn 6/240
  • 6 months 6/60 - 6/24
  • 18 months 6/9
  • 4 years 6/6 (linear)

Disadvantages of Snellen Test Type

  • Letters not of equally legible eg; O and E
  • Non-uniform progression of letter sizes
  • Unequal number of letters on each line
  • Irregular spacing between letters and lines
  • Inadequate scoring (6/9+2, etc)

RESULT= Inaccurate results, especially for those with low vision.

The LogMAR alternative to Snellen

Bailey & Lovie (1980)
We have designed ... a series of near vision charts in which the typeface, size progression, size range, number of words per row and spacings were chosen in an endeavour to achieve a standardization of the test task


logmar chart

Other researchers suggested a logarithmic progression in size of letters on test chart gives most accurate VA measurement

MAR=Minimum Angle of Resolution
-the angle subtended within the eye by a letter

How does LogMAR relate to Snellen?

  • Letter E on 6/6 line subtends an angle of 5 minutes of arc; each limb subtends 1 second of arc
  • Minimum Angle of Resolution for a 6/6 letter is 1 second of arc.
    -the log of the MAR for a 6/6 size letter is 0

Scoring LogMAR

  • Each letter has a score value of 0.02 log units; 5 letters per line
  • Each line represents a change of 0.1 log units

Advantages of LogMAR

  • Equal number of letter per line
  • Regular spacing between lines and letters
  • Uniform progression in letter size
  • Final score based precisely on total of all letters read
  • Finer grading scale allows greater accuracy and improved test/retest reliability

LogMAR / Snellen equivalent results

LogMAR Snellen Decimal
0 6/6 1.00
0.18 6/9 0.67
0.30 6/12 0.50
0.48 6/18 0.33
0.60 6/24 0.25
0.78 6/36 0.17
1.00 6/60 0.10

Other LogMAR VA tests

  • LogMAR Crowded Test (at 3m)
  • Kays Crowded Test

NB Crowded tests more accurate than uncrowded

Final thoughts

Use logMAR tests when possible, they give more accurate results.
The more you practise the more confident you will become.
When you are confident your children will relax and achieve their best results.