Development and adoption of Visual Acuity charts based on the “logMAR” principles

Duration: 
30 mins

Development and adoption of Visual Acuity charts based on the “logMAR” principles

APPLICATION OF EVIDENCE BASED PRACTICE

Example of the development and adoption of Visual Acuity charts based on the “logMAR” principles

REVIEW: Development of visual acuity charts.

The introduction to the document:

http://www.isd.mel.nist.gov/US&R_Robot_Standards/Visual_Acuity_Standards_1.pdf

gives a good concise history of how different visual acuity test charts were developed.


1862 Snellen chart  used 9 letters;  C. D, E, F, L, O, P, T, Z.

Snellen developed his own “font” there letters were on a grid 5x5 square and had very obvious serifs.

As you know the denominator is the distance from which the stroke with of the letter subtends 1’ of arc (visual angle).

The progression of sizes is somewhat arbitrary and is not a geometric progression.

Since those times research into “contour interaction” and crowding has shown that the visibility of an object is influenced by neighbouring objects in a way that depends on the proximity of the neighbour.

You can see that in the Snellen chart the larger letters have smaller spacing to letters above and below.  For a valid measure of visual acuity contour interaction needs to be controlled for.

Research in the 1950’s showed that the visibility of characters on the chart was not uniform

 

1959 Louise Sloan (an ophthalmologist working in low vision) defined a variation in the “font” used for visual acuity chart optotypes – still on a 5x5 grid but without serifs.  She selected 10 letters for use on her charts and proposed that all 10 letters should be presented with standard spacing on each row (or across two rows).

1976 Ian Bailey and Jan Lovie proposed a more manageable chart using non serif letters on a 5x4 grid.  They chose the 10 letters from the 1953 British Standard for letter charts.  They laid out the chart with controlled spacing between letters and between lines, and chose a logarithmic progression of sizes with an increment of 0.1 log minutes of arc between lines.

1979 ETDRS charts developed using the layout principles of the Bailey-Lovie chart but with Sloan letters.  These charts used in research with scoring procedures that include letter by letter counting.  Allow even minor changes in vision to be detected free of chart artefacts

 

1984  The GOLD standard design for VA charts was set when the International Council of Ophthalmology (ICO) prescribed the design requirements for modern visual acuity test charts http://www.icoph.org/dynamic/attachments/resources/icovisualacuity1984.pdf

TODAY  “logMAR” principles are widely adopted worldwide.

 

 

 

 

SUMMARY: Two reasons why the Snellen Chart and charts based on early standards such as the 1953 British standard are out-dated.

Reason 1

The spacing of letters on your chart design is quite wide when the letters are small (they are well spaced and as a consequence are easier to read).  The letters are very crowded when the letters are large (this makes them harder to identify).  This chart design provides a vastly different task to people with different levels of vision.  The task is very much easier for people with good vision and much more difficult for people with poorer vision.

 

Reason 2

A person with good vision will be presented with many more letters than someone with poorer defocused vision.

This makes the statistical analysis of data gathered from this chart virtually impossible to undertake.

A person with poor vision gets only 2 or three chances to get the letters right, whereas a person with better vision gets many more chances.  In any comparison of levels of vision, the number of letters presented must be taken into account and this is difficult when the number is variable.

 

For these two reasons it is recommended that any new chart that you design conforms to the principles in the 1984 ICO document.

In brief any new acuity chart design should have:

         The same number of letters/symbols in each line

         The same relative spacing (in terms of letter size on a row) between letters and between rows

         A geometric progression of letter sizes

         A size range that covers the range of visual performance expected from the target population.

As most young people can see very well, the minimum size of letter should be 3m when viewing is from 6m (or 1.5m when viewing is from 3m.  Letter detail will subtend 30 seconds of arc at the eye (0.5 minutes of arc or 1/120 of a degree). 

The largest size should be set by the poorest level of vision expected in the target population.

The letters used in the chart should be approximately equally legible.  (There is debate in the literature about which letter set this is).

See the attached word document for an easily downloadable copy of the information above.

Resource contributed by: A/Professor Robert Jacobs
Institution: Department of Optometry and Vision Science, University of Auckland.
Web-site: http://www.optometry.auckland.ac.nz/uoa/robert-jacobs
E-mail: r.jacobs [at] auckland.ac.nz
Notes: A/Prof Jacobs is happy for this resource to be used with attribution. If you have questions about the resource, please contact A/Prof Jacobs.
Date uploaded:  19/2/2013
Latest review:  19/2/2013
Reviewed by:  K. Challinor
Next review due:  December 2014


 

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