RETURN TO ' DISTANCE TESTS' PRICELIST
DEFINITION and SUGGESTED PROTOCOL
LogMAR stands for Minimum Angle of Resolution
The charts were designed by Bailie and Lovie and were used as a basis for the ETDRS study.
The smaller the letters on the chart, and the further away they are, the smaller will be the angle subtended to the eye by the letters and therefore the smaller the value of the LogMAR score associated with it.
The charts supplied under CAT. Nos. 397 are designed to be used at 4 metres. At this distance the top lines will give a score of 1.0. Each line below will give a score 0.1 less than the line above. Owing to the design of the chart, using a balanced distribution of Sloan or Snellen letters which are graded in difficulty, each of the five letters, in each line, count for a score of 0.1/5 = 0.02.
Therefore if a patient reads the 0.4 line in its entirety they will have a score of 0.4. If they read the 0.4 line plus three letters of the 0.3 line, they will have a score of 0.34, which results from the five letters of line 0.4 minus the score for each letter read from the 0.3 line.
0.4 - (0.02x3) = 0.34
Suggested steps to testing with the LogMAR chart
1. Set the patient/chart distance to 4 metres.
2. Slide the appropriate chart into the cabinet.
3. Cover one eye. If the patient cannot read any letters, move the chart to a distance of 1 metre from the patient and add 0.6 to the LogMAR score for the for each line. Repeat for the second eye.
4. If the patient is still unable to read any letters proceed to another means of assessment of their acuity.
Why LogMar (logarithm of the minimum angle of resolution) rather than the traditional Snellen notation? The use of LogMar allows analysis of visual acuity scores more effectively and comparisons of results more precisely. It offers this because the equal linear steps of the LogMar scale represent equal ratios in the standard size sequence.
How is the above achieved?
The charts have been designed using high contrast lettering on washable polystyrene and based on the ’Bailey and Lovie’ work and incorporating recommendations of the Committee on Vision of the American National Academy of Sciences– National Research Council. The design of these charts has been to get over the deficiencies of the acuity charts hitherto employed; deficiencies that need no amplification here.
Ten Sloan letters have been chosen for their equality of readability; the letters S,D,K.H,N,O,C,V,R and Z. All of these letters have been allocated a difficulty level. These have been
arranged on the charts in equal lines of 5. There are 252 possible line combinations using these letters Each of the chosen lines, for the Precision Vision charts, have been carefully arranged to have a mixture of letters such that the lines contain a near equal score of difficulty. 28 lines giving intermediate and near equal difficulty have been used in charts 1 and 2 .
The progression of letter height is that any line is 1.2589 times greater than the line below. This multiplier is the root of ten or 0.1 log unit. Therefore, a three line worsening of visual acuity is equal to a doubling of the visual angle regardless of the initial acuity.
The charts are designed for use at 4 metres which helps with smaller examining rooms and, of course, the size of the chart.
Using the 3 standard charts. A complete refraction can be first carried out with chart ‘R’ first ( at this stage the patient must not be able to see charts 1 and 2). To measure acuity in the right eye the left eye is covered and the subject is asked to read down chart 1 slowly letter by letter. When a letter is read
Correctly the examiner indicates this on the score sheet( this would be identical to the chart layout). Only one reading of a given letter is allowed. When the subject has difficulty, he or she is encouraged to guess. With poor acuities the chart distance can be reduced to 1 metre allowing acuities as low as 1/40 ( 6/240) to be measured. The left eye can then be tested in the same way using chart 2. Visual acuity, in each eye can be recorded as explained on the previous page.
James Collins
based on information extracted from:-
The American Journal of Ophthalmology Vol. 94, No. 1 July 1982
and Vol. 116, No. 6 December 1993