The Single Symbol Book (#250600) is used if the child/adult cannot perform in testing with line tests. It is also used if one wants to determine
what is the smallest size of optotypes recognised by a child/adult with amblyopia or impaired vision. When testing a young or visually impaired or multidisabled child for the first time the Single Symbol Book is often the first distance test to find out the child’s visual sphere and interest in small details at a far distance. The other often used test is the LEA Symbols Flash Cards.
Remember: visual acuity values measured with single optotypes or using pointing can be up to three times higher than values measured with a line test. Therefore single optotype values cannot be used as the visual acuity value for classifying visual impairment.
Establish a method of communication such as naming (signing) or pointing (matching). Decide with the child which names will be used to identify the symbols. When needed, train with the LEA Puzzle (#251600), Response Key Card (#251700), or Flash Cards (#251800).
Show the largest symbol size at near distance and then move backward to the 3-meter (10-foot) distance while watching for signs of inattention. If the child/person loses interest in the test, move closer. Always test well within the child's/person's cognitive visual sphere, the space within which the use of vision is possible.
Show one page in each decreasing symbol size and ask the
child/adult to identify a symbol. When there are multiple
symbols on a page, cover the other symbols. Continue until
the child/adult hesitates or answers incorrectly.
Turn back to the previous (larger) symbol size page(s) and
show all four symbols, one at a time. Cover all but one
symbol and ask the child/adult either to name or match the symbol.
If the child/adult identifies at least three symbols
correctly, go on to the page(s) with symbols in the next smaller
size and continue testing.
If only two out of four symbols are identified correctly,
show one of the symbols a second time to give a fifth choice.
The visual acuity threshold is defined as the level (smallest
symbol size) at which the child can correctly identify at
least three out of five symbols.
If the child/adult correctly identifies two of the five symbols,
report visual acuity as that of the previous larger symbol size.
To get more information for follow-up examinations, write down
(+2) after the visual acuity value to record that two symbols
were identified correctly in the next smaller size. For example,
"20/32(+2)" indicates the child/adult passed the 20/32 line and
also correctly named two 20/25 symbols.
When testing severely visually impaired children/adults and larger optotypes are needed, the Single Symbol Book distance test is used as a single symbol near vision test instead of the LEA Symbols Domino or Playing Cards.
Testing at Different Distances
If the test is used at a distance other than the usual 3
meters (10 feet), measure and record the viewing distance
and the symbol size (the value M*).
Viewing Distance in meters
Viewing Distance Used (meters or feet)
3 meters (10 feet)
x V.A. value for 3 meters (10 feet)
If the viewing distance used was 6 feet (180 cm) and the smallest
optotypes correctly recognized were on line 20/50 (0.4).
6 x 2/5
1.8 x 0.4
Note that it is incorrect to report ‘V.A. 20/25 at 5 feet’ if the child could read the 20/25 (10/12.5)-line
(3.8M line) at 5 feet. Visual acuity is in that case: 5’/10’ x 20/25 = 1/2 x 20/25 = 20/50. (When using the
British notation: 6/9 line at 150cm equals: 1.5m/3m x 6/9 = 1/2 x 6/9 = 6/18. When using the decimal
notation 0.8 line at 1.5m equals 1.5m/3m x 0.8 = 1/2 x 0.8 = 0.4)
When the distance is one half (or one third) of the standard distance, the visual acuity value is also
one half (one third) of the value printed next to that line.
If you do not want to do the calculations, report the result as M-unit value*, i.e. in the previous case 3.8M at 5 feet (1.5m). Visual acuity is easy to calculate based on these values:
VA = 1.5m/3.8M = 0.4 (=40/100=20/50).
The calculation is based on metric measurements. The corresponding visual acuity values in the American and in the British notation you most often find also on the visual acuity chart.
If the exactly corresponding value is not printed on the chart, calculate it as follows:
For example 0.07= 7/100= [7x3/100x3]=21/300 or 20/300;
or for the British notation: 0.07=7/100=6/86 (6x100/7=86)
You multiply both the numerator and the denominator with the number that makes the numerator equal or closely equal to 20 or 6.
Always remember to write down which test you used and at which distance. This makes the follow-up more reliable, even if the child moves to another town and therefore to an office of another doctor.
* M-unit, metric unit is the distance in meters, at which the reference optotype C is seen at a visual angle of 5'.