Single Number Book

The Single Number Book (#274000) is ideal for situations in which the child/adult cannot perform when tested with a line test. This book can help determine the best resolution of the visual pathway in a child/adult with amblyopia or impaired vision. Single optotype line sizes range from 20/200 to 20/8 (6/60 to 6/2.4) equivalent, 0.10 to 2.50, refer to the 10 feet/3 meters standard testing distance.

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Remember: when assessing visual acuity of children/adults with visual processing problems, it is important to use all three optotype tests, LEA NUMBERS®, Sloan letters, and LEA SYMBOLS® to detect problems in perception and recognition of abstract figures. An atypical development of recognition functions can affect only one of the three functions. Recognition of the LEA SYMBOLS® is the most rarely lost function, so far recorded only once in a teenager who recognized numbers and letters.

Remember 2: 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.  

The test includes isolating mask and instructions. 13 offset pages for easy flipping. 5" x 5" (12.7 cm x 12.7 cm).


  1. Establish a method of communication such as naming (signing) or pointing (matching). When matching is used, the test situation is kept purely visual. A child who has problems in remembering names of these four numbers or has problems in pronunciation should use matching. A matching card for distance vision is easy to make as a photocopy of the uppermost line of the #271200 or #271300 test.

  2. 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.

  3. Show one page in each decreasing number size and ask the child/adult to identify a number. When there are multiple numbers on a page, cover the other numbers. Continue until the child/adult hesitates or answers incorrectly.

  4. Turn back to the previous (larger) number size page(s) and show all four numbers, one at a time. Cover all but one symbol and ask the child/adult either to name or match the number.

    If the child/adult identifies at least three numbers correctly, go on to the page(s) with numbers in the next smaller size and continue testing.
  1. If only two out of four numbers are identified correctly, show one of the numbers a second time to give a fifth choice. The visual acuity threshold is defined as the level (smallest number size) at which the child can correctly identify at least three out of five symbols.

  2. If the child/adult correctly identifies two of the five numbers, 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 numbers 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 numbers.

When testing severely visually impaired children/adults and larger optotypes are needed, the Single Number Book distance test is used as a single symbol near vision test instead of the Playing Cards or Flash 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*).

V.A. = Viewing Distance in meters


V.A. = 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).

V.A. = 6f
x 20
= 6 x 2/5
= 12/5
= 12
= 12/1.2
= 10


V.A. = 1.8m
x 0.4 = 1.8 x 0.4
= 0.24

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 approximately 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'.

[ Instructions I Paediatric Vision Tests I Vision Tests ]

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