LEA SYMBOLS®
Tests for Small Illuminator Cabinet
#250400, #252400, #255100

Standard illumination is a prerequisite for standardized measurements of visual acuity. Illuminator cabinets have been used more than 25 years to guarantee even illumination of visual acuity tests. They are most often used at the maximum illumination but can also be used even at low mesopic luminance levels by reducing the luminance level with filters in front of the test. In small lightboxes there is one with self-calibrating luminance level (ESV1500) and several with fixed luminance levels, 85cd/m² and above.

It is easy to hold the covering card above the line to be read by sliding it between the test and the frame of the lightbox as shown in the picture below.

LEA SYMBOLS® distance visual acuity tests are manufactured for both the small light boxes and the large ETDRS-style illuminator cabinet. The small light box has tests for 3 meters (10 feet) [#250400 and #252400] and 4 meters (13 feet) [#255100]. The 4-meter (13-feet) tests are also used in survey projects of adult persons and by military services in many developing countries because the LEA SYMBOLS® are acceptable in all cultures.

#252400	#255100
#250400

Tests for the small lightbox ESV1500 Illuminated Cabinet. The covering card is held between the test and the frame of the lightbox. Short video of the test situation is at Follow-up of vision development >> At the age of three years >> sequence #4.

Instructions

• Establish a method of communication such as naming (signing), pointing or matching. Decide with the child which names will be used to identify the symbols. When needed, train with the LEA 3D Puzzle (#251600), Response Key Card (#251700), or Flash Cards (#251800).

• Cover the line above the line to be read with a white card and ask the child/person to identify the first symbol in each line in descending order when testing binocularly. Do not point to the symbol to be read because it makes fixation easier, especially in the case of amblyopia (lazy eye).

• Move down until the child hesitates or misidentifies a symbol.

• Move back up one line and ask the child to identify all the symbols on that line.

• If the child identifies at least 3 out of 5 symbols correctly go to the next line with smaller symbols and ask the child to identify all symbols on the line. The child may have started to focus more carefully and may be able to read that line, sometimes even the next line. Therefore say calmly “and on this line” and do not let your voice give the impression that it is too small to be read.

• If the child skips a symbol, let the child read the line to the end and then ask the child to try again while briefly pointing to that symbol and say “You hopped this picture between (e.g.) ‘house’ and ball’.” What is it?”

• A child with an amblyopic eye or irregular saccades may typically skip symbols within a line of symbols. If skipping symbols occurs, report whether it occurred in only one eye or in both eyes. It may be a sign of more general motor problems.

• Visual acuity is recorded as the last line on which at least 3 of the 5 symbols are identified correctly.

• When tested at 3 meters (10 feet) the visual acuity value is found in the margin adjacent to that line.

• After obtaining good responses with binocular testing, proceed by testing each eye separately.

• When testing monocularly, use the first symbol of each line or every second line for one eye and the last symbol of each line for the other eye to determine on which line to start testing. Then move to the small sets of test lines on each side and test the same way as when using the middle set of test lines. This way the child cannot memorise the chart. Use the right set of symbols for the testing of the right eye and the left for testing the left eye.

• Always remember to record which test you used and at which distance. This makes the follow-up more reliable, even if the child moves to another school and will be tested by another school nurse and another doctor.

Testing at Different Distances

If the chart is used at a distance other than the usual 3 meters (10 feet), measure and record the viewing distance and the symbol size (the M value*) or the visual acuity value printed at the threshold line.

To determine the visual acuity use one of the following formulas:

Note that it is incorrect to report 'V.A. 20/25 at 5feet' if the child could read the 20/25-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 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.

In the rare cases where the child/person can read all optotypes correctly on the 2.0 (20/10, 6/3) row and none on the 2.5 (20/8.0, 6/2.4) line, the test can be moved to a slightly shorter distance to record the threshold level. At a distance of 2.76 m the visual acuity value of the 2.5 line is 2.3 (20/8.7, 6/2.6).

*M-unit, metric unit is the distance in meters at which the reference optotype C is seen at a visual angle of 5'.