Distance visual acuity measurement
Ideally, the measurements should be made by using standard illumination when used for reporting purposes. Therefore the small EDTRS light box was designed for school and occupational health services. The non-illuminated visual acuity charts can be used in a place where there is good illumination. In developing countries this often means testing outside. Make sure that the child is not dazzled by the white surface of the test. It is also important to measure visual acuity available for daily function in the classroom or day care centre.
1. Start the measurement at a distance of 3 or 4 meters depending on which test you use and approach the child until the child can correctly recognise 3 symbols out of the 4 largest symbols or until the child can comfortably look at the test.
2. Recording the result: Write down the M-value and the distance at which it was recognised when using the LEA Screener. Calculate the VA-value by dividing the distance used (in meters) by the M-value, i.e. if you measured at a 1.5 m distance and the child's threshold was 15M, the VA-value is: 1.5/15 = 6/60 (0.1, 20/200). Also measure the monocular values in the same way and record the results.
If you measured at the standard distance of the charts where the visual acuity values are printed on the chart, record the visual acuity value next to the line read by the child. If you used the chart at a closer distance you need to calculate the visual acuity value by dividing it by the standard distance and multiplying by the distance used. For example, if you used a 3 meter test at 1.5 m distance, visual acuity is 1.5/3 x the value of the smallest line read by the child, or one half of the value. If the distance was 1m, the correct visual acuity value is 1/3 of the value printed next to the smallest line the child can see.
The difference between the results obtained using bright room illumination or the chart in the lightbox, is usually small, often less than one line. Thus the LEA-Screener or the 15-line folding chart combined with the near vision card are acceptable tools if the school or the tester do not have a light box.
The results recorded provide preliminary information on the size of text the child is able to see (with difficulty) written on the black board with his/her present spectacle correction (if used) or without glasses. However, since the texts on the black board are at lower contrast levels than the symbols in the visual acuity chart, it is necessary to measure the readability of the teachers' handwriting in the actual classroom situation, i.e. what is the size the child can read when seated at his/her usual place.
NOTE: If the visual acuity for near is better than it is for distance, the commonest cause is myopia, shortsightedness, for which glasses may be needed. Nystagmus, wobbling eyes may produce this effect because nystagmus is often less disturbing when looking at a near object.
Conversely, if visual acuity for distance is better than for near the child may either be longsighted or may have impaired focusing, accommodation. If you have simple plus lenses, you may find out yourself whether they help the child to see better at near distances.
Grating acuity measurements are used mostly in the assessment of vision of infants. If the child has grade III low vision it is diagnostically important to also measure grating acuity (see LEA GRATINGS) because it may be much better than expected on the basis of optotype acuity.