The large symbol cards with 40M and 60M optotypes can be presented as single cards or placed in a row for a line test at distance. If the visual acuity is so low that the cards must be held at near distance, watch whether the child/person fixates the picture to see it as a picture or traces the figure with eye movements, which is not a real recognition test situation.
The card names 40M* and 60M* have been misunderstood as referring to metric distances of testing. That is not the case.
*Louise Sloan created the term M-unit as a metric unit. 1M being = 5 min of arc at 1 meter (m).
Visus, visual acuity = (test distance in meters)/(optotype size in M-units on threshold line).
Visual acuity is calculated by dividing the test distance (m) with M-value of the threshold line.
In examination of vision impaired young children we have to measure within the distance where the child can use his vision. Visual acuity is then easy to calculate knowing the test distance and reading the M-value on the chart at the smallest line that the child could read at least three out of five optotypes (3/5). If the cards are used as single optotypes, calculation is the same. metric distance / 40M or metric distance / 60M. Metric measurements must be used.
If the cards are presented singly, the test situation is the same as when using the LEA SYMBOLS® Flash Cards choosing the presentation technique that meets the needs of the child/person. Always test within the cognitive visual sphere, the space within which the use of vision is possible..
The LEA SYMBOLS®
are calibrated with Landolt C and thus give the same visual acuity values as the Landolt C test in normally sighted persons. Visual acuity values at this low value range are measured in an area of the visual field where all optotype sets behave individually. Landolt C is a resolving optotype and measures what is the smallest gap that the person/child can perceive wheras LEA SYMBOLS®
also require recognition of the form. If a child/person uses eccentric fixation, it is usually impossible to know whether the child/person used the same preferred area of the visual field for each measurement. A shift in the fixation area is likely to result in another visual acuity value.
In clinical measurements we most often want to learn what are the smallest forms that the person can recognise. Resolving tasks are rare in every day tasks.
With this addition to the LEA SYMBOLS®
tests it is possible to measure recognition visual acuity at 3 m distance with LEA SYMBOLS®
between 2.5 (20/8.0, 6/2.4) and 0.05 (20/400, 6/126).
If the test with the 60M cards is performed at 1 m distance, then the visual acuity value is 0.016 (20/1250, 6/380).
The range of measurements with calibrated tests covers thus from 0.016 to 2.5, which is the range of visual acuity values used in clinical examinations for diagnostic work and vision rehabilitation. The uncalibrated “finger counting” can be deleted from the descriptions of visual acuity though “hand movements at __ m distance” and “light perception with or without projection” provide useful information.
The recognition visual acuity values should always be accompanied with a grating acuity value measurable with the LEA Grating Acuity test to detect changes in visibility of straight lines, which the recognition acuity does not reveal.