How to Measure Your Child’s Vision with the LogMAR Scale

Screening of Children – the New Thresholds for Referral (LogMAR).

Normal vision in children and adults is defined as 6/6 vision or decimal 1.00. The Danish Health Authority’s current guideline on “Preventive Health Services for Children and Adolescents” states that children should be referred to an ophthalmologist if the vision test result is worse than 6/9 (or decimal 0.66) in just one eye (monocular).
(On a chart designed for vision testing at a 3-meter distance, 6/9 is recorded as 3/4.5.)

On a logarithmically constructed chart, this is translated to LogMAR 0.2

Explanation

The referral threshold of 6/9 (6-meter chart) means, in plain language, that a child with vision problems can just barely see and clearly identify objects at a distance of 6 meters, while a person with normal vision can see the object clearly from as far as 9 meters away.

Transition to the logarithmic scale (LogMAR)

With the transition to reading with the logarithmic scale (LogMAR), the threshold of 6/9 remains the baseline for preschool and school children, but it is now expressed with a logarithmic value to achieve a more accurate and comparable vision test result. (Read more about the Danish Health Authority’s new requirements for the construction of vision charts below)

Construction of a logarithmic vision chart and adjustment of the threshold

Vision charts with a logarithmic scale are divided into symbol lines of letters or figures, whose size decreases by exactly 0.1 LogMAR per line down the chart. (Normal vision on the logarithmic scale = 0.0)

The value 6/9 on a 6-meter chart, just like the value 3/4.5 on a 3-meter chart, is logarithmically expressed as 0.18, but since the charts are constructed with increments of 0.1 LogMAR, the nearest reading value is LogMAR 0.2.

The threshold that should be used as a guideline for referral to an ophthalmologist on a LogMAR constructed chart is thus LogMAR 0.2 or lower..

For preschool children, LogMAR 0.3 can be accepted as the maximum, as their visual acuity often improves over time.