Development and validation of a visual acuity chart for Australian Aborigines and Torres Strait Islanders

A new visual acuity chart was designed for use with Australia's indigenous population to overcome perceived inadequacies of conventional English letter charts for this group. This chart, which incorporates a black and white turtle icon, is described, and validation data are presented.

The chart is based on logarithm of the minimum angle of resolution (logMAR) principles and incorporates a turtle symbol modified from the design of an indigenous artist. The task is one of discrimination, with subjects being required to distinguish the split tail of the turtle from its head, which has the same overall shape and average luminance; the body of the turtle provides no directional cues which might assist in this judgment. The chart was validated in two ways: Experiment I. Performance was compared with the Bailey-Lovie and Konig bar charts in terms of unaided visual acuity data for 90 subjects (mean age: 38.3 +/- 20.3 years) and Experiment II. Data were obtained for 10 young subjects for these 3 charts and an Illiterate E chart, with refractive blur imposed with trial lenses over habitual distance corrections (spherical: +0.50, +1.00, +2.00, and +4.00 D; cylindrical: +1.00 and +2.00 D, axes 45, 90, and 180 degrees). To avoid cultural and literacy issues as possible sources of differences in performance between the charts in this validation study, subjects were selected from the wider Australian population rather than specifically from its indigenous segment.

Experiment I: The Turtle chart performed most like the Konig Bar chart for this component of the validation exercise. Nonetheless, results for the Turtle chart correlated highly with those for the Bailey-Lovie chart as well as the Konig Bar chart, although there were subtle differences between charts in the rate of decline of visual acuity as visual performance decreased. Experiment II: The turtle chart behaved most like the Illiterate E chart with imposed spherical focusing errors, with the Bailey-Lovie chart showing a faster decline and the Konig Bar chart showing a slower decline in performance, with increasing defocus. All 4 charts showed similar directional biases with astigmatic defocus, being most affected by oblique (45 degrees) astigmatism.

The Turtle chart met the criteria set for its validation as a visual acuity chart in that it gave comparable results to the other commonly used visual acuity charts, both in the case of unaided vision and when refractive blur was imposed.