Vanishing optotypes. New type of acuity test letters
L. Frisen
Acuity test letters generated on a television monitor were studied with
regard to detection and resolution thresholds, both for the native
configurations and after high-pass spatial frequency filtering. Comparisons
were made with printed acuity charts of different contrast. Filtering
diminished the interval between detection and resolution thresholds. Under
some circumstances, a small (1 dB, about 26%) increment in size could make
a previously invisible optotype clearly visible, or vice versa. The term
vanishing optotype is suggested for acuity targets with this peculiar
property. Vanishing optotypes do not seem useful for routine clinical
acuity testing, but they may be advantageous, for example, in preferential
looking-acuity tests for infants and visual field examinations. Like the
present tests, such novel applications can be generated relatively easily
on personal computers.
