Landolt C acuity follows Bloch's Law for short-duration flashes

Bloch's Law states that up to a critical duration, typically between 30 and 100 ms, the detection of a flash depends on its total strength (the product of luminance and duration). This study investigated whether this principle of temporal summation extends to shape recognition, assessed via optotype-based visual acuity.

Monocular Landolt-C acuity was measured in 19 healthy participants across 25 stimulus conditions. These conditions combined five photopic luminance levels (3.3-53 cd/m²) with five presentation durations ranging from 0.125 to 512 ms. The combinations were grouped into five flash strength levels, spanning a 256-fold range from 6.7 to 1,708.2 cd · s/m² × 10^-3.

Across Flash Strength Levels 1 to 4, mean visual acuity remained nearly constant within each level, with deviations below 0.08 logMAR (less than one line on a typical acuity chart). In contrast, at Level 5 (32-512 ms), visual acuity improved significantly by two lines, with the strongest increase occurring around 100 ms.

The results demonstrate that a "Bloch-like" law also governs shape recognition. Across four octaves in luminance and three orders of magnitude in duration, reductions in luminance were effectively compensated by longer presentation times and vice versa.