The most common clinical test of visual function is visual acuity (VA) measurement. Acuity tests have evolved slowly, incorporating chart design features aimed at improving measurement accuracy and minimising test variability. However, with current gold standard logMAR charts, measurements can still be affected by testing and scoring methods, possibly attributable to variations in relative legibility between conventional letters. This limits the test sensitivity and specificity in detecting a change in clinical status. Furthermore, conventional letter charts have demonstrated an insensitivity to early visual system neural deficits. High-pass filtered letters have a design in which low spatial frequencies, where conventional letters typically vary, are removed. Constructed with a dark core and light edges, the mean letter luminance matches their grey background. This results in similar detection and recognition thresholds with foveal viewing in normal subjects such that letters appear to vanish when the resolution threshold is reached. Under extra-foveal viewing, these thresholds are seen to separate, indicating the neural sampling limited nature of resolution. This thesis investigates the functional characteristics and limits to performance of high-pass filtered letters. In laboratory-based studies, high-pass VA thresholds were found to display lower between-letter threshold variability, be more robust to the number of alternative letter choices and more resistant to optical degradations including defocus and simulated lens ageing compared to conventional letters. When a novel high-pass chart, the Moorfields Acuity Chart, was employed in clinical studies, it displayed VA scores and variability less affected by termination and scoring rules in normal subjects with uncorrected refractive error, whilst better revealing functional loss in age-related macular degeneration (AMD). Thus, it appears that high-pass letters can be incorporated in a clinical test chart offering lower variability and in which recognition thresholds are better correlated with early neural deficits in AMD, in a task already familiar to patients
