Central Distortion Screen: sources and method
Independent educational implementation by/for Dr Dean van der Westhuizen. Not affiliated with, and not a copy of, any commercial D-chart software or M-CHARTS product.

- McGowan G, Manahilov V, Strang NC, Yorston D, et al. "D-charts: a new method for quantifying metamorphopsia" (dot-chart / D-chart method), Retina 2016 (Glasgow Caledonian University / NHS).
- IOVS (ARVO) meeting abstract describing a software version of D-charts for quantifying metamorphopsia (rings of dots around a fixation point, 8 sectors, decreasing dot density).
Method used here: octagonal rings of black square elements (central 0.5-3 deg, mid 3-6.5 deg, outer 6.5-12 deg) around a red fixation target; element separation 0.4, 0.6, 0.8, 1.0, 1.3, 1.8 deg; density advances while distortion is reported. Score per segment = coarsest separation still distorted; total = sum of 24 segments. Score bands are provisional and not validated.
Screen calibration uses an 85.6 mm bank card; visual angle computed for the chosen viewing distance (default 40 cm).