This single-file HTML tool calculates a recommended tracking (letter-spacing) adjustment in em units to compensate for the irradiation illusion (bright text on dark backgrounds appears larger, making gaps feel tighter). It also checks WCAG contrast compliance.
Save this page as a single .html file to use offline. No external dependencies. The adjustment is empirical (max ~5% shift) and scales inversely with font size. For production use, combine with pair-specific kerning tables.