Lights, rhythms and light characteristics

A light characteristic combines rhythm, colour and, where published, period, possibly including a group or colour alternation. Rhythm describes light-dark relationship, grouping organisation, and period the complete cycle. Fl(2)W6s means neither a six-second light nor six flashes: it identifies two grouped flashes, white, with a six-second cycle. Figure numbers are fictional conceptual comparisons except identities expressly specified in other lessons. They are not a repertoire for an actual coastline. A count, duration and repetition interval therefore need separate labels rather than one ambiguous number.

IHO S-4 B-471.2 defines Fl with total light shorter than darkness in the cycle, Oc with light longer, and Iso with equal light and darkness. LFl identifies a flash at least two seconds long. F indicates a fixed light without periodic eclipse. These are not intensity grades or interchangeable ways of saying blink. Static depiction can show the durations without emitting light; an explicit timeline scale prevents arbitrary drawn width from resembling a published measurement. A light’s apparent brightness can change without changing its defining rhythm.