Period of light

The period of a light is given in seconds and is timed from the first flash of a cycle until the first flash of the next cycle.
For example, on the chart we see a lighted navigation aid, and written near the purple exclamation point is Gr. Fl.(4) 15s – group flashing, four flashes, every 15 seconds. This means that the light starts flashing for four flashes, followed by a period of darkness, then starts the four flashes again, followed by a period of darkness and so on.
Using a stop watch and timing a cycle from when the first of the four flashes appears, including the period of darkness, until the first of the four flashes appears again, we find that the period of that light is 15 seconds.