Phase code (light)

The phase characteristic of a light is the timed pattern of light and darkness that identifies an aid to navigation at night, what the light list states as the light’s character. The standard phases are fixed, flashing where light is shorter than dark, occulting where light is longer than dark, isophase with equal light and dark, and quick or very quick flashing. These combine into group flashing, composite group flashing, and Morse-code rhythms, each with a stated period in seconds. The mariner times the full cycle with a watch and reads color, period, and rhythm against the light list and chart to identify the mark with certainty among nearby lights.