Isophase Light (Iso)

A rhythmic light whose periods of light and darkness are equal within each cycle.

Its chart abbreviation is commonly ‘Iso’ followed by the colour and period, such as Iso.W.4s for equal two-second light and dark phases in a four-second cycle. It differs from an occulting light, which is lit longer than dark, and a flashing light, which is dark longer than lit. The characteristic helps identify a particular aid to navigation but should be matched with colour, period, sector, position and charted details. Background lights, visibility and vessel movement can make timing difficult, so identification should not depend on a single observed cycle.

Abbreviation

Iso

Class of Light

Isophase

Illustration

Period Shown