Sector lights, directional lights and leading lights guide vessels safely through hazardous waters or narrow channels at night. These lights are used when approaching or leaving harbours and are marked on charts and in nautical almanacs. How do they differ from one another?

A typical sector light has three arcs of light coloured red, white and green. In order to stay safe, as a vessel approaches the light it has to stay in the white sector. If the vessel moves into the red or green sectors, then this indicates that the vessel is no longer in the safe channel. If the vessel is within the IALA A system, it needs to alter course as follows:

• red sector – indicates the port side of the channel. The vessel needs to steer to starboard to return to the white sector.
• green sector – indicates the starboard side of the channel. The vessel needs to steer to port to return to the white sector.
• white sector – indicates safe water.

Sector lights guide vessels through hazardous channels or into harbours with narrow entrances. The boat shown here would only be able to see a white light, as it is in the white sector. 

Sector lights are shown on charts with their light characteristics. The light characteristics in the diagram above are IsoWRG3s9m8M. This means: isophase, white, red and green, flashing every 3 seconds, height 9 metres, range 8 miles. 

Learn all about buoyage and lights in our reference app IALA Buoys & Lights at Sea for all navigators at sea. The app includes a quiz to test yourself.

Directional lights

Directional lights are similar to sector lights. A directional light guides a vessel into harbour in a very narrow channel. There is less margin for error than with a sector light as the channel is so narrow. The light beam is usually flanked by darkness rather than coloured light sectors, as is the case with sector lights.

The correct angle of approach for a directional light is given as a compass bearing on charts. Unlike sector lights, directional lights are marked on charts as single, unbroken lines, with the angle also marked on the line.

Once the navigator has seen the light, they are on the correct bearing and are safe to enter the harbour.

The directional light in the diagram here flashes on a bearing of 220.5º. Its light characteristic is Fl(2) 5s 10m 12M. This means it flashes twice every 5 seconds, is at a height of 10 metres above MHWS and can be seen from a distance of 12 miles. 

Learn all about buoyage and lights in our reference app IALA Buoys & Lights at Sea for all navigators at sea. The app includes a quiz to test yourself.

Leading lights

Leading lights are pairs of beacons that help navigators keep on the correct line of approach into harbour. They work in the same way as leading marks do during the day, where two day marks or shore features give the correct course, if lined up. 

While leading lights are excellent aids to night time navigation, it is important you have identified the  correct two lights to line up, which is not always easy.  

Leading lights show the safe line to take. Here, you are too far to port if the lower light moves to the right and so you need to steer to starboard. If the light moves to the left, you need to steer to port.

Learn all about buoyage and lights in our reference app IALA Buoys & Lights at Sea for all navigators at sea.  The app includes a quiz to test yourself.

Happy boating and stay safe on the water!