Light characteristics – how do navigators identify lights at night?

By in Boat Handling, Crewing skills, Navigation, Preparation | 0 comments

How do navigators identify the different types of light around our coasts at night and what are their characteristics?

Navigating at night is made possible through an assortment of navigation aids, both ancient and modern. Lights are arguably the most important of these. Therefore, navigators need to be able identify the different types of light and what they signify. 

Electronic instruments help pinpoint a boat’s position, heading and speed. However, in order to be able to steer a safe and accurate passage, navigators must look out for and recognise the lights of navigation marks, beacons and lighthouses along their route.

How are lights identified?

Trying to identify every light you see can be difficult, especially when entering a harbour at night. In order to help with identification, each type has three main features:

  • Colour – green, red and white are most common. Yellow, blue, orange and purple are also sometimes used. 
  • Character – the rhythm of the light, or the flashing pattern it makes. 
  • Period – the length of time the light’s flashing sequence takes in seconds.

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.

Light characteristics

Unless you are unfortunately colour blind, identifying the colour of lights is not too challenging. However, identifying a light’s character and the length of time its flashing sequence takes requires good night vision and concentration.

There are a number of light patterns which are used. Navigators need to learn the different types and their chart abbreviations in order to interpret charts correctly and to recognise lights. They include:

  • Fixed (F) – a single steady light. Often 2 together.
  • Occulting (Oc) – long periods of light with short periods of darkness. This has the effect of making the periods of darkness seem like flashes. 
  • Isophase (Iso) – equal periods of light and dark.
  • Flashing (Fl) – short periods of light with long periods of darkness. Eg Fl(3) flashes in groups of three and then a period of darkness.
  • Quick (Q) – quick flashes of light varying from 50 to 300 flashes per minute.
  • Alternating (Al) – a light with different colours of alternating light. Eg (Al RG) alternating red and green lights.
  • Morse code (Mo) – lights that flash a Morse code letter.

Tip: 

Time several light sequences with a stopwatch to confirm you have identified a light correctly.

Classes of lights and their abbreviations

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.

Light abbreviations

Charts, almanacs and pilot guides use abbreviations to describe lights A typical light characteristic, including its height above sea level and range is condensed into a few letters and numbers. Here is an example together with its meaning explained:

Fl(4) WRG 20s 23m 25M

FL(4)   Character of light: regularly repeating a group of 4 flashes.

WRG   Colours of light: white, red and green. If no colour is given then the light is white.

20s   Period of time taken to exhibit one full sequence: 20 seconds.

23m   Height of light above MHWS: 23 metres.

25M   Luminous range  the distance at which the light can be seen: 25 nautical 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.

Happy boating and stay safe on the water!

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