The colours of lights are abbreviated and shown as follows:-
White W (may be omitted)
The time period of a light, is the time taken to complete one whole cycle of the characteristics of the light, that is from the start of one cycle to the start of the next. This is shown on the chart in seconds, abbreviated to 's'.
The theoretical range of visibility of a light, is given in sea miles, either as a single figure, or two figures where different colours of varying visibility are used. They are shown thus:-
Single Range eg 20M
Two Ranges eg 16/12M
Three or more Ranges eg 20-16M
Other details such as fog horn or radio beacon will be added in abbreviated form at the end of all other information concerning the light. From this information a full description of any lighthouse is shown as:-
Fl(3) WRG 20s 15m 18-12M
This would indicate that it is a group flashing three light with white, red and green sectors, the light period is twenty seconds, the elevation is 15 metres, and the range of the three different colours is from eighteen to twelve sea miles.
Admiralty List of Lights
Names, descriptions, locations in Latitude and Longitude and characteristics of individual lights can be found in the Admiralty List of Lights. The list is particularly useful for confirming the identification of a light structure in daylight. It also contains tables of distances of visibility.