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Bright flashing lights

On a paper chart there is not really enough space to describe colors and flashing sequences of a navigational light so a code was invented. ‘F’ is a fixed light, ‘Fl’ is a flashing light (which means it is off more than it is on) and ‘Oc’ is an occulting light. Interestingly an occulting light does not lean towards the dark side. It is on more than it is off. In my dictionary ‘occulting’ also means supernatural, not capable of being understood by ordinary human beings and hidden. Here is the complete list of light types:

  • Oc Occulting
  • Iso Isophase
  • Fl Flashing
  • LFl Long Flashing
  • Q Quick flashing
  • IQ Interrupted Quick flashing
  • VQ Very Quick flashing
  • IVQ Interrupted Very Quick flashing
  • UQ Ultra Quick flashing
  • IUQ Interrupted Ultra Quick flashing
  • Mo Morse Code
  • FFl Fixed And Flashing
  • FLFl Fixed And Long Flashing
  • FOc Fixed And Occulting
  • Lit Unknown details

Which seems like quite a lot to me. Then there are codes for colors. ‘W’ is white. ‘G’ is green and so on. Finally there are indicators of period (time), height and range.

Here we have some raster and vector renderings of Point Hudson. imageimage


The last one is from Nuno (we think it looks best). As you can see there is some confusion over height units but they are all basically agreed that we have a flashing red light with a period of 2.5 seconds. In Nuno we have also gone a step further and illustrated the flashing sequence with a diagram.


This is a hover tip, a little floating window that appears when you hover the cursor in one position for a few seconds. We use a lot of these in Nuno.

For the most part reading the light characteristic is pretty straightforward but there are some rules about the characteristic string than can get a bit arcane. Such as: If a Characteristics field consists of Occulting and Flashing Light Characters that have a single period and are not Alternating and have no Grouping then these are merged into a single Light Character.

For example: Fl G Oc R 5s


The use of Green and Red in combination is quite common despite the fact that red/green color blindness occurs in around 8% of males (women don’t often have this problem). With something like a traffic light it is possible to distinguish the type of light from its position but this is not so easy for marine navigation lights.

There are three types of range that get discussed with regards to lights:

The geographic range is all to do with geometry. It assumes perfect visibility and so is really to do with the curvature of the earth. Here is a typical range table:


So from the bridge of a modest size vessel you should be able to see the Point Hudson light from over ten nautical miles away. Note however that the height is referenced to chart datum so in some circumstances the tide will affect visibility. In fact at low tide you will be able to see it from further away because it will effectively be higher above you.

Luminous range is to do with the power of the light and how well it penetrates the atmosphere. So assuming that the murk in the air is the the only thing blocking the light then this is a measure of how far away the light can be seen from. Obviously the brighter the light then the further away it can be seen. But what about color? Do some colors penetrate foggy conditions better than other? Anyone know?

The Nominal Range is what is written on the chart. Technically the nominal range is the luminous range when the meteorological visibility is 10 miles. Which of course begs the question as to what meteorological visibility is. Well one definition is that it is the greatest distance at which lights of 1,000 candelas can be seen and identified against an unlit background (does that help?).


So if it is a bit murky, meteorological visibility down to 5 miles, then the Point Hudson light will only be visible from less than four miles away.



By Nuno Navigator Blog | Sat, Nov 2010 07:10 | #

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