Notebook 03 · identification at sea
Light characters: the pattern is the name
One light on a dark coast means nothing. Two identical lights are worse than none. The timed pattern is what tells a navigator which rock is speaking.
The families of character
Characters are described by the balance of light and dark and by the grouping of flashes.
- Fixed (F). Continuous, steady light. Simple, but easily confused with shore lighting.
- Flashing (Fl). Dark longer than light. A single flash each period.
- Group flashing, written Fl(3) and so on. A set number of flashes, then a longer dark interval before the group repeats.
- Occulting (Oc). The reverse of flashing: light longer than dark, with brief eclipses.
- Isophase (Iso). Equal light and dark.
- Quick and very quick (Q, VQ). Rapid repetition, used where a mark must be obvious at short range.
- Alternating (Al). Colours change between phases.
- Morse (Mo). A letter sent in light, used on some offshore marks.
The published description then adds colour and period. A character line names the family, the count, the colour and the number of seconds in a complete cycle, in that order.
Period, and how to time it
The period is not the flash. It is the whole cycle. Start the watch at the beginning of one flash and stop it at the beginning of the next identical flash. Count the flashes inside the group before the long eclipse. Do that twice. At range, and in a swell, one count is not enough.
Two mistakes recur. Counting the eclipse as part of the next group. And missing a weak flash from a panel that is partly obscured at your bearing.
Colour and sectors
White carries furthest. Red and green carry less, because the coloured glass or filter absorbs light, so a sector light will usually be given a shorter range in its coloured arcs than in its white one.
Sectors are the useful part. A sector light throws defined colours over defined bearings, so the colour you see tells you where you are. A common arrangement gives white over the safe approach and red or green over the hazard on either side. The boundaries are bearings as observed from seaward, and they are published as such. Sector boundaries are not sharp lines in practice; there is a transition, and a vessel sitting on one will see the colour flicker between arcs.
Range: three different numbers
Ranges are often quoted as one figure, which hides three ideas.
- Geographical range is set by geometry: the curvature of the earth, the height of the light and the height of the observer's eye. A tall cliff station is visible further than a low rock tower of equal power, and a bridge wing sees further than an open cockpit.
- Luminous range depends on the intensity of the light and the visibility at the time. Haze cuts it hard.
- Nominal range is the luminous range under a stated standard visibility, used so that lights can be compared on a common basis. It is a catalogue figure, not a promise about tonight.
The practical consequence: in clear weather you may raise a light beyond its nominal range as a loom on the cloud before the light itself lifts over the horizon. In poor visibility you may be well inside the nominal range and see nothing.
Sound and radio marks
Fog signals once mattered as much as the light. Sirens, reed horns and diaphones, each with its own timed blast pattern, driven by compressed air from engines in the station. Many have been discontinued as vessels came to rely on radar and satellite positioning, and because sound ranges in fog are notoriously unreliable — refraction and wind shear can leave a silent zone close inshore.
Radio and radar aids sit alongside the light now. A racon is a radar transponder at the station: when it hears a vessel's radar pulse, it replies, painting a coded Morse character outward from the mark on the radar display. Some stations also carry AIS aids to navigation, transmitting an identity and position that appears directly on the vessel's display. Differential corrections and other services have at times been broadcast from lighthouse sites because the towers already stood in useful places with power and staff access.
Where the authoritative detail lives
Characters, ranges, sector bearings, fog signals and racon codes are published, amended and withdrawn through official channels. The Admiralty List of Lights and Radio Signals, issued by the United Kingdom Hydrographic Office, is the standard reference; charts carry the same data in abbreviated form; changes are promulgated through Notices to Mariners, and urgent changes by radio navigational warning.
Practical note
Never navigate from a description on a website, this one included. Characters are altered during refits, and lights are downgraded or discontinued. Use a current chart and the relevant volume of the Admiralty List of Lights and Radio Signals, corrected up to date.
Why neighbouring lights differ
Characters are assigned so that lights visible from the same stretch of water cannot be mistaken for each other. That constrains the choice available: a busy coast quickly uses up the short, memorable patterns, which is part of why some characters are long and awkward to count. Rotation speed, panel count and eclipser design all have to be made to fit the assigned character rather than the other way round.