Light Descriptions and Light Ranges

Reading a light description

In short: a charted light is a short code in a fixed order — class, colour, period, elevation, range. Read it left to right.

Read first: light rhythms and their abbreviations (Fl, Oc, Iso, Q…) and chart datums. This topic builds on both.

• The full example Fl(3)WRG.15s21m15-11M is the standard one in INT 1, section P (chart symbols and abbreviations).
• Period: from the start of one sequence to the start of the next. Each flash or eclipse inside it is a phase; the light list gives the phases in seconds.
• Elevation is measured from the focal plane of the light to MHWS, not to chart datum. For vertical lights, e.g. 2 F R (vert), it is the elevation of the uppermost light. At any lower tide the light stands higher than charted.
• Range on the chart is the nominal range. When colours differ, the chart gives the longest and the shortest; the light list gives each colour.
• Chart or list? The chart gives the short code next to the magenta light flare. The ADMIRALTY List of Lights and Fog Signals (ALL) — 15 regional volumes, A to Q — gives the full entry: international number, position, characteristic and phases, elevation in m, nominal range, structure and its height, sectors and remarks. Quote the international number (volume letter + number) when you report a light. It is one of the compulsory bridge publications.

Sector lights

• Sector limits and arcs of visibility are given as seen from seaward, in true bearings, clockwise from 000° to 359°.
• Never rely on the limits alone: check by compass bearing of the light. The limits are rarely clear cut — a light usually fades after you cross the limit.
• At the boundary between two colours there is a small arc where the light can be obscured, of uncertain colour, or white.
• In cold or wet weather, frost or moisture on the lantern glass can widen this arc: coloured sectors may look white, and fixed or occulting lights may look flashing.
• When a light is cut off by sloping land, the bearing at which it disappears changes with your distance and height of eye.
• On the bridge: a white sector often marks the safe approach, with red and green over the dangers on each side. If the colour you see does not match your planned track, check your position at once.

Identifying a light at night

• Note the colour, count the flashes, time the period with a stopwatch, then compare with the chart and ALL. Do not make the light fit what you expect.
• You cannot judge distance from brightness: a weak light close by and a powerful light far away can look the same.
• Very short flashes may not be seen at the full range the tables give. Lights at a great elevation are more often hidden by low cloud.
• Aero lights and their looms are often the first lights seen on a landfall. They are marked “Aero” and are not maintained for ships — treat them with caution.

Luminous, nominal and geographical range

In short: brightness and today’s visibility give the luminous range; the nominal range is the luminous range in a standard 10 NM visibility; your height and the light’s height give the geographical range.

• Meteorological visibility: the greatest distance at which a black object of suitable size can be seen and recognised against the horizon sky by day (or would be at night, if lit to daylight level).
• The front pages of every ALL volume explain the terms and hold both tools: the luminous range diagram and the geographical range table.
• Nominal range is a rating, not a promise. In 3 NM of haze a 15 M light will be seen much closer than 15 M; in very clear air, further.
• Do not confuse these with the ranges of a ship’s navigation lights (COLREG Rule 22): those are minimum ranges of visibility set by Annex I.

Will I see the light, and at what range?

In short: a light is first seen at the shorter of its luminous range and its geographical range.

1. Take the nominal range and the elevation from the chart or ALL.
2. Estimate the meteorological visibility. Enter the luminous range diagram at the top with the nominal range, go down to the visibility curve, and read the luminous range on the left-hand scale.
3. Enter the geographical range table with your height of eye (top) and the elevation of the light (left). Read the geographical range.
4. Compare. The smaller figure is the range at which you can expect to see the light.

Rule of thumb for the geographical range

• Distance to the horizon ≈ 2.08 × √h NM, with h in metres (common UK rule of thumb, refraction included).
• So geographical range ≈ 2.08 × (√H + √h), where H = elevation of the light and h = your height of eye, both in m.
• The light-list tables allow slightly less refraction, so they give slightly shorter ranges. Example from a published geographical range table: light 30 m, eye 9 m → 17.2 NM (the rule of thumb gives 17.6 NM). On the bridge and in the exam, use the table.

Worked example — Fl(3)WRG.15s21m15-11M

Height of eye 9 m, light elevation 21 m: the geographical range table gives about 15.4 NM.

• The green sector (11 M nominal) will be seen later than the white in the same weather.
• Another check, from a published light list: a light with nominal range 9 M and elevation 70 m, eye 8 m, visibility 20 NM: geographical range 22.7 NM, luminous range 13 NM. It will not be seen at its geographical range — only from about 13 NM.

Ranges are approximate. Fog, haze, dust, smoke and rain cut them; abnormal refraction can stretch them. The air is not always the same between you and the light, and glare from shore lighting greatly reduces the range at which a light is picked out.

Dipping distance and the bearing-and-dipping fix

In short: the dipping distance is the geographical range for your height of eye. When a powerful light rises or dips on the horizon, its bearing plus this distance gives a fix.

• Dipping (or rising) distance = the distance at which the light just appears over, or disappears below, the horizon. Light lists call the geographical range the “dipping range”.
• Only use it when luminous range ≥ geographical range, in clear weather — typically when you see the loom of the light before it rises or after it dips. If the light is weaker, it simply fades in or out inside its geographical range and gives you no distance.
• Check it is really dipping: lower your height of eye (step down from the monkey island or bridge wing) and the light disappears; go back up and it reappears. Then you are at the dipping distance for that height.
• The fix: take a compass bearing at the moment the light rises or dips, convert it to true, and lay off the dipping distance from the geographical range table along it. Mark the fix with the time.
• Treat it as a single, rough fix. Confirm it with a second bearing or a radar range as soon as you can.
• Cautions from the light lists: abnormal refraction can increase geographical ranges; some lights whose charted range is less than their geographical range may still dip in excellent visibility; the loom of a rotating light can be seen beyond its geographical range.
• On the bridge: on a night landfall, work out the rising distance of the main light in advance and write it on the chart. When the light rises, you get a quick check of your DR or GNSS position.