Luminous, nominal and geographical range
1The lesson in plain words
How far away will you see a light? Three ranges answer different questions. The chart gives the nominal range; the real answer is the smaller of the other two.
Luminous range
Luminous range is the greatest distance at which a light can be seen at a given time, set by its intensity and the meteorological visibility, how far you can see in the present weather. It ignores the light’s height, your height of eye and the curve of the Earth. In haze it shrinks; in very clear air it grows.
Nominal range
Nominal range is the luminous range when the meteorological visibility is exactly 10 nautical miles. It is a standard measure of the light’s strength, so lights can be compared. Modern charts and Lists of Lights give the nominal range, following the IHO standard. A few national lists still use another range; the list’s introduction says which.
Geographical range
Geographical range is the greatest distance allowed by the Earth’s curve. It depends on the light’s elevation, its height above the stated sea-level reference, and your height of eye, the height of your eyes above the water. Standard tables allow for atmospheric refraction, the bending of light through the air. Raising either height increases geographical range.
Which one counts
You can see a light only up to the smaller of its luminous and geographical ranges. The List of Lights has a luminous range diagram to convert nominal range for the present visibility, and a geographical range table. A strong light first appears, or dips out of sight, at its geographical range; working a distance off from that is in the Advanced fixes unit.
Worked example
A light has nominal range 9 M and elevation 70 m. Your height of eye is 8 m and the visibility about 20 NM. The table gives a geographical range of 22.7 NM; the diagram gives a luminous range of about 13 NM. The smaller is 13 NM, so expect to see it at about 13 NM. It will not dip on the horizon: it is too weak to reach that far.
2Key data
- Luminous range
- Intensity and present visibility; ignores heights
- Nominal range
- Luminous range at 10 NM visibility; on the chart
- Geographical range
- Light elevation, height of eye and the Earth’s curve
- Visible range
- The smaller of luminous and geographical range
- Light elevation
- Above mean high water springs (MHWS), unless notes say otherwise
3Key points
- 1
Charts give the nominal range: the light’s strength at 10 NM visibility.
- 2
Luminous range allows for the visibility; geographical range for the heights.
- 3
You see a light only up to the smaller of luminous and geographical range.
- 4
A weak light may never reach its geographical range, so it will not dip.
4Frequent mistakes
Wrong: The charted range is the distance at which the light rises over the horizon.
Right: That is the geographical range. The chart gives the nominal range, a measure of strength.
Wrong: A light is always visible out to its nominal range.
Right: Nominal range assumes 10 NM visibility. Clearer air can increase luminous range; haze reduces it. The horizon may limit either.
Wrong: Raising your height of eye always increases the distance at which you can see a light.
Right: It increases geographical range, but leaves luminous range unchanged. It helps only while geographical range is the smaller limit.
5Practice case & exam keys
A light has nominal range 19 NM. Earlier visibility was 20 NM, giving luminous range about 32 NM; geographical range is 20.4 NM. At 18 NM you still cannot see it, and visibility has fallen to 5 NM. Does that prove the light has failed?
Show the answer
No. The earlier luminous range no longer applies. Use the luminous range diagram with nominal range 19 NM and the new visibility of 5 NM, then compare that result with 20.4 NM. Reduced visibility may hide a working light. Do not treat its absence alone as proof of failure.
Exam keys
- Nominal range = luminous range at 10 NM visibility; shown on the chart.
- Luminous: intensity + visibility. Geographical: elevation + height of eye.
- Visible range = the smaller of luminous and geographical range.
Finish this unit
When you've read it, match the terms, then answer three questions. Get them all right to finish the unit; opening the page alone doesn't count.
My notes for this unit
No notes