Module 12 - Visual Aids to Navigation
Light Characteristics
What you will learn
Decode fixed, flashing, isophase, occulting, quick, very quick, grouped and long-flash characters, then measure an observed period without confusing group count, flash length and cycle time.
A light character is a complete repeating sequence of light and darkness. Recognition becomes reliable only when the skipper counts the group, compares light-to-dark proportions and times from the same point in successive cycles.
Fixed (F) remains continuously lit. Flashing (Fl) is dark for longer than it is lit; isophase (Iso) has equal light and dark durations; occulting (Oc) is lit for longer than it is dark. Long flash (LFl) has a light appearance of at least two seconds. A bracketed number, such as Fl(3), counts flashes in each repeated group.
IALA defines the quick class as at least 50 but fewer than 80 flashes per minute, with 60 as the standard specification. Very quick is at least 80 but fewer than 160, commonly specified at 120. Cardinal marks then use continuous or grouped Q/VQ characters; rate alone is not enough to identify the mark.
Time from a recognisable event, such as the first flash of one group, to the same event in a later group. Measuring several periods reduces reaction error. It does not remove the need to compare colour, bearing, sector, structure and the rest of the pilotage sequence.
Light rhythms are read as repeating time patterns
Read the whole repeating cycle: mostly dark, equal, mostly light, or rapid flashes.
Worked example
Test whether an observed two-flash group matches Fl(2) W 10s.
- 1Start at the first flash of a recognisable group and count three complete group-to-group periods.
- 2The three periods take 30.6 seconds, so the average is 30.6 ÷ 3 = 10.2 seconds.
- 3The average is consistent with the charted 10-second period, but colour, bearing and surrounding aids must still agree before identification.
Sense check: Dividing by the number of flashes would be wrong: the measured 30.6 seconds contains three complete periods, not six independent periods.
| Chart abbreviation | Meaning | How to recognise it |
|---|---|---|
| F | Fixed | A continuous steady light |
| Fl | Flashing | A flash repeated regularly; darkness lasts longer than light |
| Iso | Isophase | Equal periods of light and dark |
| Oc | Occulting | Mostly lit, with regular short periods of darkness |
| Q | Quick | Rapid continuous flashes, 50-79 per minute |
| VQ | Very Quick | Very rapid flashes, 80-159 per minute |
| Fl(3) | Group flashing | Three flashes grouped together in each period |
| LFl | Long flash | A flash lasting 2 seconds or more |
Separate a group from background lights
Shore lighting hides the first flash and one hurried timing gives eight seconds for a charted 10-second character.
1. Choose the group start
Watch long enough to recognise the long eclipse between groups.
2. Average several periods
Time from the first flash of one group across three complete repeats and divide by three.
3. Rejoin the pilotage picture
Compare the result with colour, bearing, sector and adjacent marks rather than treating timing as definitive.
Sense check: If successive timings do not settle near the charted period, reopen the identification instead of averaging unlike lights.
Common mistake or limitation
- Fl(3) does not mean a three-second flash, three separate lights or a three-mile range.
- A Q or VQ rate can support cardinal recognition, but the group, period, colour, topmark and charted position identify the actual mark.
Recap
- Read the balance of light and darkness before naming the class.
- Count the group and time the complete repeating period.
- Use several cycles, then confirm the result against the whole charted context.
Optional quick check
Section 2 of 10
Three complete periods take 30.6 seconds. What period should be compared with the chart?
Common questions
What is the difference between isophase and occulting lights?
An isophase light has equal periods of light and dark. An occulting light is lit for longer than it is dark, with brief regular eclipses.
What does isophase light mean?
An isophase light has equal light and dark periods in each cycle, such as two seconds on and two seconds off in a four-second period.
What does Fl(3) mean on a nautical chart?
Fl(3) means group flashing with three flashes in each repeated group.
How do you identify a navigational light at night?
Count the light's complete period, note its colour and flash pattern, then compare those details with the charted light description.
Keep revising this topic
Sources and factual review
Reviewed 2 September 2026 - Compass Revision editorial review.
Turn light descriptions into recognition practice
The visual aids module links light rhythms, sector lights, buoyage, and chart abbreviations so they are easier to recognise at night.