Module 12 - Visual Aids to Navigation
Isophase vs Occulting Lights: Meaning and Difference
What you will learn
Distinguish fixed, flashing, isophase, occulting, quick and grouped rhythms by timing one complete repeating period.
A rhythmic character is the repeating sequence of light and darkness. Start at the same recognisable point in successive cycles, count the group and compare how long the light and dark phases last.
Every navigational light has a defined character: the repeated pattern of light and dark that lets you identify it from a chart or the Admiralty List of Lights. The most common chart abbreviations are Fixed (F), Flashing (Fl), Isophase (Iso), Occulting (Oc), Quick (Q), Very Quick (VQ), Group Flashing, and Long Flash (LFl).
The key Day Skipper distinction is between flashing, isophase, and occulting lights. A flashing light is dark for longer than it is lit. An isophase light has equal light and dark periods. An occulting light is lit for longer than it is dark, as if the light is briefly eclipsed.
Put another way: flashing is mostly dark, isophase is evenly split, and occulting is mostly light. That comparison is often the fastest way to decode a chart abbreviation before you count the period.
To identify a light at night, count one complete period from the start of a pattern until the same point repeats. Then compare the character, colour, period, height, and range with the charted description.
Light rhythms are read as repeating time patterns
Read the whole repeating cycle: mostly dark, equal, mostly light, or rapid flashes.
Worked example
Decode Fl(2) W 10s and predict what to time.
- 1Fl(2) means two flashes form one repeated group; it does not mean each flash lasts two seconds.
- 2W means the observed sector is white.
- 310s is the full period from the same point in one two-flash group to the same point in the next.
Sense check: If two flashes are seen but the group-to-group interval is about five seconds, it does not match this 10-second character.
| 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 |
Time more than one cycle
Against shore lights, the first flash is easy to miss and a single stopwatch reading gives nine seconds for a charted 10-second group.
1. Choose a repeat point
Start at the first flash of a recognisable group.
2. Count several periods
Time three complete cycles, then divide the elapsed time by three.
3. Use the whole description
Compare colour, group, average period, bearing and neighbouring lights.
Sense check: Repeated timing reduces reaction error; it does not remove the need to confirm that the correct light was observed.
Common mistake or limitation
- The number in brackets counts flashes or eclipses in the group; it is not the period or range.
- Quick and very quick are rate classes, but cardinal identification also depends on the grouped pattern and mark context.
Recap
- Fl is mostly dark, Iso equal, and Oc mostly light.
- Count the group and time the complete repeating period.
- Confirm rhythm with colour, bearing and charted context.
Optional quick check
Section 2 of 6
A light is illuminated longer than it is dark in each regular cycle. Which character fits?
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 5 July 2026 - Day Skipper Revision.
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.