Light List, Ranges, and Light Characteristics

TL;DR — Know the six primary light characteristics (Fl, Oc, Iso, FFl, Al, and group flashing variants) by their abbreviation, their light/dark ratio, and how they appear in the Light List entry. The exam will ask you to identify a characteristic from its description or abbreviation, and to read a charted string such as "Fl R 4s" correctly.

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What the Rule Says

The Light List is the authoritative publication for identifying aids to navigation by their light characteristics. Each characteristic is defined by the relationship between the duration of light and the duration of darkness within one complete period, and by whether the light is single, grouped, or alternating in colour. The following characteristics appear in the supplied source material and are directly testable on the OUPV and Master 100 GT written exams.

Flashing (Fl)

A flashing light produces a single flash that is regularly repeated. The critical defining feature is that the duration of light is shorter than the duration of darkness within each period. Light List — light-characteristic (Fl) A charted entry reads "Fl \<colour\> \<period\>s" — for example, "Fl R 4s" means a single red flash every 4 seconds. Light List — light-characteristic (Fl) When the light is white, the colour designator is omitted from the characteristic string. Light List — light-characteristic (W)

Occulting (Oc)

An occulting light is the opposite of flashing. It is a continuous light that is interrupted by regular short eclipses (periods of darkness). The defining feature is that the duration of light is longer than the duration of darkness. Light List — light-characteristic (Oc) Think of it as a light that is mostly on, briefly interrupted. The charted abbreviation is "Oc."

Isophase (Iso)

An isophase light has equal periods of light and darkness. Neither the light phase nor the dark phase dominates. Light List — light-characteristic (Iso) It is charted "Iso \<colour\> \<period\>s." Isophase sits precisely between flashing and occulting in terms of the light/dark ratio — a useful mental anchor for keeping the three straight.

Fixed and Flashing (FFl)

A fixed and flashing light is a fixed (continuous) light with a brighter flash superimposed at regular intervals. Light List — light-characteristic (FFl) The base light never goes out; the superimposed flash is simply more intense. This characteristic is used on certain channel-edge structures. Light List — light-characteristic (FFl) The abbreviation is "FFl."

Group Flashing — 3 and Group Flashing — 4

Group flashing lights produce multiple flashes per group at a fixed period, rather than a single flash. A Fl(3) light shows three flashes per group. Light List — light-characteristic (Fl(3)) A Fl(4) light shows four flashes per group. Light List — light-characteristic (Fl(4)) The number in parentheses always tells you how many flashes are in each group. Like the basic flashing characteristic, the duration of light within each flash is shorter than the intervening darkness.

Alternating (Al)

An alternating light shows different colours alternately — it does not stay one colour throughout its period. Light List — light-characteristic (Al) A common example is "Al WR," which alternates between white and red. Light List — light-characteristic (Al) The alternating characteristic is independent of the flash pattern; the light cycles through its colours in sequence.

White as the Default Colour

White is the default light colour and is omitted from the characteristic string when the light is white. Light List — light-characteristic (W) A charted entry that reads simply "Fl 4s" with no colour designator is a white flashing light with a 4-second period. When a colour other than white is present — red, green, or yellow — that colour abbreviation appears between the characteristic abbreviation and the period.

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Why It Matters on the Exam

OUPV and Master 100 GT written exams test light characteristics in several ways. Understanding the pattern of each type is essential.

Reading a charted string. Given "Fl R 4s," you must know this is a single red flash every 4 seconds, with the flash duration shorter than the dark interval. Light List — light-characteristic (Fl) Given "Iso 6s" with no colour, you must know this is a white isophase light with a 6-second period — equal light and dark — and that white is omitted because it is the default. Light List — light-characteristic (Iso) Light List — light-characteristic (W)

Identifying a characteristic from a description. If the exam states "a light in which the duration of light is longer than the duration of darkness," the answer is Occulting (Oc). Light List — light-characteristic (Oc) If it states "a light in which the duration of light is shorter than the duration of darkness," the answer is Flashing (Fl). Light List — light-characteristic (Fl) If it states "equal periods of light and darkness," the answer is Isophase (Iso). Light List — light-characteristic (Iso)

Distinguishing FFl from Fl. A fixed and flashing light never goes dark — the base light is continuous. Light List — light-characteristic (FFl) A plain flashing light goes completely dark between flashes. Light List — light-characteristic (Fl) Exam distractors will conflate these two.

Counting flashes in a group. Fl(3) means three flashes per group; Fl(4) means four. Light List — light-characteristic (Fl(3)) Light List — light-characteristic (Fl(4)) The exam may describe a light showing "four flashes followed by a period of darkness, repeated at a fixed interval" and ask for the abbreviation — the answer is Fl(4).

Alternating versus single-colour. An alternating light changes colour within its period. Light List — light-characteristic (Al) A light charted "Al WR" is not a red light and not a white light — it is both, in sequence. Exam questions may ask what "Al WR" means; the answer is a light alternating between white and red.

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Common Pitfalls

Confusing Fl and Oc. These are the most commonly reversed pair. Remember: Flashing = mostly dark (light shorter than dark); Occulting = mostly light (dark shorter than light). Light List — light-characteristic (Fl) Light List — light-characteristic (Oc) A memory device: "Occulting" shares a root with "occult" — to hide briefly. The light hides briefly, then returns. The light phase dominates.

Assuming a white light is always labelled "W." White is the default and is deliberately omitted. Light List — light-characteristic (W) If you see "Fl 4s" with no colour, do not assume the colour is unknown — it is white. Exam questions will test whether you know this convention.

Treating FFl as a type of flashing light. Fixed and flashing is a distinct characteristic. The base light in FFl is continuous; it never extinguishes. Light List — light-characteristic (FFl) A plain flashing light has no base — it is dark between flashes. Light List — light-characteristic (Fl) Do not select "Fl" when the description says the light is always visible with a brighter flash superimposed.

Misreading the number in group flashing. Fl(3) is three flashes, not a 3-second period. Light List — light-characteristic (Fl(3)) The period in seconds appears after the colour designator, not in the parentheses. The parenthetical number is always the flash count.

Assuming Iso means the period is always 2 seconds. Isophase means equal light and dark phases, not any specific total period. Light List — light-characteristic (Iso) The period is stated separately in the charted string.

Confusing Al with a sector light. An alternating light changes colour as part of its characteristic — it cycles through colours over time at any bearing. Light List — light-characteristic (Al) A sector light shows different colours in different arcs of bearing. These are different concepts; do not conflate them.

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Quick Check

Q1 — A light is described as "a continuous light interrupted by regular short eclipses, with the duration of light longer than the duration of darkness." What is the correct abbreviation?

Oc — Occulting. The duration of light exceeds the duration of darkness, and the light is continuous with brief interruptions. Light List — light-characteristic (Oc)

Q2 — A charted entry reads "Fl 6s" with no colour designator. What colour is the light, and what does the characteristic mean?

White, single flash every 6 seconds, with the flash duration shorter than the dark interval. White is the default colour and is omitted from the characteristic string. Light List — light-characteristic (W) Light List — light-characteristic (Fl)

Q3 — What distinguishes a Fixed and Flashing (FFl) light from a plain Flashing (Fl) light?

In FFl, a continuous base light is always present; a brighter flash is superimposed at intervals. The light never goes completely dark. In a plain Fl light, there is no base — the light is completely dark between flashes. Light List — light-characteristic (FFl) Light List — light-characteristic (Fl)

Q4 — A light shows four flashes, then a period of darkness, then four flashes again, repeating at a fixed period. What is the correct Light List abbreviation?

Fl(4) — Group Flashing, four flashes per group. Light List — light-characteristic (Fl(4))

Q5 — A light is charted "Al WR." What does this mean?

The light alternates between white and red. The "Al" abbreviation denotes an alternating light showing different colours in sequence. Light List — light-characteristic (Al)

Q6 — Which light characteristic has exactly equal durations of light and darkness?

Isophase (Iso). Equal periods of light and darkness define this characteristic. Light List — light-characteristic (Iso)

Q7 — Rank Flashing, Isophase, and Occulting in order from the characteristic with the least light (relative to darkness) to the most light.

Flashing → Isophase → Occulting. Flashing has light shorter than dark; Isophase has equal light and dark; Occulting has light longer than dark. Light List — light-characteristic (Fl) Light List — light-characteristic (Iso) Light List — light-characteristic (Oc)