Light Characteristics

Especially during the night, when most of the coastline features are not visible, lights are a great help to the navigator as reference and bearing points.

In order to distinguish one light from another and to allow a unique identification during the night, each light has distinctive optical characteristics. This includes a distinctive color and a distinctive sequence of light and dark intervals. This characteristic sequence is called phase characteristic.

Color Characteristics

The colors used for the identification of lighted beacons or marks are white, red or green. Some special beacons carry a yellow light. No other color is used for lights.

Besides the color of the beacon light, which is only visible between sunset and sunrise, beacons are painted with distinct daylight colors. Beacons may be painted with the following color combinations:

• complete red or red with an horizontal green band (lateral marks)
• complete green or green with an horizontal red band (lateral marks)
• a combination of horizontal yellow and black bands (caridinal marks)
• black with an horizontal red band (isolated danger marks)
• vertical red - white stripes (safe-water marks)

Phase Characteristics

One of the principal characteristics of a light is the duration of the characteristic sequence. The sequence duration or period is expressed in seconds and is the first identification characteristic. The period of a light should be measured with a stop watch to get a reliable identification.

The characteristic sequence of a light consists of an alternating pattern of light and dark intervals or phases. Depending on the ratio of light to dark phases, lights are distinguished in: fixed lights (F) : they are always on and have no dark phase; occulting lights (Oc): the duration of light phases is greater than the duration of the dark phases; flashing lights (Fl): the duration of the light phases is less than the duration of dark phases; isophase lights (Iso): the duration of light phase is equal to the duration of the dark phase. Depending on the structure of the characteristic light sequence, occulting and flashing lights may show single or group patterns. Single-occulting and single-flashing lights have a simple sequence of one light and one dark phase. Group-occulting and group-flashing lights have more complex sequences of multiple light and dark phases. Even more complex sequences are shown in composite group-occulting or composite group-flashing lights. Quick lights are special flashing lights. The flashing rate is higher than 60 flashes per minute. These lights may have a continuous (Q) or interrupted flashing (IQ) pattern. Very quick flashing lights have a rate of more that 120 flashes per minute. These lights also may have a continuous (VQ) or interrupted flashing (IVQ) mode. A light showing different colors alternatively is described as alternating light (AL). The alternating color may be combined with an additional light phase characteristic.

From the interactive lights page examples of lights with different phase and color characteristics can be selected and examined. It can be used as training aid for the determination of lights characteristics.

Almost all lights today are operated automatically. They are extinguished automatically during daylight hours and switched on during the night by light sensitive switches. Due to the variations of the sensitivity of the light switches, all lights are not switched on or off at the same time. Mariners should account for this when identifying navigation aids during twilight periods when some lights are on while other lights are extinguished.

Light Sectors

Sectors of colored glass or plastic are sometimes placed in the lanterns of certain lights to indicate dangerous waters. These lights show different colors when observed from different bearings. A sector changes the color of a light, but not its characteristic, when viewed from certain directions.
Sectors may sometimes be only a few degrees in width or extend in a wide arc from deep water toward shore. Bearings referring to sectors are expressed in degrees true as observed from an approaching vessel.

In most cases, areas covered by red sectors should be avoided. The nature of the danger can be determined from the chart. In some cases a narrow sector may mark the best water across a shoal, or a turning point in a channel.

The colors of different sectors change through an arc of uncertainty of 2° or greater, depending on the optical design of the light. Therefore bearings to lights should not be determined by observing the color change (sector boundary) but conventionally by using a hand-bearing compass.

Light Ranges and Weather Conditions affecting Light Characteristics

The condition of the atmosphere has considerable effect upon a light's range. Sometimes lights are obscured by fog, haze, dust, smoke or precipitation. On the other hand, refraction may cause a light to be seen farther than under ordinary circumstances.

Unfavorable weather conditions, may even change some of the lights characteristics. Haze and distance may reduce the apparent duration of a light's flash. In some conditions of the atmosphere, white lights may have a reddish hue. In clear weather green lights may have a more whitish hue. In regions where ice conditions prevail, an unattended light's lantern panes may become covered with snow or ice. This may reduce the light's luminous range and change the light's observed color.

The distance from a light cannot be estimated by its apparent brightness. There are many factors which can change the perceived intensity. Also, a powerful, distant light may sometimes be confused with a smaller, closer one with similar characteristics. Every light sighted should be carefully evaluated to determine if it is the one expected.

The presence of bright shore lights may make it difficult to distinguish navigational lights fro the background lighting. Lights may also be obscured by various shore obstructions.

A light's loom is seen through haze or the reflection from low-lying clouds when the light is beyond its geographic range. Only the most powerful lights can generate a loom. The loom may sometimes be sufficiently defined to obtain a bearing.

At short distance, some of the brighter flashing lights may show a faint continuous light, or faint flashes, between regular flashes. This is due to reflections of a rotating lens on panes of glass in the lighthouse.
The apparent characteristic of a complex light may change with the distance of the observer. This is because for a given candlepower, white is the most visible color, green less so and red least of the three. This fact also accounts for the different ranges given in the "Light List" for dome multi-color sector lights. The same lamp has different ranges according to the color imparted by the sector glass.