Monday, March 2, 2009

Predicting the Visual Range of Lights

Usually a navigator wants to know not only the identity of a light, but also the area in which he might reasonably expect to observe it. His track is planned to take him within range of lights which can prove useful during periods of darkness. If lights are not sighted within a reasonable time after prediction, a dangerous situation may exist, requiring resolution or action to insure safety of the vessel. However, the range at which a light can be sighted is not subject to exact prediction.

The approximate area in which a light can be observed is normally a circle with the light as the center, and the visual range as the radius. However, on some bearings the range may be reduced by obstructions. In this case the obstructed arc might differ with height of eye and distance. Also, lights of different colors may be seen at different distances. This fact should be considered not only in predicting the distance at which a light can be seen, but also in identifying it.

Visual Range of Lights
The condition of the atmosphere has a considerable effect upon the distance at which lights can be seen. Sometimes lights are obscured by fog, haze, dust, smoke, or precipitation which may be present at the light, or between it and the observer, but not at the observer, and possibly unknown to him. Although a light of low luminous intensity is more subject to being obscured by anyone of these conditions than is a light of high intensity, the visual or sighting range of even a light of very high luminous intensity is considerably reduced in such conditions. There is always the possibility of a light being extinguished. In the case of unwatched lights, this condition might not be detected and corrected at once. During periods of armed conflict, certain lights might be deliberately extinguished if they are considered of greater value to the enemy than to one's own vessels.

On a dark, clear night the visual range is limited primarily by one of two ways:
1. Luminous intensity
2. Curvature of the earth

A weak light cannot normally be expected to be seen beyond a certain range, regardless of the height of eye. This distance is called luminous range. Light travels in almost straight lines, so that an observer below the visible horizon of the light should not expect to see the light, although the loom extending upward from the light can sometimes be seen at greater distances. Table 8 (Bowditch) or Geographic Range Table from the Coast Guard Light List gives the distance to the horizon at various heights. The tabulated distances assume normal refraction. Abnormal conditions might extend this range somewhat (or in some cases reduce it). The geographic range, as the luminous range, is not subject to exact prediction at any given time.

The luminous range is the maximum distance at which a light can be seen under existing visibility conditions. This luminous range takes no account of the elevation of the light, the observer's height of eye, the curvature of the earth, or interference from background lighting. The luminous range is determined from the known nominal luminous range, called the nominal range, and the existing visibility conditions.

The nominal range is the maximum distance at which a light can be seen in clear weather as defined by the International Visibility Code (meteorological visibility of 10 nautical miles.)

The geographic range is the maximum distance at which the curvature of the earth permits a light to be seen from a particular height of eye without regard to the luminous intensity of the light. The geographic range sometimes printed on charts or tabulated in light lists is the maximum distance at which the curvature of the earth permits a light to be seen from a height of eye of 15 feet above the water when the elevation of the light is taken above the height datum of the largest scale chart of the locality.