Marine Glossary

|| || || || ||

visual range

1. The maximum distance (usually horizontal) at which a given object can be seen by day in any particular circumstance, as limited by the atmospheric transmission. The distance is such that the contrast of the object with its background is reduced by the atmosphere to the contrast threshold value for the observer. This term should not be confused with visibility. See also contrast threshold.

2. Of a light: the predicted range at which a light can be observed. The predicted range may be either the luminous range or the geographic range. Therefore, in predicting the range at which a light can be seen, one first determines the geographic range to compare this range with the luminous range, if known.

If the geographic range is less than the luminous range, the geographic range must be taken as the limiting range. If the luminous range is less than the geographic range, the luminous range must be taken as the limiting range. 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, using the Luminous Range diagram (G198). 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 and terrestrial refraction permit a light to be seen from a particular height of eye without regard to the luminous intensity of the light.

Note that the geographic range sometimes printed on charts or tabulated in light lists is the maximum distance at which the curvature of the earth and refraction permit 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.