Wednesday, March 4, 2009

How to Compute the Visibility of a Light

The following terms and their definitions as used in the Light List are used in connection with the range of visibility of a light.

Horizon distance (or distance of visibility) is the distance, expressed in nautical miles, from a position above the surface of the earth along the line of sight to the horizon. It is the approximate range of visibility of an object of known height to an observer whose eye is at sea level. Horizon distances for various heights are given in Table 8 of Bowditch (Green). A similar table can be found in the introductory pages of each volume of the Light Lists. These tables are calculated for standard conditions of weather and refraction.

Nominal range is the maximum distance at which a light may be seen in clear weather (meteorological visibility of 10 nautical miles, (see International Visibility Code), expressed in nautical miles. Nominal range is listed for all Coast Guard lighted aids except range and directional lights.

Luminous range is the maximum distance at which a light may be seen under the existing visibility conditions. Luminous range may be determined from the known nominal range and the existing visibility conditions. Nominal and luminous ranges take no account of elevation, observer's height of eye, or the curvature of the earth.

Geographic range is the maximum distance at which a light may be seen under conditions of perfect visibility, limited only by the curvature of the earth. It is expressed in nautical miles for a height of observer's eye at sea level.

Computed range is the geographic range plus the observer's distance to the horizon based on his height of eye.

Computed visibility is the visibility determined for a particular light, taking into consideration its height and nominal range, and the height of eye of the observer.

These are some Coast Guard visibility of light problems that you may have on a exam and a format that may help. Remember: D = √ (square root) ht x 1.17