水平線の視認と見張り

Abstract

The visual perception of the targets at sea is the most important factor for safety and for a good lookout in preventing maritime traffic accidents. This study shows the conditions for being able to perceive the visible horizon between the sky and sea surface and this. Author shows the visual and nautical conditions thereof ; they are as follows; 1. The conditions of visual perception of the line of division between the sky and sea surface is that the luminance difference of the sky (about 2 degrees over horizon) and sea surface (about 2 degrees under horizon) is more than the luminance difference threshold. 2. When nobody can perceive the above line of division (horizon), the visual and nautical environment is indicated as follows; (1) The luminance of the sky (about 2 degrees over horizon) is 0.3cd/m^2. (2) The luminance of the sea surface (about 2 degrees under horizon) is 0.03cd/m^2. (3) The difference luminance threshold is 0.27cd/m^2. But in the case of 0 degree which is a relative direction towards the direction of the sun. 3. A sun altitude with above the mentioned condition is -6〜-9 degrees (1/3〜1/2 duration of astronomical twilight), which is connected with the relative direction towards the direction of the sun. 4. The luminance of the sky and sea surface and the illuminance of the sky have gone up and down with the sun's altitude, and they change rapidly and widely with the case of the time period from sunset and sunrise to -9 degrees of the sun's altitude. 5. When under way the time period from sunset and sunrise to -9 degrees of the sun's altitude is needed to keep a special caution for a good lookout and the safety of the ship.