The principal cause of the ship collision is "Lookout insufficiency". Why cannot another ship be watched visually immediately before the collision in the place where the ship collides? This study uses "visibility level" (hereafter, it is written as VL) as an index to express the easiness to watch the ship at sea to clarify the cause. It is a purpose of this study to understand the realities of the lookout based on the VL, and to propose navigator's lookout points in addition. To examine the visual and perceptional conditions of the ship by the influence of the sun light at sea, the visual environment at sea (luminance of the sky, luminance of the sea surface, and luminance of the ship) was measured by the experiment on board. Author shows the items of the results on this study as follows; 1. The relation between VL and the distance to the ship. 2. The visual perception on the ship and the relation to VL.
