The Marine traffic system consists of human being, vessels and environment. This study shows the physical characters of the visual environment which is the background of the visual perception of the targets at sea for safety and for a good lookout in preventing maritime traffic accidents. Author shows three points. The first is the illuminance at the place of fore inside of the navigation bridge. The second is the luminance at the view point (the sky) about 2 degrees over horizon. The third is the luminance at the view point (the sea surface) about 2 degrees under horizon. All measurements were carried out through the windshield glasses of the navigation bridge. The main results are as follows; 1. The illuminance at the place of fore inside of the navigation bridge is standardized by the sun's altitude. The relation between illuminance at the navigation bridge and the sun's altitude is under; (Sun's Altitude in degree)=(Illuminance of approximate value in lux : lx) : (more 10 degrees)=(1000〜10000 lx) (-3 degrees)=(10 lx) (10 degrees)=(1000 lx) (-6 degrees)=(1 lx) (0 degree)=(100 lx) (-9 degrees)=(0.01〜0.1 lx) 2. The luminance at the view point (the sky about 2 degrees over horizon) towards the sun's direction in case of fine weather and in summer is about 7, 000〜27, 000 cd/m^2. This luminance can be shown by the approximate formula Y=a/X+b (Y : luminance in cd/m^2, X : sun's altitude in degree, a : constant=one million, b : constant=-5, 000). 3. The luminance at the view point (the sea surface about 2 degrees under horizon) towards the sun's direction in case of less than 30 degrees of sun's altitude has sun glitter's influence which makes watch officers to prevent his visual perception.
