Abstract
To the navigation officers of the watch (OOW) on the navigation bridge the environmental condition of visual acuity is the most important factor for keeping a proper look-out, as regulated by the International Maritime Organization's Rule 5 which is collision prevention rule as called "COLREG". Navigation officers are required to keep a proper look-out for prevention of (ships) ollisions. There are many collision incidents at sea, and especially so under conditions of good visibility. This paper has two topics related to the illuminance and luminance from sunlight, as follows: The first topic is the introductory explanation of the illuminance inside the navigation bridge. The second topic is the sky luminance condition as seen by the OOW from the navigation bridge. These two topics are fundamental factors related to visual perception at sea. The OOW on the bridge has to understand that every care must be taken, especially in fine weather conditions.
