The purpose of a navigation light is to show presence, aspect and occupation: can you see it—can he see it—do you both know what you are seeing?

Radiation is measured by ‘luminous flux’; the unit of which is the ‘lumen’, but that is not what we need to know. We need ‘luminous intensity’, the amount of light emitted in one particular direction and normally called the candlepower, the unit for which is the ‘candela’. Figure 1 shows how brilliance, in candela, affects range. For the purposes of Imco calculations a transmissivity factor of 0·8 is used; K = 0·74 corresponds to the Iala standard and 0·9 to the old ‘dear dark night’ standard used for oil lanterns. A sailing yacht has also to consider voltage drop and Fig. 2 shows how a lead acid battery while being charged might produce 14 volts, giving a 10 W bulb a brilliance of 22 cd, whereas the same battery not being charged might give as little as 9 cd.