Page 159 - Offshore Electrical Engineering Manual

146 CHAPTER 14 Offshore Lighting

FIGURE 2.14.1
Illumination effect of a point source of light.

unit for luminous intensity I is the candela (cd) or lumen per steradian; the imperial
unit used prior to this was the candlepower.
The illuminance, denoted by E, is the luminous flux falling on a surface. The unit
of illuminance is the lumen per square metre, such that:
I cos θ
E =
r 2 (2.14.1)
The inverse square law is only strictly applicable to point sources of light, but can
be applied for practical purposes to large diffusing sources provided that r is more
than five times the largest dimension of the source.
A typical point source calculation sheet is shown in Fig. 2.14.2.

LINEAR SOURCES
Large fluorescent lamps are obviously not point sources and any calculation must
take this into account. In Fig. 2.14.3 a linear source of length l is represented by AB.
The illuminance is required on P in the plane CD which is parallel to the source.
APEB is a plane which passes through the axis of the source, is at an angle θ to the
vertical and makes an angle ϕ with the normal PN to the plane CD.