Page 164 - Offshore Electrical Engineering Manual

Lighting Calculations 151

UTILISATION FACTORS FOR INTERIORS AND AVERAGE ILLUMINANCE
The utilisation factor UF(S) for a surface S is the ratio of the total flux received by S
to the total lamp flux of the installation. Utilisation factors are used to calculate the
number of luminaires needed to provide a given illuminance on a surface. UFs vary
according to the light distribution of the luminaire, the geometry of the room, the
layout of the luminaires and the reflectance of the reflecting surfaces.
The average horizontal illuminance E (S) produced by a lighting installation, or
h
the number of luminaires required to achieve a specific average illuminance, can
be calculated by means of utilisation factors using the ‘lumen method’ formula as
follows:-
F × n × N × MF × UF (S)
E (S) =
A h (S)
where F is the initial bare lamp flux, n is the number of lamps per luminaire, N is the
number of luminaires, MF is the maintenance factor associated with the deterioration
due to dirt, dust and lamp ageing, A (S) is the area of the horizontal reference surface
h
S and UF(S) is the utilisation factor for the reference surface S.
Although utilisation factors may be calculated by the lighting designer (see
Bibliography), most manufacturers publish utilisation factors for standard conditions
of use and for three main room surfaces. The first of these surfaces, the C surface
is an imaginary horizontal plane at the level of the luminaires, having a reflectance
equal to that of the ceiling cavity. The second surface, the F surface, is a horizontal
plane at normal working height which is usually assumed to be 0.85 m above the
floor. The third surface, the W surface, consists of all the walls between the C and F
planes. A typical table of utilisation factors is shown in Fig. 2.14.5.
The room index is a measure of the angular size of a room, and is the ratio of the
sum of the areas of the F and C surfaces to the area of the W surface, each rectangular
area of room being treated separately. For rectangular rooms, the room index is given
by:-
LW
RI =
(L + W) H
where L is the room length, W is the room width, and H is the height of luminaire
plane above the horizontal reference plane. The effective reflectances (ratio of
reflected flux to incident flux) are needed for the wall surface, ceiling cavity and the
floor cavity.
The wall surface will consist of a series of areas A to A of different reflectances
1
n
R to R respectively. The effective reflectance of a composite surface is the area-
n
1
weighted average R , given by:
a
n RkAk
∑
k= 1
n
R a = ∑
k= 1 Ak