 
2. Lumen Method
Calculations
The lumen method is based on
fundamental lighting calculations. The lumen method formula is easiest to appreciate in
the following form.
(1)
where E = average
illuminance over the horizontal working plane
n = number of
lamps in each luminaire
N = number of
luminaire
F = lighting
design lumens per lamp, i.e. initial bare lamp luminous
flux
UF
= utilisation factor for the horizontal working plane
LLF = light loss
factor
A = area of the horizontal working plane
2.1 Light Loss Factor
Light loss factor (LLF) is
the ratio of the illuminance produced by the lighting installation at the some specified
time to the illuminance produced by the same installation when new. It allows for effects
such as decrease in light output caused by
(a) the fall in lamp luminous flux with
hours of use,
(b) the deposition of dirt on luminaire,
and
(c) reflectances of room surfaces over
time.
In fact, light loss factor is the product
of three other factors:
(2)
where LLMF = lamp lumen maintenance factor
LMF = luminaire maintenance factor
RSMF = room surface maintenance factor
2.1.1 Lamp Lumen Maintenance Factor
Lamp lumen maintenance factor
(LLMF) is the proportion of the initial light output of a lamp produced after a set time
to those produced when new. It allows for the decline in lumen output from a lamp with
age. Its value can be determined in two ways:
(a) by consulting a lamp manufacturer's
catalog for a lumen depreciation chart, and
(b) by dividing the maintained lumens by
the initial lamps.
2.1.2 Luminaire Maintenance Factor
Luminaire maintenance factor
(LMF) is the proportion of the initial light output from a luminaire after a set time to
the initial light output from a lamp after a set time. It constitutes the greatest loss in
light output and is mainly due to the accumulation of atmospheric dirt on luminaire. Three
factors must be considered in its determination:
(a) the type of luminaire,
(b) atmospheric conditions, and
(c) maintenance interval.
2.1.3 Room Surface Maintenance Factor
Room surface maintenance factor
(RSMF) is the proportion of the illuminance provided by a lighting installation in a room
after a set time compared with that occurred when the room was clean. It takes into
account that dirt accumulates on room surfaces and reduces surface reflectance. Figure 4
shows the typical changes in the illuminance from an installation that occur with time due
to dirt deposition on the room surfaces.
2.2 Utilisation Factor
Utilisation factor (UF) is
the proportion of the luminous flux emitted by the lamps which reaches the working plane.
It is a measure of the effectiveness of the lighting scheme. Factors that affect the value
of UF are as follows:
(a) light output ratio of luminaire
(b) flux distribution of luminaire
(c) room proportions
(d) room reflectances
(e) spacing/mounting height ratio
2.2.1 Light Output Ratio of Luminaire
Light output ratio of
luminaire (LOR) takes into account for the loss of light energy both inside and by
transmission through light fittings. It is given by the following expression.
(3)
Example 1
The total, upward
and downward lamp output from a lamp are 1000 lm, 300 lm and 500 lm respectively.
Calculate upward light output ratio (ULOR), downward light output
ratio (DLOR), light output ratio (LOR) of luminaire and percentage of light energy
absorbed in luminaire.
Amount of light energy absorbed in
luminaire = 100  80 = 20 %
A greater DLOR usually means a higher UF.
A simple classification of luminaires
according to their distribution is based on flux fractions, as shown in Figure 5. Upward
flux fraction (UFF) and downward flux fraction (DFF) are used as a
basis of comparison.
Example 2
For data given in Example 1
determine upward flux fraction (UFF), downward flux fraction (DFF) and flux fraction
ratio (FRR).
Figure 5 Flux Fraction of Various Luminaires
2.2.2 Flux Distribution of Luminaire
Direct ratio is the proportion of the total
downward luminous flux from a conventional installation of luminaires which his directly
incident on the working plane. It is used to assess the flux distribution of luminaire.
Since the intensity distribution pattern of the light radiated from a luminaire in the
lower hemisphere will affect:
(a) the quantity of the downward flux falls
directly on the working plane and
(b) the quantity of flux available for
reflection from the walls in a given room,
Direct ratio depends on both the room
proportions and the luminaires. Direct ratio has a low value with a narrow
room (small room index) and a luminaire which emits most of its light sideways (BZ 10),
and on the contrary, a high value with a wide room (large room index) and a luminaire
which emits most of its light downwards (BZ 1).
2.2.3 Room Proportion
Room index (RI) is the ratio
of room plan area to half the wall area between the working and luminaire planes.
(4)
where L = length of room
W = width of room
Hm = mounting height, i.e. the vertical
distance between the working plane and the luminaire.
2.2.4 Room Reflectances
The room is considered to consist of three
main surfaces:
(a) the ceiling cavity,
(b) the walls, and
(c) the floor cavity (or the horizontal
working plane).
The effective reflectances of
the above three surfaces affect the quantity of reflected light received by the working
plane.
2.2.5 Spacing to Height Ratio
Spacing to Height ratio (SHR
or S/Hm) is defined as the ratio of the distance between adjacent luminaires (centre to
centre), to their height above the working plane. For a rectangular arrangement of
luminaires and by approximation,
(5)
where A = total floor area
N = number of luminaires
Hm = mounting height
Under a regular array of luminaires the
illuminance on the working plane is not uniform. The closer spaced the luminaires for a
given mounting height, the higher the uniformity; or the greater the mounting height for a
given spacing, the greater the uniformity. If uniformity of illuminance is to be
acceptable for general lighting,
(a) SHR should not exceed maximum
spacing to height ratio (SHR MAX) of the given luminaire as quoted by the
manufacturer, and
(b) geometric mean spacing to height ratio
of the luminaire layout should be within the range of nominal spacing to height
ratio (SHR NOM) of the given luminaire as quoted by the manufacturer, i.e.
(6)
