16394 Environmental Engineering Design
Lighting Design Lecture
Introduction
The prime objectives behind the design of a lighting system are as follows:
the safety and comfort of occupants – the nature of a task or process performed in a
space will dictate the illuminance level which must be provided by the lighting system
(lx or lm/m2). Tasks involving high degrees of visual acuity will require higher
lighting levels.
the minimisation of energy consumption – minimisation of energy consumption
involves the development of the most energy efficient lighting systems which is
suitable for the task, this can be achieved by selecting high efficiency equipment and
making use of available daylight.
colour rendering or the creation of a specific atmosphere – the colour
characteristics of a lighting scheme will affects tasks performed when the lighting
system is on. For example tasks which require the accurate representation of colour
require a light with the spectral characteristics of daylight. Alternatively,. to create a
“warm atmosphere” in a restaurant requires the selection of lights skewed to the red
end of the spectrum.
A lighting design has several stages. These are as follows:
1) Identification of the requirements for the lighting system, illuminance levels,
colour requirements, available space, etc;
2) Selection of equipment, lamps, luminaires: lighting systems consist of numerous
components, the two most important of which are: lamps, which influence the
lighting level, colour characteristics and efficiency of the lighting system;
luminaires affect the efficiency with which the light is distributed and so affect
lighting efficiency and uniformity
3) Design of the lighting system: lighting systems are designed to achieve a
reasonably uniform distribution of light on a particular plane (usually horizontal),
avoidance of glare with a minimum expenditure of energy. The most rudimentary
form of lighting design is done using a manual calculation – the lumen method.
However lighting design is increasingly done by computer.
4) System control: once a lighting system has been designed it can be controlled in
such a way as to make maximum use of available daylight, through selection of
appropriate switching mechanisms and daylight responsive controls.
16394 Environmental Engineering Design
Identification of Lighting Requirements
Lighting requirements are primarily dictated by the function of a space or the tasks
being performed within it. Lighting requirements are usually specified by the required
lighting level and the colour rendering requirements.
For example:

a general office requires an illumination level of 500 lux; colour rendering is mot
particularly important;

an architects drawing office requires higher lighting levels of 750 lux due the
more visually tasks being conducted within the office. In addition as colour work
is being done the lighting system must have good colour rendering.
Examples of lighting level requirements are given in the table below:
Building Area
Entrance hall
Corridors
Kitchens
General offices
Drawing Rooms
Standard Maintained
Illuminance (lux)
200
100
300
500
750
Classrooms
Lecture Theatres
Art rooms
300
300
500
Colour Rendering
Good colour rendering
may be required
Excellent colour rendering
required
Table 1 Lighting level requirements.
Lighting requirements are summarised in the CIBSE Code for interior lighting,
available from the library.
The colour properties of a particular lamp are supplied by the manufacturer in the
form of a lighting catalogue: the Philips lighting catalogue is available in the design
room alternatively the catalogue is available on-line at: www.philips.com
The colour rendering properties of a particular lamp are described by its colour
rendering group or the CIE number (Commission Internationale de L’Eclairage):
Colour
rendering group
1A
CIE colour rendering index
(Ra)
Ra > 90
1B
80 < Ra < 90
2
60 < Ra < 80
Comments
Used wherever accurate colour
matching is required
Used when accurate colour
matching is required e.g. clothes
shops
Where moderate colour rendering is
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3
4
required
40 < Ra < 60
Where colour rendering is
unimportant but distortion of colour
is unacceptable
60 > Ra
Where marked colour distortion is
acceptable e.g. street lighting
Table 2 Colour rendering groups
Selection of Components
Selection of components follows from the identification of systems requirements.
The luminaires are normally chosen first: lighting catalogues usually describe the uses
for particular types of luminaire, these also come with different types of reflectors1 for
different applications e.g. low glare reflectors for computer rooms.
casing
lamp
reflector
Figure 1 The components of a typical luminaire.
Lamps are selected based on those which are compatible (lamp type, dimensions,
frequency of operation, etc) with the selected luminaire and which have the
appropriate colour-rendering index.
For example the Philips TCS 660 luminaire comes with a choice of 8 different
reflector types.
The lamps that can be fitted to this luminaire must be Philips high frequency
fluorescent tubes that can be 1200mm or 1500 mm long. The luminaire can be fitted
with a maximum of 2 fluorescent tubes.
1
Reflectors govern the light output characteristic of a luminaire.
16394 Environmental Engineering Design
Design of the Lighting System
A simple means of designing lighting systems is achieved by means of the lumen
method; this is a simplified design approach to enable the designer to achieve an even
light distribution in spaces of reasonably simple geometry (i.e. rectangular).
The basis of the lumen method is the following equation:
N
E A
n  F  MF  UF
(1)
N - is the number of luminaires required;
E - is the required illuminance (lux);
A - is the area to be lit;
N - is the number of lamps per luminaire;
F - is the lamp lumen output (lumens);
MF - is known as the maintenance factor, which is a combination of three
factors;
UF - is the utilisation and is a function of the luminaire properties and room
geometry.
Utilisation Factor (UF)
The room geometry is a crucial factor in determining the utilisation factor term in the
lumen equation. Several parameters are important.
length L
width W
ceiling cavity
ceiling plane
hm
working plane
floor cavity
Figure 2 important dimensional parameters for the Lumen Method
In the Lumen method of design the room geometry is characterised by a room index
(K):
K
L W
( L  W )hm
(2)
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The reflectances (ρs) of surfaces are also important properties in the calculation of
terms for equation (1). The reflectances of walls floors and ceilings is represented by
a number between 0 and 1: 0 – no light is reflected back from the surface, 1 – all
incident light is reflected back from the surface.
The calculation of surface reflectances is more complicated for imaginary planes such
as the luminaire and working planes, in these cases the reflectances of these planes
must be calculated as an equivalent cavity reflectance (ρe):
n
e 
A
s 1
n
i
i
A
s 1
i
(3)
Where n is the number of surfaces in the cavity and A is the area of the surface within
the cavity.
The utilisation factor can be obtained once the surface reflectances (or effective
reflectances) are known along with the room index (K); each luminaire produced by a
manufacturer has a lookup table for UF (UF values in bold):
Reflectances
Room Index (K)
Ceiling
Wall
floor
0.75
1.00
…
5.00
0.7
0.3
0.2
0.45
0.51
…
0.69
Table 3 Utilisation Factors
Maintenance Factor (MF)
The maintenance factor is a value designed to account for the reduction in light output
from a lighting system due to: the ageing of the lamps and the accumulation of dirt
and dust on the light fittings and room surfaces. The MF is therefore time varying and
is the product of 4 factors:
Lamp lumen maintenance factor (LLMF) – a value between 0 and 1 which accounts
for the degradation of lamp output over time:
lumen output at time t
initial lumen output
The LLMF for any time t can be obtained from manufacturer’s data.
LLMF (t ) 
Lamp survival factor (LSF) – this accounts for the failure of lamps over time, if failed
lamps are replaced immediately this factor can be ignored.
Luminaire maintenance factor (LMF) – a value between 0 and 1 that accounts for dirt
and dust accumulation on the luminaire. The luminaire maintenance factor