Chartered Institute of Building Service Engineers

(CIBSE)

‘code for interior lighting design’

The maintenance factor gives an estimate of how lighting

conditions will deteriorate through everyday use.

Dust and dirt

Aging of lamps

Cleaning of room surfaces

It should be noted that without detailed knowledge of the maintenance plan a maintenance factor of 0.8 must be used.

MF = RSMF x LMF x LLMF x LSF

Where:

RSMF:

Room Surface Maintenance Factor

(dirt on the surfaces of the room)

LMF

Luminaire Maintenance Factor

(dirt on luminaire)

Where:

LLMF:

Lamp Lumen Maintenance Factor

(dirt on the lamp)

LSF

Luminaire Survival Factor

(reduced light output due to lamps failing)

UF = lumens received on working plane

lumen output of luminaires

Utilisation factor takes account of the loss of

light due to absorption on room surfaces

Type of luminaire

Room index

Reflectance of room surfaces

A luminaire with a concentrated light output

directed on the working plane will have a

higher UF than a luminaire with a dispersed

output

This takes into account the length and

width of the given room, also the height of

the luminaires above the working plane

(Hm).

RI = L x W

(L + W) Hm

Brighter colours with high reflectance result

in a higher UF.

Note: A high UF means fewer lamps are

needed (see formula) resulting in a more

energy efficient light design.

Ceiling Walls

Floor

Room index

A garage 20m long by 12m wide with a ceiling

height of 6m is required to be illuminated to a

level of 200 lx at a working plane height of 1 m.

The luminaires will be suspended 2m below the

ceiling height. The reflectance's are as follows:

Ceiling 0.5, Walls 0.5, Floor 0.2.

The utilisation factor can be calculated from the

previous table.

Room index:

RI = L x W

(L + W) Hm Therefore:

= 20 x 12

(20 + 12) 3

= 240

96

= 2.5

Therefore UF is ……..

C.W.F

RI

UF

A garage 26m long by 16 m wide with a ceiling

height of 7m is required to be illuminated to a level

of 500 lx at a working plane height of 1.5 m. The

luminaires will be suspended 1.5m below the

ceiling height. The reflectance's are as follows:

Ceiling 0.3, Walls 0.5, Floor 0.2.

Calculate the utilisation factor from the previous

table.

Room index:

RI = L x W

(L + W) Hm Therefore:

= 26 x 16

(26 + 16) 4

= 416

168

= 2.48 (2.5)

UF

This is the ratio of space between luminaires

to their installed height above the working

plane.

Manufactures will generally specify a

recommended SHR to ensure their products

best performance.

Example

Room dimensions: Ceiling height = 4m

Working plane = 1m

(recessed luminaire)

Manufactures recommended SHR 2:1

Therefore: SHR = 2:1

= 6:3

Calculate the number of luminaires required for a given room. Dimensions: L = 20m W = 40m H = 8m • Luminaire suspended 1.5m from ceiling • Working plane 1m Manufactures recommended SHR (1.5:1)

Gap from working plane to luminaire = 5.5m

Therefore: SHR 1.5 : 1

8.25 : 5.5

Width = 40m = 4.8 luminaires

8.25

Length = 20m = 2.4 luminaires

8.25

s o u rce I ca n d e la

flo w lu m e n s

i l l u m in a n ce E lu x

Fig . 1 .9

Luminous Intensity

Luminous Flux

Illuminance

Luminance

Luminous Intensity

The measure of light from a source .i.e. a lamp. It

is measured in candelas (cd).

Symbol: I

Luminous Flux

It is the rate at which light flows from a source. It is

measured in lumens (lm).

Symbol: F

Illuminance

The measure of the amount of light falling on a surface. It is

measured in lux (lx).

Symbol: E

Luminance

Is the measure of the light intensity leaving the surface which

has been illuminated by a given source. It is measured in

candelas per meters squared (cd / m²)

Symbol: L

Typical recommended maintained Lux levels:

Corridors and stairs 100

Warehouses 100 - 200

Medium bench and machine work 500

Fine painting spraying and finishing 750

Printing inspection 1000

Proof reading / drawing offices 750

General offices (desk based) 500

General offices (screen based) 300

Supermarkets 750

E = (F x n) x N x MF x UF

A

Where:

E = Average Illuminance (lux)

F = Initial lamp lumen output (lm)

N = Number of luminaires

n = Number of lamps per luminaire

MF = Maintenance factor

UF = Utilisation factor

A = Area (m²)

This method is most suitable for interior lighting design,

where a high proportion of light on the working plane is

reflected by internal surfaces.

The lumen method, sometimes called the luminous flux

method of calculation, is normally used to calculate the

average illuminance (Lux) on working planes, or to calculate

the number of luminaires required providing a specified

average illuminance in rooms.

A factory floor, 30m x 20 m, is to be illuminated

with 2 x 58 W LPMV luminaires. Calculate the

number of luminaires needed to maintain an

average illuminance of 500 lux. The initial lamp

output is 5,200 lumens. Calculations are to be

carried out using a MF of 0.8 and a UF of 0.7

E = (F x n) x N x MF x UF

A

Therefore:

N = E x A

(n x F) x MF x UF

N = 500 x (30 x 20)

(2 x 5,200) x 0.8 x 0.7

= 51.51 (52 luminaires)

A hotel reception area, 18m x 11 m, is to be

illuminated with 40, 70 W, 2 pin pod MBI

luminaires. The initial lamp output is

6,600lm, the MF is 0.8 and the UF is 0.65.

Calculate the average illuminance in the

reception area. Also calculate the efficacy of

the lamp.

E = (F x n) x N x MF x UF

A

= 6600 x 40 x 0.8 x 0.65

18 x 11

= 693.33 lux

Efficacy = 6600 / 70 = 94.3 lm/W