How to Find No. LED Luminaries

Lumen method formula

The average illuminance E(h) over a reference surface s can be calculated from the "lumen method" formula.

E(h) = F x n x N x LLF x UF(s)
--------------------------
area of surfaces

where:

F = the initial bare lamp flux (lumens)
n = the number of lamps per luminaire
N = the number of luminaires
LLF = the total light loss factor
UF(s) = the utilization factor for the reference surface s of the chosen luminaire

Utilization factors can be determined for any surface or layout of luminaires. The "UF" symbol is normally shown followed by an extra letter in brackets, to denote the surface, for example, UF(F) is the utilisation factor for the floor cavity and UF(W) is the utilisation factor for the walls.

Utilization factors are, in practice, only calculated for general lighting systems with regular arrays of luminaires and for three main room surfaces. The highest of these surfaces, the C surface (for ceiling cavity), is an imaginary horizontal plane at the level of the luminaires having a reflectance equal to that of the ceiling cavity. The lowest surface, the F surface (for floor Cavity), is a horizontal plane at normal working height (i.e. table height), which is often assumed to be O.85m above the floor. The middle surface, the W surface (for walls), consists of all the walls between the C and F planes.

Although the lighting designer can calculate utilization factors, lighting companies publish utilization factors for standard conditions for their luminaires. To use this table, it is only necessary to know the Room Index and the effective reflectance of the three standard surfaces (floor cavity, walls and ceiling cavity).

Room Index

The Room Index is a measure of the angular size of the room, and is the ratio of the sum of the plan areas of the F and C surfaces to the area of the W surface. For rectangular rooms the room index is given by:

RI = L x W / (L + W)Hm

Where:

L = the length of the room
W = the width of the room
Hm = the height of the luminaire plane above the horizontal reference plane.