Calculation of mean illuminance
In practice, it is often necessary to obtain an estimate of the mean intensity of illuminance (En) for a given level of electrical power supplied, or the electrical power P required for a given level of illumination. En and P can be estimated from the formula in
En= Nominal illuminance (lx)
P = Connected load
P* = Specific connected load (W/m^2)
A = Room floor area
k = Corection factor
The specific power p* required for this calculation depends on the type of lamps used and relates to direct illumination. The correction factor k depends on the size of the room and the reflection levels of the walls, ceiling and floor.
If the calculation is to be made for rooms with different types of lighting, the components are calculated individually and then added together.
Calculation of the illumination using the specific power is also applicable to offices, In the example, a rectangular room with an area of 24m^2 is equipped with 4 lights. From above formula with 2 x 36W lamps (connected value, including 90W ballast), an illuminance of ca.375lx is achieved.
In offices, in addition to conventional louvered mirror lighting, square louvered lighting with compact fluorescent lamps or structured lighting are frequently installed. Lighting structures use a combination of power supply rails to carry spotlights.
The luminous flux required for lamps used to floodlight a building can be calculated from the formula in
Ø =π x L x A / Ƞʙ x ϱ
Ø = Luminous flux required
L = Mean luminance (cd/m^2)
A = Surface to be flooding
Ƞʙ= Lighting efficiency factor
ϱ = Reflection factor for the material
The luminance should be between 3cd/m^2 (free standing objects ) and 16dc/m^2 (objects in very bright sorroundings)