Methods to Calculate the Illumination of Light

For a commercial lighting design to be successful, each area of the building must have a sufficient level of light to allow users to perform their tasks. Thus, to determine the desired illumination, proper lighting calculations are to be made.

There are a number of methods employed for lighting calculations, the three main methods are given as follows −

  • Watt per Square Meter Method

  • Lumen Method

  • Point to Point Method

Watt per Square Meter Method

The watt per square meter method is fundamentally a rule of thumb method. This method of lighting calculation comes in handy when doing a quick calculation or double-check. It entails allocating watts per square metre of the area to be illuminated in accordance with the desired illumination, based on an average figure of overall system efficiency.

Lumen Method

The lumen method, also called the light flux method, is the method employed for lighting calculation is applicable to those cases where the sources of light are such as to produce an approximate uniform illumination over the working plane or where an average value is required.

Calculation −

In lumen method of lighting calculation, the total lumens output is determined from the size of the lamp or lamps employed and from their efficiency. Now, the lumens received on the working plane are determined by multiplying the total lumens output from the source with the coefficient of utilization.

In the case, when the lamps and surroundings are not perfectly clean, then in the determination of the lumens received on the working plane, the depreciation factor or maintenance factor should be included.

Thus, the lumens received on the working plane is given by,

$$\mathrm{\mathrm{Lumens \:received}\:=\:\mathrm{Number\:of\:lamps}\:\times \:\mathrm{Wattage\:of\:each\:lamp}\:\times\:\mathrm{Efficiency\: of\: each \:lamp\: in \:terms \:of\: lumens\: per\: watt}\:\:\times\:\mathrm{\left ( \frac{\mathrm{Coefficient\: of\: utilization}}{Depreciation\:factor} \right )}}$$

Also, the lumens received on the working plane may be given as,

$$\mathrm{\mathrm{Lumens \:received}\:=\:\mathrm{Number\:of\:lamps}\:\times \:\mathrm{Wattage\:of\:each\:lamp}\:\times\:\mathrm{Efficiency\: of\: each \:lamp\: in \:terms \:of\: lumens\: per\: watt}\:\times\:\mathrm{Coefficient\: of\: utilization}\:\times\:\mathrm{Maintenance\:factor}}$$

Using either of the above expressions, the lumens received on the working plane can be determined.

Inverse Square Law Method

The inverse square law method, also known as point-to-point method, is applicable where the illumination at a point due to one or more sources of light is required. Though, the candle power of the sources in the particular direction under consideration being known.

If a polar curve of a lamp and its reflector giving the candle power of the lamp in different directions is known, then the illumination at any point within the range of the lamp can be determined from the inverse square law.

In case, when two or more than two lamps are illuminating the same working plane, the illumination due to each lamp can be calculate and added by this method. However, this method is not much used due to its complications and heavy applications.