A number of methods have been employed for lighting calculations, among which may be mentioned as: 1. Watts for Square Meter Method 2. Lumen or Light Flux Method 3. Point to Point or Inverse-Square Law Method.
1. Watts for Square Meter Method:
This is principally a “rule of thumb” method, very handy for rough calculation or checking. It consists in making an allowance of watts per square meter of area to be illuminated according to the illumination desired on the assumption of an average figure of overall efficiency of the system.
2. Lumen or Light Flux Method:
This method 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.
From the size of lamps employed and from their efficiency total lumens outputs are determined. Multiplying the total lumens output from the source by coefficient of utilisation, the lumens received on the working plane are determined. If the lamps surroundings are not perfectly clean, then in determination of lumens received on working plane, the depreciation factor and maintenance factor should be included i.e. lumens received on working plane.
3. Point to Point or Inverse-Square Law Method:
This method is applicable where the illumination at a point due to one or more sources of light is required, the candle power of the sources in the particular direction under consideration being known.
If two and more than two lamps are illuminating the same working plane, the illumination due to each can be calculated and added. This method is not much used because of its complicated and cumbersome applications. It is employed only in some special problems, such as flood lighting, yard lighting etc.