Illumination in Electrical Engineering


The Illumination at a point on a surface is defined as the luminous flux falling per unit area on a surface and it is expressed in lumen/m2.

Lux: Lux is the unit of Illumination. It is the Illumination. Created on the inner surface of a sphere having a one-meter radius by a point source of light energy of one candle power.

Lumen: Lumen is the unit of luminous flux which is equal to the flux emitted per unit solid angle from and uniform source of one candle power.

Luminous Flux: It is the light energy radiated per second from a luminous body in the form of light waves. It is measured in lumen.

Luminous Intensity: Luminous flux emitted by the sources per unit solid angle in any particular direction is known as luminous intensity. Candela is the unit of luminous intensity.

Luminous Efficiency: It is the ratio of the luminous flux obtaining to the power intake of the light energy source. The unit of luminous efficiency is Lumen-Watt.

Glare: Glare means the brightness within the field of vision to a lower level. It is not desirable as it causes discomfort and fatigue ness in our eyes. This effect of glare can be avoided by using either pearl or opal incandescent lamps or fluorescent lamps.

Space/Height ratio: It is the ratio of the horizontal distance between two lamps to the mounting height of the lamps above the working planes. More uniform illumination can be obtained with a lesser space/height ratio. For indoor lighting generally the Space/height ratio is about 0.5 to 2.0.

Utilisation Factor: It is the ratio of the total lumens that are being utilized on the working plane to the total lumens emitted by the lamp. The utilization factor varies from 0.3 to 0.6 for direct lighting and for indirect lighting the value of the utilization factor varies from 0.2 to 0.3.

Maintenance Factor: The total Lumens emitted per watt of the Lamp throughout its whole life can’t maintain its original illumination. The total Lumens emitted per watt decreases for dirt, dust, etc. With time this factor varies from 0.6 to 0.8. The reciprocal of maintenance factor is known as the depreciation factor. It is varying from 1.25 to 1.6 normally.