Basic Electrical Engineering

Illumination – Introduction, Important terms in illumination


Almost all human activities are based on light. Natural light is obtained from the sun. Artificial light plays an important role in our everyday life. In places where natural light is not available, artificial light is obtained by electric lamps. Lighting plays an important role because of its belief, consistency, simple control and low price. The electrical lighting is mainly used for domestic purpose, decorative purpose, advertising, traffic- control, medical field and for street lighting also. When light falls on a surface, it becomes visible and the phenomenon is called as illumination. It is denoted by E and is measured in lumen per square meter.

Important terms in illumination

Plane angle

The angle subtended at a point by two converging lines lying in the same plane is called plane angle, and is measured in radians. It is equal to the ratio of the length of the arc to its radius.

Solid angle

Solid angle is measured in steradians. Solid angle is the ratio of area of the surface to the square of radius of sphere.

Luminous flux

It is the light energy radiated out per second from the body in the form of luminous light waves. The unit of luminous flux is lumen (lm).


It is the unit of luminous flux. One lumen is defined as the luminous flux emitted per unit solid angle from a point source of one candle power.

Luminous intensity (I)

Luminous intensity or Candlepower of a point source in any particular direction is given by the luminous flux radiated out per unit solid angle in the direction.


The amount of light that causes a luminous flux over a square meter surface is called lux.

Mean Horizontal Candle Power (MHCP)

It is the mean of the candle powers in all directions in the horizontal plane containing the source of light.


Glare is difficulty seeing in the presence of bright light such as direct or reflected sun light or artificial light. It causes annoyance, discomfort or interference with vision or eye fatigue.

Space height ratio

Space height ratio is defined as the ratio of the distance between adjacent luminaries (center to center) to their height above the working plane.

Utilization factor

Utilization factor or coefficient of utilization is defined as the ratio of total lumens reaching the working plane to the total lumens given out by the lamp.

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