Introduction and Basics

Introduction:

Light is the prime factor in the human life as all activities of human beings ultimately depend upon the light.
Light on surfaces on which it falls. Thus, the illumination makes the surface look more or less bright with a certain colour and is this brightness and colour which the eye sees and interprets as something useful, or pleasant or otherwise. Where there is no natural light, use of artificial light is made. Artificial lighting produced electrically, on account of its cleanliness ease of control, reliability, steady output, as well as its low cost is playing an increasingly important part in modern every day life. Light may be produced by passing electric currents through filaments as in the Incandescent lamps, through arcs between carbon or metal rods, or through suitable gases as in neon and other gas tubes. In some forms of lamps the light is due to fluorescence excited by radiation arising from the passage of electricity through mercury vapour. 

Specifications of good interior lighting scheme:

A well-designed lighting scheme will satisfy the following conditions:
· Adequate illumination according to the recommended lux value.
· Uniform distribution of light all over the working plane.
· Comfortable colour lighting of the visible objects.
· No glare or hard shadows.
· Minimum or economical cost scheme. 

Basic Definitions:

Light:

It is defined as the radiant energy from a hot body, which produces the visual sensation upon the human eye.

Luminous Flux (Φ):

It is defined as the total quantity of light energy emitted per second from a luminous body. It is represented by symbol F and is measured in lumens. 

Illumination (E):

The number of lumens, falling on the surface per unit area. It is denoted by symbol E and is measured in lumens per square meter (lu/m2) or (lux).

Luminous Intensity (I):

The luminous flux from a source in a specified direction inside a small solid angle and is measured in lumen per steradian (lu/ster) or candela (cd)

Luminance (L):

It is the intensity per apparent unit area of the surface of the actual light source and is measured in candela per square meter (cdm-2)

Reflectance:

The ratio of reflected flux to incident flux (either luminous or radiant)

Luminous Efficacy (η):

It is defined as the ratio of the luminous flux to the power input and is measured in lumen per watt (L/W). 

Utilization factor (U.F):

The ratio of the lumens actually received by a particular surface to the total lumens emitted by a luminous source. This factor is varies widely according to the following factors:
a-The type of lighting system whether it is direct or indirect.
b-The shape and relative dimension of the room.
c-The wall surface and its colour fit reflectance of light. 

Maintenance factor (M.F):

During the deterioration of lamp accumulation of dust on the globes and reflectors, the efficiency of reflection of light from walls due to the insufficient clearing. The illumination under normal conditions usually less than that when every thing is perfectly clear the relative between the two cases is the maintenance factor.