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Virtually all buildings have electric lighting which serves two purpose. It help us to recognise objects quickly and in sufficient detail to learn all we need to know about them, and it contributes to making buildings safe and pleasant place in which to work or take part in other activities.

There must always be enough light to make object visible but other factors are no less important. The directions from which light come, the brightness and color contrasts created between details of interest and their background, the presence or absence of bright reflections in the part of the object being looked at, and changes in color resulting from the type of lamp used can all effect ease of recognition.

Some of the more common terms used in lighting design and the associated units are given below
Luminous flux. The light emitted by a source, or received by a surface. It is expressed in lumens. Symbol: Ø
Lumen. This is the SI (Standard International) unit of luminous flux. An ordinary 100W lamp for example emits about 1200 lumen. One lumen is the luminous flux emitted within solid angle (one steradian) by a point source having intensity of one candela. Symbol: lm
Luminous intensity. The quantity which describes the power of a source or illuminated surface to emit light in a given direction. It is the luminous flux emitted in very narrow cone containing the given direction divided by the solid angle of the cone. The result is expressed in candelas. Symbol: I
Candela. The SI unit of intensity. It is lumen per steradian. Symbol: cd
Illuminance. The luminous flux density at a surface, i.e. the luminous flux incident per unit area. The quantity was formerly know as the illumination value or illumination level. It is expressed in lux (lumens/m2 or lm/m2). Symbol: E
Lux. SI unit of illuminance. It is equal to one lumen per square meter.
Room index. An index related to the dimensions of a room, and used when calculating the utilization factor and the characteristics of a lighting installation. It is given bellow
lw / hm (l + w)
where l is the length and w the width of the room and hm the height of luminaires above the working plane.

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