**Definition of luminous intensity**

Luminous intensity – symbol l, unit candela (cd)

Luminous intensity is the concept for the concentration of light in a specific direction, radiated per second. It is designated by the symbol l. The unit is the candela (cd). The luminous intensity can be defined as :the luminous flux in a certain direction, radiated per unit of solid angle (steradian).

This brings us to the concept of the solid angle, and its unit: the ‘steradian’. A solid angle can best be described as a measure for a portion of space around a point bounded by the surface of a cone whose top (vertex) is at the point.

In general a light source will not radiate its luminous flux uniformly in all directions. If, however, we imagine a sufficiently narrow cone, with its vertex at the light source (which is considered as a point ), then the luminous flux contained in this cone will approximate a uniform distribution. The concentration of luminous flux within this narrow cone can now be defined as the luminous flux in this cone, divided by the opening of the cone, expressed in terms of the solid angle of the cone. The result is called the luminous intensity (l), measured in candelas (cd), in the direction of the center-line of the cone.

A Steradian is a measure for a solid angle

The si2e of the solid angle is expressed in a similar manner as with a plane angle, either in degrees or radians. Imagine a sphere of arbitrary radius (r), with the top of the cone as its center. The part of the surface of the sphere which is enclosed by the cone is then proportional to the solid angle (w) of the cone. If the area of the surface of the sphere covered by the cone is equal to the square of the radius (r2 ), then the corresponding solid angle is termed a steradian/. So, if the enclosed area of the surface of the sphere is not equal to r2 , but is, say, equal to A, then the solid angle:

*A whole sphere contains 4p steradians*

*The maximum possible solid angle will enclose a whole sphere. As the surface of a
sphere of radius r is 4pr2 , this solid angle will be equal to:*

*Hence, a half- sphere will contain 2p steradians.*

*The concept of luminous intensity is of tremendous importance in lighting technology, as virtually no piece of lighting equipment emits its luminous flux equally in all directions. This quite deliberately so, for some directions are far more important to the observer than others, and some even have to be avoided (glare!).The luminous intensity distribution of a lamp or luminaire, therefore, gives a good indication of its practical luminous efficiency and area of application.*

Bicycle headlamp without reflector, in any direction | 2,5cd |

Bicycle headlamp with reflector, centre of beam | 250cd |

In the open under a heavily-overcast sky | 5000 lux |

Incandescent reflector lamp PAR38E Spot 120W,centre of beam | 10 000cd |

Lighthouse, centre of beam | 2 000 000cd |