Luminous Flux: Luminous flux is a form of energy of the light waves which is radiated per second from the luminous body.

Example: When electric power supplied to an incandescent lamp, the whole electric power is not changed into luminous flux. Some power is lost by heat conduction and absorption, remaining radiant flux only a fraction of it is in the form of light waves in between wavelength 4000 A⁰ and 7000 A⁰.

Luminous Intensity: The luminous intensity may be defined as the flux emitted by the source per unit solid angle.

Let, a point O is a source flight.
And dF = luminous flux crossing any section of a narrow cone of solid angle.
dw= Solid angle of the cone.
I= Luminous intensity.
Therefore, the luminous intensity (I) in the direction of the cone is the ratio of flux dF to the solid angle dw.

Lumen: Lumen is defined as the luminous flux per unit angle from a source of 1 candlepower.

Illumination:Illumination is defined as the luminous flux fallen per unit area of the surface.

Let, incident luminous flux (DF) falls on a small area (DA).
This is shown in the fig.

R is the distance between the area and the point where the point where the solid angle is formed. If the area is in square ft. then the unit of illumination is lumens per sq. ft on foot-candle.

1. Luminous Flux , Luminous Intensity, Illumination
2. Plane and solid angle of light