Introduction to Photometry

It is the branch of optics that deals with the study of measurement of visible light.
Luminous body: A body is said to be luminous if it posses light of its own. Example: Sun, star, candle etc
Solid Angle: The angle subtended by the surface at that point is called solid angle at that point. It is represented by l. It is measured as the ratio of surface area to the square of distance between the points.

i.e., Solid angle (l)=Ar2

Unit of solid angle is steradian.
1 Steradian is the angle subtended by the surface at the point when the area is 1m² and radius is 1 metre.
Radiant energy: The total energy radiated by a source is called radiant energy.
Luminous energy: The radiant energy which is capable of producing the sensation of vision to our eyes is called luminous energy.
Luminous flux: The luminous energy emitted by the source per second is called luminous flux. It is denoted by Q. Its unit is lumen.
Luminous intensity: The luminous energy emitted by the source per unit solid angle is called luminous intensity.

L=Q4π

The unit of luminous flux is candela.
If we consider total solid angle, then n = 4 π
or Φ = 4 πI