Correct Answer - Option 2 : 4π
Luminous Flux (Φ):
- It is the light energy radiated out per second from the body in the form of luminous light waves.
- Since it is a rate of flow of energy, it is a sort of power unit.
- Unit of luminous flux is lumen (lm) and it is defined as the flux contained per unit solid angle of a source of one candela or standard candle.
Luminous Intensity (I) or Candle-power:
CP of a point source in any particular direction is given by the luminous flux radiated out per unit solid angle in that direction.
In other words, it is the solid angular flux density of a source in a specified direction.
If dΦ is the luminous flux radiated out by a source within a solid angle of dω steradian in any particular direction, then,
\(I=\frac{d\phi}{d\omega}\)
If flux is measured in lumens and solid angle in steradian, then its unit is lumen/steradian (lm/sr) or candela (cd).
If a source has an average luminous intensity of 'I lm/sr' (or I candela), then total flux radiated by it all around is,
Φ = ωI = 4πI lumen
