Stated briefly, Radiant/Luminous Flux represents the total emitted optical power generated by a light source that is collected and measured with no regard to the direction of the flux. Radiant/Luminous Intensity is the amount of emitted optical power generated by a light source in a specific direction and conical shape angle directed toward the observer.
Flux is the basic unti of optical power,
expressed in watts (radiometric flux) or lumens (luminous flux)
For ease of understanding, picture an incandescent bulb as a light source that emits a finite amount of individual light rays. Imagine the rays are straight lines extending equally in every possible direction from the bulb. The total number of these light rays represents the total optical power (electromagnetic energy) of that light source. Both Radiant & Luminous Flux represent the total power of a light source; however Luminous Flux factors in the human eye response. The unit for Radiant Flux is watt (W), and Luminous Flux is lumen(lm). An integrating sphere is often used to collect the light emitted in all directions.
Now picture an imaginary sphere that surrounds the light source, and the light source is emitting a countable amount of individual rays within a small conical angle in a specific direction.
Both Radiant & Luminous Intensity represent the total amount of optical power within this coned shaped sector of the total space around the light source; however Luminous Intensity factors in the human eye response. The coned shaped sector is known as a steradian. The unit for Radiant Intensity is Watt/steradian (W/sr) and Luminous Intensity is Lumens/steradian (lm/sr). The Candela (cd) unit is also used for Luminous Intensity, and 1cd=1lm/sr.