1. What is luminous flux and what is luminous intensity? How are they related? 2. What is the definition of lumen (unit of luminous flux)? How is it related with candela? There are a lot of definitions in my books but I am a bit confused, particularly over the definition of lumen. One definition says that the amount of light emitted by a source of luminous intensity 1 cd and passing through a unit solid angle is 1 lumen and hence a source of 1 cd luminous intensity emits luminous flux of 4pi lm. But luminous flux is the total amount of light emitted by a source per second and luminous intensity is luminous flux per solid angle, right? So, aren’t the definitions of the quantity luminous flux and its unit lumen self-contradictory?