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What’s the difference between Theoretical & Operational lumens?

A Lumen is the unit of measurement of the complete quantity of light emitted by a light source – the luminous flux.

Lux is the quantity of luminous flux on a surface. It decreases by the square of the distance (inverse square law) of the surface from the light source:

E(lx) = luminous flux (lm) / area (m2).

What is the difference between a Lumen and a Candela?

Lumen = Luminous flux = The complete quantity of light emitted by a source.

Candela = Luminous Intensity = Quantity of light radiated in a particular direction.