BG483222118 Lumen (unit) unity The lumen (symbol: lm) is the unit for luminous flow. More information: Small units, factor... The lumen is a measure of the total amount of visible light emitting a light source in all directions. The lumen is a derived unit from the SI system. 1 lumen is the lumen of a pointed light source whose light is combined in 1 steradial and emits 1 candela. More generally, the lumen flux is equal to the product of the luminous luminous intensity of the source in candela and the space angle in which the light is emitted in steradial. One lumen is therefore equal to one CD sr. In other words, since a sphere is 4π steradials, a light source emitting a uniform luminous intensity of 1 candela in all directions, a total luminous flux of 4th lumen The intensity of light (as electromagnetic wave) is usually represented by the Poynting vector in W to m². Since the eye can only detect a part of the EM spectrum (most sensitive to 550 nm (green) waves), the lumen is also defined as equivalent to a power of 1 to 683 watts at this wavelength. The luminous flow is the total amount of light in a beam. If one looks at a part of a beam, that part has a smaller luminous flux (in lumen) but (in principle) the same luminous intensity (in candela) as the whole bundle. The luminous flow depends on the power of the source and on the colour of the light. A LEDLAMP gives a luminous flow of 40 to 100 lumen per watt.