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That is, a light source that uniformly radiates one [[candela]] in all directions radiates a total of 4π lumens. If the source were partially covered by an ideal absorbing hemisphere, that system would radiate half as much luminous flux—only 2π lumens. The [[luminous intensity]] would still be one candela in those directions that are not obscured.

 

 

If a light source emits one [[candela]] of luminous intensity uniformly across a [[solid angle]] of one [[steradian]], its total [[luminous flux]] emitted into that angle is one lumen. Alternatively, an [[isotropic]] one-candela light source emits a total luminous flux of exactly <math>4\pi</math> lumens. The lumen can be thought of casually as a measure of the total "amount" of visible light in some defined beam or angle, or emitted from some source. The number of candelas or lumens from a source also depends on its spectrum, via the nominal response of the human eye as represented in the [[luminosity function]].