הלומן (סֵמֶל: lm) היא היחידה הסטנדרטית לשטף האור של מקור אור. It is an SI derived unit based on the candela. It can be defined as the luminous flux emitted into unit solid angle (1 sr) by an isotropic point source having a luminous intensity of 1 קנדלה. The unit lumen is then equal to cd x sr.
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.
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יחידות פוטומטריה SI
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