定义：单位面积上的光能量。

在基础物理中通量被定义为一些辐射或粒子流的时间积分量。在光学中，光脉冲的光通量F被定义为单位面积上输送的光能。其最常见的单位是J / cm2。 
与光强一样，光通量也是一个与位置相关的数值。对于激光而言，光通量通常在光轴处最大，在远离光轴的地方则会降低。 
在某些情况下，对光通量的最大值更为感兴趣。对于高斯光束，峰值的光通量为总光能量除以π w2 / 2，其中w是高斯光束的光束半径。 
如果知道与时间相关的光强，则可以通过对脉冲的时域积分得到光通量。 

光通量的用途 
当一个强的超短脉冲使得激光晶体或者有源光纤发生增益饱和时，其脉冲脉宽通常远小于上能级寿命。饱和的程度只取决于脉冲光通量，而不是光强的时间分布。增益介质的一个重要特性就是它的饱和通量。 
对于慢的可饱和吸收体，也会发生与增益介质类似的情况。 
在激光脉冲导致激光损伤的情况下，损伤阈值也常被成为光通量。然而损伤阈值并不是一个与脉冲脉宽无关的量，通常情况下，临界的光通量随着脉冲脉宽的上升而上升。
 

Definition: optical energy per unit area

Formula symbol: F

Units: J/m², J/cm²

In general physics, the fluence is defined as the time-integrated flux of some radiation or particle stream. Specifically in optics, the fluence F e.g. of a laser pulse is the optical energy delivered per unit area. Its most common units are J / cm² (joules per square centimeter).

In the same way as an optical intensity, the fluence is a position-dependent value. For a laser beam, the fluence is often highest on the beam axis and lower at positions somewhat away from that axis. For continuous-wave beams, the term fluence is meaningful only in combination with some irradiation time.

In some cases, one is interested in the peak fluence, which is the highest fluence value occurring within the laser beam profile. For a Gaussian beam, the peak fluence is the total optical energy divided by π w² / 2, where w is the Gaussian beam radius.

From the time-dependent optical intensity, one can obtain the fluence by temporal integration over the full pulse duration.

Common Uses of Fluence Values

If an intense short or ultrashort pulse saturates the gain e.g. of a laser crystal or active fiber, the pulse duration is often far below the upper-state lifetime. The local degree of saturation then depends only on the pulse fluence, and not on the temporal distribution of the intensity. An important property of any laser gain medium is its saturation fluence.

For slow saturable absorbers, essentially the same remarks apply as for gain media.

In the context of laser-induced damage by laser pulses, one often specifies the damage threshold of a material as a fluence. This does not mean, however, that the damage threshold is independent of the pulse duration; usually, the critical fluence value rises for increasing pulse durations.