定义：由于一些特定过程导致的光子能量的降低

有各种光学过程，其所产生的光辐射的光子能量与入射的光辐射相比变低了，或者其波长变长了。这种光子能量的损失通常被称为斯托克斯频移，用来纪念是爱尔兰物理学家乔治·斯托克斯。斯托克斯频移会在如下情况下发生： 

1. 当一些材料被激发以发射荧光时，所述荧光的平均光子能量通常比入射光的小，其能量差被转移为微观振动（加热）。然而，这并非总是如此，参阅光学制冷。 
2. 由光泵浦激光器发射的光子的能量通常比在泵浦的光子能量小。激光器中最小的斯托克斯频移（参阅量子缺陷）可小于泵浦光子能量的1％，但其常见的值为是百分之几至40％之间。 
3. 在拉曼放大器和拉曼激光器以及在基于布里渊散射的器件中，被放大的光的光子能量比所述泵浦光光子能量小。 
4. 在拉曼光谱中，大部分散射光的具有变小的光子能量。但经常也可以检测到一些反司托克斯光，其光子能量则大于入射光的光子能量。

Definition: the reduction in photon energy in certain processes

There are various kinds of optical processes where the resulting radiation has a decreased photon energy (or an increased wavelength) compared with the incoming radiation. Such a loss of photon energy is often called Stokes shift, referring to the Irish physicist George G. Stokes. Stokes shifts occur e.g. in the following cases:

• When some material is excited to emit fluorescence, the average photon energy of the fluorescent light is usually smaller than that of the incoming light, and the energy difference results in microscopic vibrations (heating). However, this is not always the case, see the article on optical refrigeration. The Stokes shift is always positive if it is defined via the absorption and emission band maxima for a given transition.
• The energy of photons emitted by an optically pumped laser is usually smaller than that of the pump radiation. The smallest possible Stokes shifts (→ quantum defect) for lasers (e.g. with ytterbium-doped laser gain media) correspond to less than 1% of the pump photon energy, but more usual values are between several percent and 40%.
• In Raman amplifiers and Raman lasers and also in devices based on Brillouin scattering, the photon energy of amplified light is also smaller than that of the pump light.
• In Raman spectroscopy, most of the scattered light has a reduced photon energy. Some anti-Stokes lines with increased photon energies can also often be detected.

A Stokes shift may be quantified in terms of a difference in photon energies (i.e., energy per involved atom), or alternatively a difference of optical frequencies.