定义：
改变激光光束形状的棱镜对。

失真棱镜对是指通常用来改变激光光束形状的一对棱镜。例如，激光二极管输出的椭圆光束可以采用一对棱镜作为光束扩展器，并且只扩展一个方向上的光束，于是光束截面变为圆形，这在很多应用中会用到。也可以只是改变椭圆度，或者将圆形光束转变为椭圆光束。 

图1：失真棱镜。输出光束比入射光束窄很多
采用失真棱镜对改变光束形状的原理并不是聚焦效应（也就是改变波前的曲率），而是由于在棱镜界面处存在折射因而光束半径发生改变。
这种变化在任何棱镜界面处都存在（正入射情形除外），因为根据斯涅耳定律，入射角与折射角是不同的（参阅折射）。然而，如果光束路径是对称的，也就是光束与入射和出射平面之间的夹角是相同的，光束半径发生的两次变化可以相互抵消。
因此需要采用非对称结构（如图1）。如果界面的一端入射角为布儒斯特角而另一端（这要求棱镜表面的夹角等于折射角）为正入射（或者角度很小）。
这是只需要第二个界面采用抗反射涂层，这是p偏振的布儒斯特角入射的光在该界面上的损耗最小。在上面描述的结构中，单个棱镜的放大率（输出光束与输入光束半径的比值）等于棱镜材料的折射率，或者是折射率的倒数（取决于方向）。
如果不能找到与放大率相匹配的棱镜材料，可以通过采用不同的入射和出射角度来调整棱镜。 

图2：失真棱镜对，折射率为1.5，每个棱镜的一侧都是布儒斯特角，另一侧为正入射。显示了两束平行光束通过棱镜对。两个光束之间的距离和光束半径都发生了改变。
因此棱镜对可看做左边入射光束的光束扩展器。垂直于所画平面的光束半径是不变的。 
改变光束在一个方向上的半径只需一个棱镜就够了，而且还会改变光束方向。采用失真棱镜对，则可以得到方向不变的输出光束，只是位置发生了评议。
（如图2）两个棱镜的指向也做了改变这样对光束半径的改变发生在同一个方向。因此总的放大率为折射率的平方，或者折射率平方的倒数。如果还需要避免上面光束的偏移，需要采用四个棱镜。 

各种问题 
失真棱镜可以作为单个器件进行购买，还可以购买已经仔细对准的棱镜对。还有一些装置配有可旋转的支架，这样用户可以自由调节棱镜的方向。 
由于棱镜材料具有色散，单个棱镜以及棱镜对的光束偏转角是依赖于波长的。色散效应在某些应用中是非常有害的。色散连同抗反射涂层的有限带宽共同限制了工作波长范围。 
需要注意的是，激光二极管中产生光束的散光是不能通过失真棱镜对校正的。从另一个角度看，采用一个不会引入散光的装置而不仅仅是两个棱镜的结合，是非常有利的。

Definition: prisms pairs for reshaping laser beams

An anamorphic prism pair is a pair of prisms which is usually used for reshaping the profile of a laser beam. For example, the elliptical beam from a laser diode can be transformed into a beam with circular cross-section, as is needed for many applications, by using a prism pair as a beam expander for only one direction (e.g., the horizontal one). Of course, one can also just modify the ellipticity, or convert a circular beam into an elliptical one.

Figure 1: An anamorphic prism. The output beam is narrower than the input beam.

The principle of beam shaping with anamorphic prism pairs is not based on focusing effects (i.e., changes of wavefront curvature), but rather on changes of the beam radius for refraction at flat prism interfaces. Such changes occur at the interfaces of any prism (except for normal incidence), because the angle of the beam against the surface-normal direction is different inside and outside according to Snell's law (see the article on refraction). However, for a symmetric beam path, where the beam angles against input and output face of the prism are identical, the two changes in beam radius cancel each other. Therefore, one has to use an asymmetric configuration (see Fig. 1). It can be convenient, for example, to have normal incidence (or some small angle) at one interface and Brewster's angle at the other one; that requires an angle between the prism surfaces which equals the internal angle of refraction. Only the former interface (with normal incidence) then requires an anti-reflection coating; the losses on the Brewster interface are minimized for p polarization. In the described configuration, the demagnification factor (ratio of output to input beam radius) of a single prism is equal to the inverse refractive index of the prism material, or the inverse of that for the other orientation. If a prism material with suitable refractive index for the wanted magnification cannot be found, one may arrange the prism for different input and output angles.

Figure 2: An anamorphic prism pair with refractive index of 1.5, where Brewster's angle is used on one side of each prism, and normal incidence on the other one. Two parallel beams passing through the prisms are shown. Their distance changes, and likewise their beam radii in the direction of the plane are changed. The prism pair thus works as a beam expander if the input beam comes from the left side. Of course, the beam radius in the direction perpendicular to the drawing plane is not changed.

A single prism is sufficient for changing the beam radius in one direction, but it also changes the beam direction. By using an anamorphic prism pair, one can obtain an output beam with an unchanged direction, only a position offset. (This is illustrated in Fig. 2.) The two prisms are of course oriented such that they change the beam radius in the same direction. The overall magnification is then the square of the refractive index, or the inverse of that. If the mentioned beam offset also needs to be avoided, one may use a combination of four prisms.

Various Practical Aspects

Anamorphic prisms can be purchased as single items, but also as mounted and properly aligned prism pairs. One may, for example, have a cylindrical package containing a prism pair and being easier to mount in a setup than it would be when using separate prisms. There are also setups with rotatable mounts, where the prism orientations can be precisely realigned by the user.

Due to chromatic dispersion of the prism material, the beam deflection angles of a single prism as well as of a prism pair are somewhat wavelength-dependent. This dispersive effect might be detrimental in some applications. This effect in addition to the limited bandwidth of anti-reflection coatings limits the range of operation wavelengths.

Note that astigmatism e.g. of beams from laser diodes can not be corrected with an anamorphic prism pair, as it does no focusing. (For that purpose, one could e.g. use precisely positioned cylindrical lenses.) On the other hand, it can be advantageous to have a setup which cannot introduce astigmatism – other than e.g. the combination of two lenses, which is an alternative realization of a beam expander. Another advantage of the anamorphic prism pair over a pair of cylindrical lenses is that the transverse positioning is much less critical.