The polarization ratio of crystal monochromators

A tabulation of some 40 measured values of the polarization ratio K (a measure of the fractional polarization introduced into an X-ray beam by a crystal monochromator) is presented. The values may be represented through the parameter n given by K = cosⁿ2θ_M. The measured values of n cluster around unity and the theoretical rationale for this result is discussed. Possible explanations of outlying values are considered. A recommendation is made that the polarization ratio of an apparatus be measured using a direct method whenever possible, but methods for estimating a value of K are also given.