Definitions and terminology used by AXCO in optical specifications

The following definitions are made to provide an unambiguous reference for the terms used by AXCO in the specification and publication of its optics and their application. Many of these terms are often used loosely by different groups within the X-ray analysis community and we strongly recommend examination of these definitions, especially prior to making performance comparisons between our products and any other configurations or products.

X-ray Flux - total photons per unit time through a specified aperture, collimator or system.

X-ray Intensity - total photons per unit area per unit time. Equals X-ray flux per unit area.

Peak Intensity - maximum value of intensity for a nonuniform (spatial or angular) intensity distribution. Assumes a discrete sample granularity over the distribution.

[Intensity] Gain - ratio of X-ray intensity at one specified position in an optical system to the X-ray intensity at an equivalent specified position in another optical system. Conventionally the common specified position is the specimen plane but other locations may be specfied where a beam focus does not coincide with the specimen (e.g. focussed beam diffraction).

Peak Gain - intensity gain ratio of peak intensities between two optical systems having nonuniform (spatial or angular) intensity distributions at some specified equivalent position in each system.

Flux Gain - intensity gain averaged over a specified area. Equals intensity gain for flat intensity distributions at the specified position.

Insertion Gain - intensity gain of an optical system with an optic present with respect to the same optical system with the optic removed. All other optical elements and component separations remain unchanged. Note that this commonly used definition of "gain" produces large numerical values with laboratory equipment because removal of the optic from the system without relocating the X-ray source results in considerable inefficiency due to the inverse-square-law.

Instrumental gain - intensity gain between two significantly different optical systems ("instruments") at a specified position common to the two systems. This may be used for a comparison of two different optical solutions to the same analytical task, a comparison between an existing instrument and a proposed improvement using an optic or a comparison between an ideal system using an optic and the best-case rearrangement of the same source and specimen within contraints of equipment dimensions and required beam properties. Note that this definition is meaningless out of context and its use requires careful specification of the two cases.

Equivalent collimator gain - intensity gain between an optical system employing an optic to produce a beam with a specified divergence (or convergence) limit at a specified position compared to a system using the same X-ray source alone, relocated to produce the same specified limit at an equivalent position ("collimation").

Full-width at Half-maximum (FWHM) - the width (spatial or angular) of a distribution at half of its maximum value. One of the most evil definitions commonly used in optics. In the context of either beam intensity or divergence distribution, this definition must assume a finite width over which the peak "maximum" is sampled, which will vary with sample granularity and is sensitive to noise or count statistics significant within that granularity, often leading to significant error in the location at which the full-width is taken. Of even greater concern is the application of this definition to highly non-Gaussian distributions such as those often encountered in focussed-beam spatial or angular intensity distributions, where a large proportion of X-ray flux may exist in "tails" of the distribution below the half-maximum value. See also FWAB.

Full-width Above Background (FWAB) - the width (spatial or angular) of an X-ray intensity distribution above the background level. This instrumental definition of beam width includes the whole of an X-ray flux distribution having a statistically significant value above the background count rate within the sample granularity. Importantly, this definition will overestimate the beam width for coarse sample granularity and still include the whole of the statistically significant flux distribution.

(c) copyright 2004 AXCO Pty. Ltd.