X-ray tubes  produces two kinds of beams,the point-focused and line-focused beams,which are nearly identical ,but have different brightness's-the point focus is brighter than the line focus and therefore different applications.

For example,in powder diffraction the use of line focus is justified by the need to maintain as many particles in the irradiated volume of the specimen as possible.The line of focus is typically 0.1-0.2 mm wide and 8-12 mm long.In single-crystal diffraction point focus is employed because a typical size of a specimen  is as small as 0.1-1 mm. The high brightness of a point -focused beam enables one to achieve a high scattered intensity in a single-crystal diffraction experiment.