It is the method for Determination of three Dimensional Structure of a Protein. The basic mechanism of this technique is diffraction of X-rays on the molecular arrangement of any given sample and than intensities of spots produced on the photographic plate is measured.
Diffraction of rays – Braggs equation  

Bragg Equation: nλ     =     2dsinθ
Where         λ    =    Wavelength of rays
            θ    =    Angle between incident rays and the surfaced
d    =    Distance between the layer of atom
n    =    An integer
This Equation determines the angles θ, under which the first, second, third,. . . order reflections occur, for n = 1, 2, 3,.. . The greater the wavelength, the larger the angle for reflection on the same plane.
Every crystal has a specific arrangement of molecules and each molecule appear to reflect X-ray beams at certain angles (theta). d is the distance between atomic layers in a crystal and the lambda is the wavelength of the incident X-ray beam, n is an integer. 
X-rays are normally produced, when high-speed electrons collide with a metal target. As a source of electrons hot tungsten (W) filament is used which emits high-velocity electrons between the cathode and the anode and these electrons are a strike on a metal target like Cu, Al, Mo, Mg. 

In this technique, X-rays produced by cathode tube are reflected towards the crystals to be studied. These X-rays are converted into a monochromatic ray by use of filters. For detection of a single molecule, it is mounted on the goniometer. This equipment is used for the stabilization of a molecule in a particular orientation.
When these X rays are bombarded onto the crystals, few of the rays get diffracted and rest of the rays are passes through crystal undiffracted. Several 2D images are taken from different planes and a combined 3D image is formed. 

Application :
Measurement of average spacing between layers or rows of atoms 
Determination of orientation of a single crystal or grain-like preferred orientation, thermal expansion.
Determination of crystal structure and orientation of unknown material. 
Measures the size, shape and internal stress of small crystalline regions.
Measurement of thickness of thin films and multi-layers.
Determination of atomic arrangement.

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