Geometry
The geometry of an electron diffraction experiment is shown here.
The Bragg Law for small angles approximates to: 
l = 2dq 

From the diagram: 

Therefore: 

or 

 The distance, r, of a diffraction
spot from the direct beam spot on the diffraction pattern, varies inversely with the
spacing of the planes, d, that generate that
spot.
Note: no lenses have been shown. They merely alter the effective camera length, L. Often the value of lL is referred to as the camera constant of the
microscope.
Any 2D section of a reciprocal lattice can be defined by two vectors so we
only need to index 2 spots. All others can be deduced by vector addition.
If the crystal structure is known, the ratio procedure for indexing is:
 Choose one spot to be the origin. Note: it does not matter which spot you choose.
 Measure the spacing of one prominent spot, r_{1}.
Note: for greater accuracy measure across several spots in a line and average their
spacings.
 Measure the spacing of a second spot, r_{2}.
Note: the second spot must not be collinear with the first spot and the origin.
 Measure the angle between the spots, f.
 Prepare a table giving the ratios of the spacings of permitted diffraction planes in the
known structure. Hint: start with the widest spaced plane (smallest r). You only ever need to do this once for each
structure. Blank tables are provided here...
 Take the measured ratio r_{1}/r_{2} and locate a value close to this in
the table.
 Assign the more widelyspaced plane (usually with lower indices) to the shorter r value.
 Calculate the angle between pair of planes of the type you have indexed.
Equation and example...
 If the experimental angle,f , agrees with one of the possible
values  accept the indexing. If not, revisit the table and select another possible pair
of planes.
 Finish indexing the pattern by vector addition. Example
of indexing provided here...