In order to calculate the lattice spacing in our sample we need to know the ‘camera length’.
Camera length = the distance from the sample to the projected image.
The diffraction pattern (formed in the back focal plane) forms the projected image on the screen and can be recorded, hence the term ‘camera length’ since this is where the camera is positioned.
The projection process enlarges the distance between the reflections in the diffraction pattern.
To obtain precise measurements from a diffraction pattern using a TEM, one must precisely know the camera length.
A small camera length provides a pattern with little space between the reflections and a large camera length provides a pattern with large spaces between the reflections.
So, too small a length and the diffraction image only fills up a small region of the projection screen. Too large and part of the diffraction pattern can be lost at the edges beyond the projection screen.
It must be remembered that the camera length must be calibrated for accurate measurements. This can be done by using a calibration standard sample for which the lattice spacing is known.
Once an accurately calibrated diffraction pattern is achieved, the information in the pattern is used to determine lattice planes and in the indexation of diffraction patterns [see useful links for information on how to index diffraction patterns].
The images are convergent beam electron diffraction patterns (CBED) from a ZnO crystal. With a larger camera length we see more detail from the discs.