Practical Electron Microscopy and Database

Pole (plane normal) is a concept, for instance, used in electron stereographic projection or so-called stereogram. Like drawing a map of the earth, you can imagine a crystal located inside a sphere as shown in Figure 4119. The first step to obtain a stereographic projection is to draw lines normal to corresponding crystal planes from the center of the sphere to intersect the sphere at point N_s (N₁ to N_n) in the northern hemisphere (There are 5 Ns shown in the crystal example). Similarly, there are 5 Ss (S₁ to S₅) shown in the southern hemisphere and there are 6 Ps (P₁ to P₆) shown on the equatorial plane. These lines are so-called plane normals, poles, or face poles.

Figure 4119. The spherical projection. The crystal is located at the center of the sphere.

Assuming [UVW] is the electron beam direction, the poles (plane normals) at the UVW zone axis represent the possible diffraction planes for this zone. Therefore, when [UVW] is in the center of the projection, all the hkl reflections should be around the circumference of the projection. This is the reason that electron stereographic projection can be used for interpretation of electron diffraction patterns and determination of crystalline orientations.