Diffraction mode of TEM

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Modern electron microscopes have several different modes of operation: imaging mode, diffraction mode, spot mode, micro-diffraction mode, nano-diffraction mode, scanning mode, rocking-beam mode, and all sorts of other flavours and variations.

Before going into diffraction mode, insert a selected area aperture. The SAD aperture is the lowest one in the column.

One thing has changed: the projector system has changed setting so that instead of focussing on the first image plane (imaging mode), it is now focussed on the back focal plane of the objective (diffraction mode).

The back focal plane is that place where beams that come out of the specimen parallel to one another cross- over one another.

When we press the diffraction button, the strength of next electron lens below the objective is decreased, so that it is now focussed on the back focal plane of the objective lens as shown below.