In Central dark Field (CDF) imaging in a TEM (Transmission Electron Microscope) the dark field image is made from a weaker spot / reflection. In one of the textbook it is mentioned that it is done so because after tilting a weaker spot to the centre it becomes strong and hence the contrast is improved. I want to know how/why after tilting a weaker spot to the centre it should become strong whereas by tilting a strong spot to the center it becomes weak?
I do not know all the particulars to your specific question.
However, The technique requires indirect elimination of the specimen. Too much probe elimination can override the dark field effect.
However, in light microscopy, it is possible to use two eliminating techniques at the same time. (Say, …dark field and partly-crossed polarizers, or dark field and interference contrast, which can produce very interesting effects, and can brighten an image a little, while producing a similar or related effect. I have not tried this “multiple technique” with either TEM or SEM, but it might be very interesting. ???
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