What is the relationship between the contrast and the beam current in the scanning electron microscop or (FIB)?

Which contrast? Beam current is another word for amount of electrons. Contrast is the term describing the gradient in electron detection. From my simple point of view, there is no influence on the gradient if you change the amount of electrons. It becomes better visible but the contrast is still the same. This is totally different for the electron energy since then you really change the physical interaction. Beam current only (primarily) means the same physics but only more often....

Dear Gert Nolze

Thank you for your attention. But I mean here why the value of the contrast changes with the different value of the current for the same acceleration scanning voltage

There are indeed some other effects which might have impact like at high magnification the reduction of resolution. In case of a wider beam theoretically also the contrast might be influenced since the probe size changes. Even the very simple influence "focusing" at different beam currents might be one factor. Moreover, if I take my common image optimization into account...I do not know actually which gain (contrast) I used for the imaging after optimization. In other words, how do you know that the contrast improves? Do you really "save" the electron signal as numbers, or only the image itself? If there is less noise the contrast simply seems to be improved, but actually it isn't. You also should answer my first question :-) . WHICH contrast? About which images you are talking? SE (which one), BSE (Which one)? At which acceleration voltages you are talking? I assumed common conditions like 20keV, 1nA...

However, in order to prevent any speculations you should post both images with all available parameters selected during imaging.

BTW: For FIB the entire discussion has to start again since the imaging formation is different in case of Ga ions and electron imaging.

Dear Gert Nolze

See attached file