Some aspects of image projection in the field-ion microscope

The projection of ions in the field-ion microscope image of a tungsten specimen has been measured for a number of different conditions. It is well known that the relationship L=kθ holds, where θ is the angle from the tip axis. However, we have found that for a typical field-of-view the L=kθ relationship also holds to a fair degree of accuracy for all angles in the image. The effect of off-axis projection is found to match a simple geometrical model, in which the ion trajectory with relation to the tip is assumed to be unaffected by the position of the screen or counter electrode. These results are important in determining the model to be used for accurate reconstruction of ion positions in the three-dimensional atom probe.