A Comparison of the Diffraction Conditions for Obtaining Optimum Fringe Contrast in Bright and Dark Field Images of Stacking Faults

The effects of systematic reflections on stacking fault contrast has been studied from the symmetry position to the reflection 3g in the Bragg condition. Optimum contrast in the bright field was obtained when the reflection g was close to the Bragg condition. When the crystal was tilted away from this orientation a marked decrease in contrast occurred with the familiar result that stacking faults become more difficult to observe. Stacking faults in dark field images, on the other hand, exbihited good contrast and were thus relatively easy to observe over the entire range of orientations studied. An explanation for the difference between bright and dark field is given in terms of the important Bloch waves excited at different orientations of the crystal.