Abstract
The contrast of thickness fringes in thick crystals has been studied for various diffraction conditions. The results show that, under certain circumstances, it is possible to obtain fringes in considerably thicker crystals than is normally possible. These fringes arise from the interference of two well transmitted Bloch waves as opposed to the familiar thickness fringes which result from the interference of one Bloch wave which is well transmitted and the other which is highly absorbed. The effects of absorption under these circumstances are similar to those obtained when normal absorption only is present.