DWSI = Double Wall Single Image
These sketches give a rough schematic representation of the overall form of the radiographs produced by the different DWSI geometries discussed.
Known in the Trade as a "D-shot" because of the "D" shaped appearance of the image. Only the straight sharply-focused film-side weld image is interpretable.
The shadow of the source-side weld casts uniformly over the weld and HAZ of the interpretable film-side weld image. Which gives an acceptable shot. Control of exposure conditions can produce an image of correct density and good subject contrast. The shadow will be very uniform indeed in any situation of a competently executed weld.
This is what you get if you send the centre of the X-ray beam through the same location you would place a Gamma source for DWSI.
The should-be interpretable region is part shadowed and part not shadowed by the source-side weld. It is not going to be possible to meet the aim of a uniform film-density in the specified range and a contrasty image. And the eye will not naturally be able to scan for defects in this unbalanced image.