You're still making a 2 dimensional image of a 3D object, so as you say, no two parts of the object are exposed onto the same piece of film. If light hits a part of the object underneath another which has already been imaged, the first piece is blocking the now illuminated part (from lens viewpoint), so it is never imaged.
Originally Posted by polyglot
There are some issues with the technique:
Because camera, light sheet (& focus) are all set, and the image is only made at that plane as the object passes through it, magnification is constant, and therefore perspective doesn't work as it normally would. "Leading lines" no longer exist, and objects look a bit flattened.
A sphere would lose it's expected shape, I imagine.
To create a light sheet, often two boards in the shape of a donut are sandwiched 1 or 2mm apart from each other (we used brass spacers), then 3 lights are placed 120deg from each other around the edge, directed into the centre.
This works well, but if the object has holes or depressions in it, they may not be illuminated as it passes through the sheet - the end result is an object with a deep shadow in the said depressions.
But, for a sphere, this isn't an issue.
Not sure if that makes things any clearer or worse.