This answer is going to explain partly why lighting from behind doesn't work, but I'm going to duck around a lot of the 'science' because though I understand the effect, I can't describe it in "maths".
When light hits an edge, it bends slightly; it doesn't all bend at the same amount, reds bend differently to blues. This is far less of an effect than a rainbow generated by a prism, but it's there nonetheless. This generates a 'half-lit' part called a penumbra.
Add to this that unless your light source is a theoretical single point, then you will get light coming from all different angles, some can get around the edge of your object, some can't, with a fairly even distribution. This is going to add more of that 'fun' penumbra stuff, leading to vague edges.
Thirdly, unless you have a small-enough aperture to get the entire depth of your object precisely in focus, you will also be getting softer focus areas… which further exaggerate this effect on edges.
Add this all up & you get vague, fuzzy half-lit, half-leaked light around the edges.
This is a crop from a bit of camera lighting gear, against a cloudy sky. It is, believe it or not, pretty much in focus… but not right to the edges of the object. This is just a screenshot crop from a larger image, towards the top left of frame, which will further emphasise some of these aspects.
Any inaccuracies in my photographic technique here will just emphasise the errors you would get under any circumstance using back-light.
Firstly, if you look at the bit of screw-thread near the bottom. This is tight in focus, yet you can still see light spilling around it & slightly blurring the edge. You're also seeing some reflective aspect off the surface, even though this is an almost matt-black object.
Up towards the top of the frame, around the ⊃ shaped gap, you can see where it's not quite so in focus. At the bottom of the gap is slightly red-tinged & the top slightly blue. This is a lens effect exacerbating the penumbra effect already in play.
All in all, it doesn't make for a great way to analyse small gaps in a structure like this.
On the other hand, turning round the same 'stage' so that your big, broad, flat light is now behind you & putting a simple sheet of white paper behind the object, suddenly everything springs into view.
The penumbra from before now aids our shadows - the light is so broad that there is absolutely no identifiable shadow falling on the paper. The object itself is very evenly lit. We can see detail, even though it is almost solid black.
This is still far from a 'perfect' photograph, again a crop form a larger image of approximately the same bit of lighting holder. The centre is still in better focus than the edges & I don't have enough depth of field to get the whole thing sharp. Front to back depth of the object is about 1" 25mm. I'm focussing on the face of of the screw-thread from about 4 feet away. Aperture set to f11, which is as small as you can really get it without diffraction becoming more of an issue than depth of field. The entirety of the crop you see is an area approx 1" 25mm square. This is one thing about 'macro' (close-up) photography; your depth of field is very low. If you need to get the entire object in sharp focus you will probably end up having to focus-stack.
I didn't use a dedicated macro lens or tripod for this, just a long zoom already on the camera, hand-held - this was just a quick snap to prove a point.
Even with this rough & ready approach, you can see we have more detail, better [if not yet perfect] sharpness at the edges, and almost coincidentally, a tiny gap visible between two of the sections. Near the screw thread you can also see dust on the object & tiny imperfections in the edges, many of which were hidden when back-lit.
If anyone is interested in seeing the uncropped images - these are ⅓ original size. Click to see at that size.
& no, I've no idea why I turned the camera portrait for the 2nd one… too late now;)
