This does sound like one of those things that the marketing department sometimes wander down to engineering discuss and leave feeling very put upon when all the engineers laugh.
Put it this way : possible in theory, maybe if someone finds the right materials ( which have to be able to not just switch from blocking photos to passing them, but also counting them and some interface to deliver a measure of that count. That's four things the sensor has to do ( and they just about manage two of those now ).
However an OVF right behind the sensor would probably make a somewhat awkward handling position ( compared to the evolved position of the SLR OVF ). SO we might still end up with a mirror or prism to shift the OVF to a more conventional position.
But if we did that we'd be no better off, as we're still using a mirror or prism.
Also if the viewfinder is behind the sensor we'd end up with a thick camera body ( as the whole OVF now sticks out the back ). Again, not very nice for handling and not very friendly for packing.
Lastly, tell me what happens when I shoot video ? The sensor clearly can't be transparent for that, so I'm back to an LCD. But where does the LCD go now that the back of the camera had a honking great viewfinder in it ?
So I don't see a transparent sensor as particularly useful in practice compared to some of the mirror based alternatives.
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