Need a plastic, glass or other flat transparent material, which when projected on from a DLP projector or galvo laser projector, would act like any other projection screen, but the transparent part would remain transparent.
I've used several material before ranging from transparent polyethylene sheets, translucent fabrics to acrylic to slightly frosted glass but while they all work they suffer from two issues, always one more than the other:
1) not transprent enough
2) translucent where not needed
I think its time I actually learned what's the proper material to use since I've been doing this for quite some time.