Hi all,
motivated by @Alain_Oguse, I attempted above (post 341) a simple privacy screen for a phone on top of my diffused light source and posted a video to show how effective this was. I then ordered some CineStill CS-LiteBrite+ sheets since that seemed purposely build to provide some collimation and increased brightness. I now tested these and frankly, the results are rather disappointing: Yes, it increases the light by some 25% or so, but it does not seem to provide any collimation (in contrast to CineStill claims). If anything, it makes the high contrast edges even worse (in my case). Here’s a video showing the effect to adding a CS-LiteBrite+ film on top of my light source:
In comparison, I re-tested also the phone “privacy screen” to make sure that what I showed previously was indeed correct:
Indeed, the high contrast edge definitions become less “smeared” using the privacy screen, but not so with the purpose-built CineStill sheets. So my conclusion is, short of doing proper collimation, to advice to purchase a phone privacy screen (they can be had pretty cheap for older generation phones) since this provides a cheap and simple way to increase the resolution when scanning with a diffused source. The one I used was made by JETech for an iPhone 6.1-inch and it seems a proper slab of glass (it had two of them in one box):
One detail that was asked above is whether it works in both directions: Yes, when it’s placed on my phone, it blocks the view in all side directions. These screens do reduce the brightness somewhat, but that is easily countered by a longer shutter speed.
EDIT: Read also my post below: Let's see your DSLR film scanning setup! - #356 by ArnoG