I recently posted the idea of a "magnifying spherical shell"
where (parallel) rays of light from any direction focus on a
point in the centre. I thought that idea could be modified to
make a one-way-ish window.
Start with an array of hemispherical magnifying shells on a
sheet of glass. Each
hemispherical shell having a black dot in
the centre. Looking at a hemispherical shell from any angle
will look black (since the black dot is magnified).
See cross-section diagram.
View from the front
Looking at the front of the array will look black because of
each magnified black dot.
Light will also be transmitted through the window. This will
be out of focus.
So each hemispherical shell will be pixel of black plus the
out of focus light.
View from the back
Looking at the back of window, you will see an array of tiny
un-magnified black dots. You will also see all (non-parallel)
light which has made its way through the lenses. All the
parallel light (that is light from infinity) gets absorbed by
the black dot.
Each hemispherical shell will transmit (mainly out of focus)
The window will look mainly black from one side so you can't
really see through it. The other side will allow you to see
through (albeit pixellated).
Might be useful as a shower screen, or anywhere else an
imperfect one-way-ish window could be used.