"its equivalent to top layer being transparent that lets light through one way but doesn't let it reflect back"
That would be a violation of the laws of thermodynamics. It's impossible to make a one way mirror. If you could, you could allow light through from one side, and collect free energy, since one side would be hotter than the other.
You can turn them around, and they will work unchanged.
They transmit exactly the same amount in both directions.
The only reason they seem to work is that one room is well lit, and the other is dark. So it's hard to see anything from the well lit room.
They are half silvered, so they reflect half, and transmit half. The well lit room then reflects lots of light, but is receiving little light from the dark room.
The dark room reflects little light, but is getting lots from the well lit room.
But it's not "imperfect". It's 0. It's not one way in the slightest.
If I turn the lights off in my house and you can't see me through an open window will you tell me the open window is a one way window?
Imperfect implies say, 49% of light goes one way and 51% goes the other. Such a mirror does not exist.
And please remember the context (although I think the poster was embarrassed and deleted his original comment), we are talking about making black colors on metal, and he was wondering if it's because it's a one way mirror.
