I envision astral perception sort of like changing everything physical into various shades of fog. You know when you're driving through fog, and your high beams create an impenetrable wall in front of you, but turning them down to daytime running lights allows you to see the furthest? I imagine astral space to be much the same, but light is the emotion evoked by the object in question. The fence of a prison yard would be opaque, because of the intense feelings of confinement, of being trapped, that the fence evokes in hundreds of people every day. The concrete walls of say, a long abandoned temporary camp, however, might be totally transparent because they've caused no emotion or feelings at all in a hundred years, and weren't that important even then.
Just my personal interpretation based on the descriptions in the 4e and 5e books. Hopefully it helps.