About 20 years ago when I was just starting out in photography, I was at a portrait photographer's studio. He was showing me some of his equipment. One thing that I found interesting was a set of square filters he used. They were made of solid glass, about 1/8" to 1/4" thick and appeared to have metal flakes at different depths in the glass. What was odd was that the filters were inserted between the lens and the camera body rather than being mounted to the front of the lens. If I recall correctly, they gave a glowy, softening effect that I've heard videographers refer to as the "Barbara Walters" effect.
What are these filters called? I've never seen anything similar since then, though I don't do much portrait photography.