Isn't there a subtle distinction between transparency and alpha? I think transparency means see-through-ness whereas alpha means is it there or not. Then there's translucency to complicate matters which I think means whether or not a backlight can cast an image of itself on the material such the image can be seen looking at the front of the material. One of the things I've been playing around with in different apps is putting a bulb light inside a sphere and then seeing if you can see any light seeping through the sphere surface (from the outside) with different transparency/translucency/alpha settings. The result seems to vary somewhat between apps.