If one wishes to acquire lucite Mezuzah cases, meaning that the Shaim Sha-dai would be visible (as recommended by the Shulchan Aruch) they should only be affixed in a place where the proper respect will be given to them. For instance - inside a bedroom where one undresses, a lucite case should not be used. However, most door posts are located outside the room. In this case, lucite would not pose a problem. For an outdoor Mezuzah where the sun can shine directly on it, lucite cases are not recommended. The reason being, is that the sunshine can eventually cause the parchment to crack, invalidating the Mezuzah.