Edit: So maybe you do have better control, but what if the etchings are so small that there's no significant contrast change when viewing from different angles?

Edit: So maybe you do have better control, but what if the etchings are so small that there's no significant contrast change when viewing from different angles? I'm just basing my speculation on the fact that these guys probably have tried a lot of things and this is most likely the best way they've found. It would be interesting to hear from them what were the results of other setups.