[popo]

It isn't a coating, but a new material with different refractive qualities making it useful to give a lens designer more possibility to correct for colour. @BC I wasn't saying that not having this part automatically means you will have a problem with blue. I was saying, in practice, I observe the deep reds as being particularly off on some of the lenses I use. It may well be that using BR, the blue end may be corrected more easily in one way, opening up the possibility of other degrees of freedom to better correct other parts of the spectrum than otherwise would be possible.

Still, all these are far enough in the future it isn't worth worrying about right now.