I am familiar with this type of "self-protective behavior" as my husband is a diagnosed paranoid schizophrenic. A psychiatrist would likely tell you what waverider just has, although in a much less cute manner :
Does it do any harm just to humor him, rather than trying to talk sense into him? Much the same way you might do a child who thinks there are monsters in the cupboard.
Don't try to talk him out of it (that would be invalidating his reality). But don't go the other route and appear to agree with his view (as that would be invalidating your reality). Interestingly, a few American psychiatrists in the 1970s tried the experiment of appearing to agree with the perceptions of their psychotic patients, in an attempt to "gain their trust" so that subsequent therapy would be better tolerated. This experiment proved unsuccessful.
I believe the best thing you can do is not draw too much attention to this strange "projection shield" and not act too alarmed over his paranoia.
I have skimmed a few of your earlier posts, and it sounds as though you probably don't have any firm diagnosis for your husband's mental health issues and he may not be interested in seeking professional help for them. If that's the case, then practicing all the self-care strategies you will find on this forum is probably your best bet.
Sorry for the creepiness factor of the "harmful rays" belief. It's a bit of a shock when it manifests itself in such a concrete way, isn't it?