To satisfy my very questioning mind I have been looking for investigations of orbs. How's is this for coincidence - the first site I find is http://www.theavalonfoundation.org/docs/orbexp.html.
But all this does is confirm for me that the 'scientific' explanation of dust and so on just does not stand up to testing, and it seems that orbs are not confined to a specific time or specific conditions..
I continue to look and to gather information!