In almost all older religious paintings that I've seen, holy figures with halos have gold/glowing disks behind their heads, like this:
or sometimes more pronounced, but still clearly behind the head:
However, halos in modern times are almost always portrayed as a yellow ring floating above the head, like this:
How, when, and why did this change happen? Was there a specific reason for the change, or did the symbol just shift over time? I could not find any information online, so any help is greatly appreciated.