Toward the end of the 1960's, when recycling of paper became more and more common, the phosphorescent tagging was becoming ineffective. The reason was that as envelopes began to be produced from recycled content, the fluorescence of the paper began to increase, from the whitening agents used to bleach the pulp. This reaction tended to over-shadow the glow from the tagging, requiring a much stronger and brighter tagging that would be visible to the mail cancelling machines, even when the envelopes were on hibrite paper.
Thus fluorescent tagging came into being. Fluorescent tagging is much brighter than phosphorescent tagging. Generally, most fluorescent tagging glows either a bright yellow, a bright green, or a bright greenish yellow under long-wave ultraviolet (black) light. Australia used a chemical called Helecon (a zinc sulfide compound), which glows a deep orange red. On Australian stamps, Helecon was either incorporated into the ink, the paper, or both the ink and the paper.
The key difference between phosphorescent tagging and fluorescent tagging is that there is no afterglow whatsoever, when the light source is disconnected. Also, unlike phosphorescent tagging, which is usually visible on the surface of the stamp in normal light, fluorescent tagging is generally invisible in normal light. Sometimes you can see shiny bands on the stamp surface, but usually you will not be able to see the tagging clearly without a UV lamp.
When the tagging completely coats the stamp, the paper is often referred to as fluorescent coated paper. An example of this on a stamp from Brazil is shown below: