Hi-brite generally refers to the type of paper a stamp is printed on. It can get confusing and seem complicated if you get to fixated on the term "Hi-Brite". In Boerger's book, "Handbook On U.S. Luminescent Stamps", he has a section on Fluorescent Paper. In these early tagged stamps he breaks fluorescent paper down into LO (low bright), ME (medium bright) and Hi-Brite. The fluorescent type papers he refers to and what they look like are not the same as more modern stamps, which is one point of confusion. The brightness of these papers is very even overall.
In the early 80's and possibly earlier you see paper that would be considered non-fluorescent but because luminescent fibers have been added the paper appears luminescent when viewed from a distance. The amount of fibers then impacts how bright the fluorescence appears.
For very modern stamps, last 5 - 10 years hi-bright paper is very very bright white under LW UV and unmistakable.
Brixton-Chrome has an excellent write-up on their website describing luminescent paper types.https://brixtonchrome.com/pages/und...nced-version
While it describes Canadian stamp paper, they same descriptions can be applied to US stamps.
Keep in mind SW UV is used for viewing US tagging while LW UV is needed for paper identification.
Bottom line don't sweat the details. If you are interested in paper varieties the tagging DB many many stamps with paper varieties list. For a specific stamp, just look for the difference in brightness to separate them.
One big caution, used stamps can easily fools you and appear as hi-brites. Two things can cause a used stamp to fluoresce brightly. One is optical brighteners used in soaps/detergents added to make thing look brighter. Soap reduces the water surface tension and reduces the soaking time to remove a stamp, but the stamp absorbs these brighteners making them highly fluorescent.
The other cause is transference of the optical brighteners used on modern envelops. Modern envelops are extremely white under LW UV due to the brighteners and these brighteners can leach off of the envelop onto the stamp during soaking. These stamps appear to have a swirly bright luminescence to them.