Optical Brightening Agents
Fluorescent additives in paper that absorb ultraviolet light and re-emit it as blue, making the sheet look whiter.
In depth
Brighteners make a print appear crisper on a shop shelf and cause two problems: the paper looks different under light sources with different ultraviolet content, and the agents degrade over years, leaving the sheet progressively yellower. Papers marketed as archival are usually free of them for this reason.
Examples in practice
A print on brightened paper looks neutral in daylight and distinctly warm under gallery LEDs with no ultraviolet output.