Optical brightening agent.

A fluorescent additive in the substrate that absorbs ultraviolet and re-emits it as visible blue light at roughly 400 to 450 nm, so the sheet returns more visible light than the visible light falling on it and reads as whiter than white. The effect only fires when the light source carries UV, which is why the same stock shifts appearance between a viewing booth, a window and a shop's overhead lamps, and why two spectrophotometers can report different values for one sheet. ISO 13655 answers that with the M conditions, M1 being defined to cut the instrument-to-instrument variation that fluorescence causes, and ISO 3664 was tightened in 2009 to control the UV content of viewing light. Ignore all of it and the proof that matched under the booth gets rejected on the customer's desk, which is a rerun of a job that measured correct.