Optical brighteners

What is actually happening

They are not cleaning agents — nothing comes off the fibre when one is applied. Carpet is dyed, so a brightened spot now behaves optically differently from the carpet around it. It often goes unnoticed at first because the brightener is masking the very spot being treated, then becomes obvious immediately after a professional cleaning takes the surrounding soil out and exposes the contrast. At that point it is a halo no cleaning process addresses, because it is not soil sitting on the fibre.

How you prevent it

Test any product before it touches carpet. Spray it on a white coffee filter, rinse with clean water, let it dry, and shine a blacklight on it in a dark room. A blue-white glow means brighteners. It costs nothing and it works regardless of what the label claims.

Fluorescent dyes found in some store-bought spotters. They absorb ultraviolet light and re-emit it as visible blue-white, so a treated patch looks brighter without any soil having left the fibre. The effect does not rinse out, which leaves a permanently lighter patch that shows worse after a real cleaning.