Surveying with a UV lamp
Find the contamination before treating it, because the visible mark and the actual deposit are frequently in different places.
In detail
Before any treatment for animal contamination, a professional darkens the room and goes over the floor and furnishings with a UV lamp. Deposits that never produced a visible stain fluoresce, and those are usually the ones still producing odour — treating only what can be seen means treating the wrong area and then wondering why the smell persisted. The survey defines the work: its extent, whether it has reached seams or edges, and whether it is worth attempting at all. Where fluorescence covers a large area or follows a seam, the contamination has probably reached the backing and the layer below, which changes the answer from cleaning to replacement. One finding makes this method genuinely counter-intuitive. A treated area frequently STILL fluoresces afterwards, because phosphorus left from the breakdown of phosphocreatine bonds to the fibre like a dye. So the lamp is a good instrument before treatment and a misleading one after it, and a householder using one to check a professional's work will read a stain that is no longer urine as urine.
Sources for this page
Sources
Consumer Floor and Furnishings Care Information: carpet topics
Institute of Inspection, Cleaning and Restoration Certification (IICRC) · Trade or professional body
Relied on for: That cleaning professionals use a black or UV light to identify urine areas that cannot be seen with the naked eye because not all urine stains show, that this is the proper way to identify affected areas, that a treated area may still fluoresce afterwards from phosphorus bonded to the fibres, and that this does not mean urine removal was incomplete. (states this directly)