The tracers emit light in the visible spectrum when stimulated by a UV light source. The technique of applying a luminescent marking is useful in applications where there is significant variation in the background that needs to be ignored, such as text and graphics printed on labels. A luminescent mark can be printed using an invisible ink anywhere on a label without affecting the aesthetics of the label, while allowing a luminescence sensor to detect the mark to verify presence, orientation and positioning of the label.
Often, clear films such as those used in tamper-evident seals contain optical brighteners that cause them to luminesce in the presence of a UV light source.Many commonly used packaging and production materials use luminescent tracers as a means of providing presence verification. Since many of these materials are clear or nearly clear, other types of sensors are not suitable for reliable verification.