Invisible marking means the application of photoluminescent materials that cannot be easily seen in normal daylight, but become visible when exposed to a beam of long wave ultraviolet that is sufficiently intense to cause visible fluorescence to appear at the marked article. The final product is not damaged in any way allowing non-destructive quality control and automation.

Process Control Automation with Fluorescent Markings.

Observations and automatic scanning work best with subdued visible ambient lighting. Users select invisible marking because they do not want the mark to be seen for appearance reasons or where they do not wish knowledge about marking to be easily discernible.

Automation in Scanning

The purpose of scanning is for automation. The scanner verifies that an invisible mark is either absent or present and causes some action to be taken based on the signal. The action taken obviously depends on the application, but could take the form of sending a product on a selected path in a conveyor system of to trigger a subsequent processing operation.
Invisible marking is also used where marks need to be placed on multicolored surfaces that make automating scanning difficult because light from the background prevents a clear signal from being received by the scanner. It also can be used to guide manual operations as in garment fabrication using black light to guide cutting and sewing.