Reading faded signature using enhanced light?

[drc]

At a crime scene, CSI forensic experts have special lights that give a broad range of life-- infrared to all the colors of the visual spectrum to ultraviolet to colored lenses. They use this to pick up hidden details. They will then photograph it for documentation. Used to identify blood drops on a dark surface, eyelashes on a rug, etc. Under certain lighting, an item will stand out.

So it could be UV that made the signatures stand out, could be infrared, could be an visible color that picks them up. With the CSI light machine, they will go through the whole range of light until something is picked up. I've used infrared light to pick out old writing on baseball bats. It often works well.