Marking Cards Only For Your Eyes With Invisible Ink
Luminous work consists of light green ink applied to the red areas on the back of a playing card. To the naked eye, the green markings were overwhelmed by the card’s legitimate red ink. But viewed through a filter that only lets red light through, any colour other than red – including the green ink – shows up as a clearly visible dark spots. Wearing a red-tinted monocle is likely to get you noticed, but many players wear sunglasses to hide their eyes, so the cheats used to build the red filter into a pair of shades. Only they could read the marks. Although eventually supplanted by better marking systems, luminous work can still “get the money” today against unsuspecting novices.
Although touted as “invisible” by the catalogues, the reality is that luminous markings were not truly invisible. They were difficult to see, but if you were familiar with the concept and looked closely enough, the green ink could be read without filtered glasses.