The playing cards for your eyes only


Cheating by tampering with the deck is almost as old as cards themselves. The only thing that has changed is the technology used to escape detection. Jason England explains.


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No one knows exactly when and where playing cards were first invented. In fact, “invented” is probably the wrong word. The most likely scenario is that what we would now recognise as a deck of cards evolved from larger and bulkier gaming pieces similar to dominos. What is almost certain is that whenever playing cards appeared, cheaters invented the first marked deck about five minutes later.

The casino industry has been dealing with the problem of marked cards for more than 100 years. In the world of private card games it’s even worse. The earliest known reference to marked cards is from 1408. One of the biggest problems is that cheaters have huge financial incentives to invent new techniques to mark cards.

For centuries, a marked deck consisted of little more than some extra ink added to the backs of the cards, or a small portion of ink removed from the design. However, in the early 20th century these simple but effective techniques began to give way to the clever application of scientific principles.

In America, gambling supply houses openly offered cheating supplies from the mid-1800s. Their catalogues sold just about everything you can think of that could be used to cheat at a gambling game, from loaded dice to gimmicked roulette wheels. The catalogues had always offered marked decks, but in the mid-1920s a new technique was created and offered for sale. It was called “Luminous Card Work”.

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.

This all changed in the 1980s. The cheaters’ dream of a marking system that was invisible to the naked eye became a reality with the invention of secret inks that fluoresce outside the range of human vision. By using inks that emit in either the ultra-violet or infrared spectrums, cheaters created marks that were truly invisible to the naked eye. These marks can only be read by secret cameras with optics that can “see” the UV or IR spectrum. A colleague would hide nearby and read the marks on a monitor. Then the information on how to play would be transmitted back to the table using hidden earpieces.

Luminous work can still ‘get the money’ against novices.

As technology has advanced, so have cheating techniques. Invisible ink systems are still in use today in very sophisticated forms. The biggest advance in recent years has been with the use of “optical character recognition” (OCR) software that can be built into everyday objects. OCR software reads the cards and “talks” directly to the players with the earpieces – the only confederate the player needs now is his smartphone, which won’t demand a cut of the winnings. Unfortunately for the cheats, if a player suspects the cards are marked and seizes them, the marks can and will be found. The ultimate deck of marked cards is one that can be plainly read by the cheaters, but impossible to detect by anyone after the game. Such a system exists. Using a thermal-imaging camera, it’s possible to detect a small heat differential to a very high degree of accuracy. With cameras hidden above the table, a team of cheaters needs only to slightly warm the target cards to make them “visible”. In a blackjack scenario, where the aim is to beat the dealer by finishing with the hand closest to but not exceeding 21, the high cards would be warmed and the low cards cooled slightly before bringing them to the table.

A confederate looking at the output from the thermal camera would know the approximate value of both the top card before it’s dealt, and the dealer’s face down “hole card”. The heat eventually dissipates and the entire deck returns to room temperature. At that point the cards are completely immune to any sort of forensic analysis.

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