Landis & Gyr
Usually called also optical cards, they are manufactured by Landis & Gyr in Switzerland, UK, and Taiwan. They work by reflecting a laser beam off diffraction patterns embossed into an aluminium strip on the card. Units are cancelled by a hot probe which physically destroys the pattern. Early cards didn't had the usual white stripe so whether an earlier card has been used or not can be seen by holding the card at the correct angle to the light when the little marks made by the hot probe in cancelling the units can usually be seen. After a period, cards were produced with a white 'Thermographic Band' over the optical strip which shows a black mark where units have been cancelled and used cards are thus easily distinguished.
The Optical Track is the strip of metallic foil on which the pattern on an optical card is embossed and it runs across the card about 15mm from the top edge. The ends of the track can be seen projecting into the margins of the card. It is covered by a Band or Optical Band of white thermographic paint which should not be confused with the track itself. The widths of the optical bands have varied - 1.5mm, 2mm, 4mm and then 3mm - at different times and otherwise similar cards with bands of different widths may differ in value. More recent cards are the so-called full-face: the front of the card is fully covered by the image and, due to an innovative technique, the usage of the card can be seen on the back. Up to 1988, silk-screen printing was the only process to manufacture the cards; in 1988 appeared the first cards produced in offset printing, and since 1991 all cards are printed with this process.
On the back of the card a control number is usually impressed. By reading the first three digits of the control, we can easily give an age to the card. In fact, the initial digit stands for the year of manufacture and the following pair of digits before the letter indicate the month. That is, for instance, 306H...means that the card has been produced in 1993, June (06). The only problem is that there are no differences in controls of a card issued in 1988 and one issued in 1998, but if you look also at the kind of printing, you'll be able to identify it easily.
