Abstract: The card is particularly intended to give a card-holder access to a service such as pay-TV (broadcast or cable), but in principle could be adapted to any lock-and-key type of system, e.g. credit cards, cards giving access to computer terminals or rooms, etc. The data is encoded on the card in the form of data-significant marks and spaces (2, 3). For example, the marks (2) may be distributed along two parallel tracks with marks in one track representing "1" bits and marks in the other track representing "0" bits. Dummy marks (20, 21, 30, 31, 32, 33) are also made on the card. The dummy marks have the same appearance to the human eye and to photographic and photocopying equipment as the data-significant marks. However, they are printed using inks having a different response in an invisible part of the spectrum, so that card reading equipment can be made to treat dummy marks differently from the way it treats data-significant marks. This makes the cards difficult to fake using commonly available equipment.