1. Field of the Invention
The invention relates to a method for changing the optical characteristics of a display field on a carrier by releasing erase energy in the region of the display field.
2. Background of the Invention
Increasingly, for cashless payment of goods and services, credit payment means in the form of tokens, cards or similar carriers which have a number of units of value ave being used. If the units of value ave present on the carriers in the form of value characters which can be recognised optically and by machine, optical reading devices are also needed to read the data stored on the credit payment means and to check their authenticity, and cancelling devices are needed to cancel the respective number of units of value needed for payment from the credit means. To cancel the value character, energy usually has to be supplied while, for example, sufficient thermal energy is conducted to the location of the value character to locally soften the carrier material and destroy the value character in question.
CH-PS 635 949 discloses a credit payment means wherein the value characters ave contained in the form of optical markings on a card in accordance with the DIN standard 9781, Part I. For cancelling, an erase head according to CH-PS 640 075 is used which is brought directly to the value character to be cancelled and which is heated by an electrical current impulse in order to increase the temperature locally in the credit card in such a way that the optical character beneath it is destroyed. With this solution, the entire energy to be expended for erasure purposes has to be supplied from the outside in the form of thermal energy.
With solutions of this kind, great losses occur. Thus, a large part of the thermal energy is lost by heating the erase head and by the conduction of heat from the erase head into the surroundings. Further losses occur in overcoming the thermal contact resistance. Approximately ten times the energy has to be expended as is needed for cancelling a value character.
A credit payment means is also disclosed in CH-PS 604 279, wherein the carrier material which contains the value characters in the form of optically recognisable markings has a form reminder capacity which can be excited by the supply of energy and which produces a change in the form of the carrier material and also of the markings. By way of this solution, the optically recognisable markings are, admittedly, able to be erased using a lesser amount of externally supplied energy. However, special materials have to be used which have a form reminder capacity.