In 1987 British Telecom started trialling a new kind of credit card. Recall, invented by the Drexler Corporation of California, could store 2.6 megabytes of data, equivalent to 800 pages of text or 8 video pictures.

Storage was optical not magnetic. The card was coated with a thin film of metal which deformed in 5 micrometre spots when heated with a laser beam. This varied the reflectivity to allow readout by a lower-powered laser.

The cards came from California and the drives from Nippon Conlux in Japan. High precision opto-mechanics were needed to step the laser across the card in increments of 12 micrometres to create a horizontal grid of 2500 parallel tracks. This complexity pushed up the price but BT reckoned that if demand justified mass production, card readers could be made for around £500, with blank cards costing less than £2.

BT’s trial involved 200 volunteers, all pregnant women attending an unnamed West London hospital. The hospital would record their medical records on cards which the women then carried in their wallets or handbags. BT wanted to find out whether the cards could survive nine months of scuffing and bending, and said it hoped that one day everyone would carry a card which stored their full medical history. But until the system was proven, BT and the hospital played safe and duplicated the patients’ records on paper.

Even as BT launched the trial, optical card technology was facing competition from smart cards, with embedded computer and memory chips. Smart cards were still expensive but Sky soon adopted the technology for its satellite subscription service, which drove down the price. BT never even announced the results of the Recall trial.