It's just another day at the supermarket: you walk in, pick up a handheld scanner for the bar codes on the produce, and start walking around. Before you begin putting items in your trolley, though, you tell the scanner to alert you if you buy baby food containing peanuts - as your child is allergic to them - or adult foods with high fats, as you're on a diet.
As you go round, the scanner will warn you if you try to buy peanut butter, and keep a count of the calories you have purchased for yourself.
Such "smart scanners" may sound like a futuristic fantasy, but Sainsbury has already laid the groundwork for that system, and could implement it within the year if enough customers demand it.
The existing scanners simply read the bar code and total the cost of goods collected. They were first introduced by Safeway in July 1995 at a store in Wales. Sainsbury introduced them in November 1996, and now offers them in 24 stores. Each has a radio frequency connection to a computer in the store: when the in-built laser scans a code it queries the computer, which sends back the relevant price information.
Sainsbury is already planning to double the number of stores with handheld scanners, adding another 25 stores this year of its total of 388 outlets.
However, scanners have two disadvantages: every chain in the highly competitive supermarket business has now introduced them, and so far they only offer one feature - the price and total of goods. They do not even save customers time because it takes more time to scan individual items while collecting them than to move them past a scanner when you have your load.
But Sainsbury's computer department is excited about the possibilities of adding new functions and information to the existing system. Jill Lucas, the development manager, said: "The sky's the limit. The scanner could bleep for allergy-inducing foods, or for high-fat foods when someone's on a diet."
The obstacle to implementation is getting enough information into the store's central database. However, a Sainsbury spokeswoman said: "The technology exists to define all sorts of things. The next step would be to find out what our customers actually want."