On the computer screen full identification of both book and borrower appear and a keystroke records the book as borrowed on a given date, lists it as out on loan in the computerized card catalogue. Librarians will tell you that the pen is a light emitting diode, LED for short. Examination of an LED shows that it is a solid structure, not at all like our familiar light bulbs. Computers are very efficient at memory storage and retrieval. In stores they perform well at the tasks of keeping track of inventory and providing price information.
Humans are much less efficient at such routine tasks and more apt to make careless errors.
UPC symbols, or barcodes, identify each product and its manufacturer. These symbols eliminate the need for individual price stickers on products, or the need to change prices manually in the event of a sale. While we humans can look at the UPC barcode in ordinary sunlight or electric light and clearly see its pattern, the computer cannot. It must have some way of clearly distinguishing black from white. Lasers and their accompanying photoreceptors provide the tools necessary to accomplish this.