The part of the overall operation that is of particular interest to us is getting in the computer to 'see' or distinguish, the black and white lines in the barcode. This device is the photocell. Before learning more about photocells and how computers see, recall how we see. Humans see an object when light from the sun or some other source strikes the object and then reflects back to the eye, striking the retina which sends a series of electro-chemical signals to the brain. Figure 1.3 Light from the barcode is not the only light entering the eye.
Light also enters from the rest of the can, the counter, and other objects.
We humans can distinguish the light from the barcode from other light because our brains work with our eyes to sort out objects and to form patterns. The human eye-brain system is a truly marvelous pattern recognizer which we cannot yet, or possibly ever, duplicate with technology. Since this is the case, we need to provide a special kind of light and a special kind of pattern for a computer. We have just discussed the special patter, the UPC code, and now we need to investigate the special light that allows the computer to distinguish that code from other objects in its environment. Let's look at what happens when a computer 'sees' a barcode under regular store lighting.