In one line
A barcode reader sweeps a laser beam across the black and white bars, and a photodetector turns the pattern of reflected light into electrical pulses that are decoded into the product number.
Explained simply
Black surfaces absorb light and white surfaces reflect it. When a laser spot slides across a barcode, the reflected light flickers "dark, bright, dark" in a rhythm set by the bar widths. The reader just measures this rhythm, like reading Morse code with light.
How it works
- A laser (He-Ne in older scanners, a red laser diode in modern ones) produces a narrow, intense beam.
- A rotating polygon mirror or oscillating mirror sweeps the beam across the barcode, often in several directions so the product need not be perfectly aligned.
- Dark bars absorb the light; white spaces reflect it diffusely.
- A lens collects the reflected light onto a photodiode, which produces a current proportional to the light.
- The electronics convert this into a train of pulses whose durations match the bar and space widths.
- A decoder translates the widths into digits (for example a UPC or EAN code) and sends them to the computer, which looks up the price and stock.
A laser is used because its small, bright, monochromatic spot resolves thin bars and can be separated from room light with a filter.
Quick example
At a supermarket billing counter, the cashier passes a packet over the scanner window. Within a fraction of a second the item name and price appear, and the shop's inventory is updated automatically.
Common mistakes
- Saying the bars reflect and spaces absorb; it is the reverse.
- Forgetting the scanning mirror, which is what moves the spot across the code.
- Confusing laser scanners with camera-based (image) scanners used for QR codes.
How MU asks it
Asked as a short note: "Explain the working of a barcode reader using laser" (3 to 5 marks). Draw a block diagram (laser, rotating mirror, barcode, lens, photodetector, decoder, computer) and write the steps in order.