Question
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How Does a QR Code Work Technically?
Short answer
A QR code stores characters as a black-and-white pattern of square modules. The three large squares in the corners serve for alignment, allowing it to be read even when tilted or rotated. Built-in error correction ensures that the code still works even if part of it is damaged or obscured.
Structure
- Position Detection Patterns – the three large squares in the corners. A reader recognizes the position and rotation from them.
- Alignment Patterns – smaller squares that compensate for distortions, such as on a curved bottle.
- Data Area – this is where the actual characters are stored.
- Error Correction – additional information that allows damaged areas to be reconstructed.
The Four Error Correction Levels
| Level | Recoverable | Suitable for |
|---|---|---|
| L | about 7 % | Screen display, clean environment |
| M | about 15 % | Standard for printed advertising |
| Q | about 25 % | Stickers, packaging |
| H | about 30 % | Industry, outdoor use, logo in the code |
A logo can be placed in the center because the error correction compensates for the obscured area. However, this is only possible up to the limit of the chosen level – using a large logo at level L produces a code that sometimes works and sometimes does not.
Why Short Addresses Are Better
The more characters are stored, the finer the pattern becomes. A fine pattern requires more printing area and is harder to read. This is where dynamic QR codes have a technical advantage: they contain only a short forwarding address, regardless of how long the actual destination is.
Standard
QR codes are standardized in ISO/IEC 18004. They can be used free of charge – the inventor Denso Wave has waived the enforcement of its patent rights.
Key facts
- Standard
- ISO/IEC 18004
- Error Correction
- four levels: about 7 %, 15 %, 25 %, and 30 %
- License
- free to use