QRコードをスキャン

A QR code is a two-dimensional grid of black and white modules arranged inside a square boundary. Three large finder patterns sit in the top-left, top-right, and bottom-left corners. These squares tell the scanner where the code begins and how it is oriented, so codes can be read at any angle. Smaller alignment patterns appear in larger codes to correct for surface curvature or printing distortion.

The remaining modules carry the actual data, encoded in one of four modes: numeric (digits only, up to 7,089 characters), alphanumeric (digits plus uppercase letters and a few symbols, up to 4,296 characters), byte mode (any UTF-8 text, up to 2,953 bytes), or Kanji (Japanese characters).

The scanner reads format information near the finder patterns to determine the error correction level and masking pattern, then decodes the data stream module by module. Version 1 codes use a 21x21 grid, while the largest Version 40 codes use 177x177 modules. Most real-world codes fall between Version 2 and Version 10, which balances data capacity against physical size.

QR codes use Reed-Solomon error correction, which means they can still be read even when part of the code is damaged or obscured. There are four levels: L restores up to 7% of lost data, M handles 15%, Q handles 25%, and H can recover from 30% damage.

Higher error correction sounds better, but it comes with a trade-off. More error correction data means fewer modules are available for your actual content, so the code either holds less data or needs to be physically larger. Level M is the default for most generators, and it works well for digital screens and clean print environments.

Level Q or H makes sense for codes that will be printed on rough surfaces, placed outdoors, or partially covered by a logo. If you plan to put an image or icon over the center of your QR code, use Level H. That center overlay effectively damages 10-15% of the code, so you need the highest recovery margin available.

Successful scanning depends on three factors: camera resolution, the physical size of the QR code, and the distance between them. A general rule is that the scan distance can be roughly 10 times the width of the code. A 3 cm code scans reliably at about 30 cm. A poster-sized 30 cm code can be read from 3 meters away.

Modern smartphone cameras (12 MP and above) handle QR codes easily. The limiting factor is usually focus distance rather than resolution. Phones struggle with very small codes (under 2 cm) because the autofocus cannot lock on at close range.

Lighting matters more than people expect. Glossy surfaces create reflections that confuse the scanner, and low-light conditions reduce contrast between black and white modules. Matte print finishes scan more reliably than glossy ones. If you are placing codes in a dimly lit environment, increase the physical size by 50% and maintain strong contrast between the foreground and background colors.

When a QR code will not scan, the cause almost always falls into one of five categories. First, the code is too small for the viewing distance. Second, there is not enough contrast between the code and its background. Dark blue on black or light gray on white will not work. Stick to a contrast ratio of at least 4:1. Third, the code is damaged, wrinkled, or partially covered. This is where error correction level matters.

Fourth, the URL or data encoded is so long that the code has too many tiny modules, making them hard for cameras to distinguish. Shorten the URL or reduce the text. Fifth, the code was generated as a low-resolution raster image (PNG at small size) and then enlarged, which blurs the module edges. Always generate at a high resolution or use SVG format, which scales without quality loss.

If you have tested all five and the code still fails, try scanning with a different app. Some older scanning apps do not support newer QR code versions or certain encoding modes.