QR code

How to read it

QR stands for quick response. Denso Wave, then part of Denso, announced it in 1994; it became an ISO standard, ISO/IEC 18004, in 2000, and Denso Wave has said it won’t exercise its patent on the standardised code. Each black or white square is a module. The smallest QR code, version 1, is 21 by 21 modules and each version adds four modules a side, up to version 40 at 177 by 177, which GS1’s specification says can hold 7,089 digits or 4,296 letters and digits.

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What the parts of a QR code mean

The three square-in-a-square marks are the position detection, or finder, patterns. Their developer Masahiro Hara chose squares as the pattern least likely to appear on business forms, Denso Wave records, and the team picked the proportions of black and white across each mark, 1:1:3:1:1, after surveying printed matter for the least used ratio. Keyence’s guide describes the rest: a row and a column of alternating black and white modules between the finders are the timing patterns, which give the grid’s coordinates; the format information holds the error correction level and mask, and is read first; an alignment pattern corrects for distortion; and a blank margin of four modules runs all round.

Error correction levels L, M, Q and H

A QR code repeats its data in a Reed-Solomon error correction code, so part of it can be dirty or missing and still read. There are four levels: L restores about 7% of the codewords, M 15%, Q 25% and H 30%, GS1’s specification says. A higher level makes a larger code for the same data. Denso Wave says M is the one most often chosen, with Q or H for dirty factory settings.

Look-alikes

A Data Matrix code has a solid L along two edges instead of corner squares. Micro QR, a smaller variant, has one finder square, not three.