QR (an abbreviation for "Quick Response") codes were originally designed for use in the automotive industry in Japan by a subsidiary of Toyota, Denso Wave.   The code is patented, but the patent holder has publically declined to exercise their patent rights and the specification is published as an ISO standard  ISO/IEC 18004

QR code labels are optical, machine-readable labels that consist of black and white squares arranged in a square grid on a white background.

The process of reading a code involves locating three distinctive squares at the corners of the image, and then using a smaller square near the fourth corner to normalize the image for size, orientation, and angle of viewing. The small dots are then converted to binary numbers and validity checked with an error-correcting code.

The information encoded in the QR code may be made up of almost any type of data: alphabetic, numeric, binary or Kanji (Japanese characters).  The amount of data that can be contained in a QR code varies by size.   The coarsest code is version 1 (21x21 picture elements) and the largest is version 40 (177x177).   In general, the optics on mobile devices are capable of correctly processing QR code densities up to version 10 (57x57) codes.

The capacity of QR codes is very large:  when encoding conventional latin-1 letters and numbers, a total of 4,276 characters can be put in a  version 40 code of about 3 inches in size.  In practical terms, the capacity of a version 10 code 3 inches square is about 400 characters.

As an example, the sample pharmaceutical label below is 1 inch (2.5 cm) square.  It fits on the bottom of a standard prescription bottle and can be read with the QR code reader on any mobile device (click here for a free QR code reader for the iPhone, iPad or iPod.)

This code contains 221 characters of content from real prescription label.  It reads: Joseph P. Customer, Penicillin V Potassium Oral, 100 mg tablet; take twice per day until gone. Exp. 2/28/2014.  Dr. Jerry Munden. 704-503-1015. Information: http://nlm.nih.gov/medlineplus/druginfo/meds/a685015.html

This small code, thus, includes the name of the customer, identifies the medication, dosage and usage information, gives the name and phone number of the prescribing physician and a link to the National Library of Medicine page that has the complete information about the particular formulation of the medication described (information which is normally provided in tiny type on a very large piece paper) and all in a small QR code

QR codes are designed to be read correctly or not at all.  They are formatted algorithmically using Reed-Solomon error correction. QR codes support four levels of error correction to enable recovery of missing, misread, or obscured data. Greater redundancy is achieved at the cost of being able to store less data. 

With appropriate error correction, surprisingly large portions of the code may be obscured and still have the code remain readable -- up to 30% data loss depending on the options used during code generation.   This feature has been exploited by some advertisers who put their own graphic logos in the middle of the QR code, obscuring some of the dots that comprise the code.

QR codes that are damaged beyond the limits of the error recovery algorithm do not give incorrect results; they simply fail to read.

To make your own QR code(s), click here.