Paper data storage
Paper data storage refers to the use of paper as a data storage device. This includes writing, illustrating, and the use of data that can be interpreted by a machine or is the result of the functioning of a machine.  A defining feature of paper data storage is the ability of humans to produce it with only simple tools and interpret it visually.

Though now mostly obsolete, paper was once an important form of computer data storage as both paper tape and punch cards were a common staple of working with computers before the 1980s.

History
Before paper was used for storing data, it had been used in several applications for storing instructions to specify a machine's operation.  The earliest use of paper to store instructions for a machine was the work of Basile Bouchon who, in 1725, used punched paper rolls to control textile looms.  This technology was later developed into the wildly successful Jacquard loom.  The 19th century saw several other uses of paper for controlling machines.  In 1846, telegrams could be prerecorded on punched tape and rapidly transmitted using Alexander Bain's automatic telegraph.  Several inventors took the concept of a mechanical organ and used paper to represent the music.  

Binary punched card

In the late 1880s Herman Hollerith invented the recording of data on a medium that could then be read by a machine.  Prior uses of machine readable media, above, had been for control (automatons, piano rolls, looms, ...), not data.  "After some initial trials with paper tape, he settled on punched cards..."  Hollerith's method was used in the 1890 census.  Hollerith's company eventually became the core of IBM. 

Other technologies were also developed that allowed  machines to work with marks on paper instead of punched holes.  This technology was widely used for tabulating votes and grading standardized tests.  Banks used magnetic ink on checks, supporting MICR scanning.

In an early electronic computing device, the Atanasoff–Berry Computer, electric sparks were used to singe small holes in paper cards to represent binary data.  The altered dielectric constant of the paper at the location of the holes could then be used to read the binary data back into the machine by means of electric sparks of lower voltage than the sparks used to create the holes.  This form of paper data storage was never made reliable and was not used in any subsequent machine.

Modern techniques

1D Barcodes
Barcodes make it possible for any object that was to be sold or transported to have some computer readable information securely attached to it. Universal Product Code barcodes, first used in 1974, are ubiquitous today.  Some people recommend a width of at least 3 pixels for each minimum-width gap and each minimum-width bar for 1D barcodes. The density is about 50 bits per linear inch (about 2 bit/mm).

2D Barcodes

2D barcodes allow to store much more data on paper, up to 2.9 kbyte per barcode. It is recommended to have a width of at least 4 pixels—e.g., a 4 × 4 pixel = 16 pixel module.
For a typical black-and-white barcode scanned by a typical 300 dpi image scanner, and assuming roughly half the space is occupied by finder patterns, fiducial alignment patterns, and error detection and correction codes, that recommendation gives a maximum data density of roughly roughly 2 800 bits per square inch (about 4.4 bit/mm2). With a typical 300 dpi scanner, one can stack up to 24 high-density QR-codes on an A4/letter page.
One can use color in the data encoding scheme, by doing so, one further increases the maximum density, as for example in German administration's JAB-Code shown at the left.

Limits
The limits of data storage depend on the technology to write and read such data. The theoretical limits assume a scanner that can perfectly reproduce the printed image at its printing resolution, and a program which can accurately interpret such an image. For example, an 8″ × 10″ 600dpi black-and-white image contains 3.43 MiB of data, as does a 300dpi CMYK printed image. A 2400ppi True color (24-bit) image contains about 1.29 GiB of information; printing an image maintaining this data would require a printing resolution of about 120,000 dpi in black and white, or 60,000 dpi with CMYK dots.

See also

Banknote read by a vending machine
Book music
Digital Preservation
Edge-notched card
Index card
Kimball tag
Machine-readable medium
Magnetic ink character recognition
Mark sense
Music roll
Optical mark recognition
Paper disc
Perfin
Perforation
Punched tape
Spindle (stationery)
Stenotype
Ticker tape

References

External links
 DataGlyphs
 PaperBack data storage

Data management
Storage media