An optical-digital archival media system using Kodachrome film as a write-once, read-many permanent digital repository

Précis for an invention


Kodachrome film emulsion, unique in that it features three independent, silver-halide emulsion layers, sensitized to red, green, and blue — not one sensitizing layer, vis-à-vis black and white film or other colour film stocks. Lab testing over the last 70 years reveal that Kodachrome, whose coloured dye couplers are added during processing, is exceptionally stable over time. As a beneficial consequence, properly-stored Kodachrome transparencies produced in the late 1930s and early 1940s, when viewed today, retain nearly all of the original analogue information — in this case, colour tonality, balance, and faithfulness of the original image.

Digital data storage — magnetic media, non-volatile flash memory, write-once media (CD-R, DVD-R, etc.), and so on — faces a continuing logistical challenge of long-term stability and preservation for posterity. The very real worry of degraded media, format obsolescence, or electromagnetic disruption become motivators to find long-term archival storage methods to assure that digitized data are not lost. Sometimes, venerable technologies can bridge this challenge.

Introducing a new means of storing digital media information — by optical-digital means — on Kodachrome emulsion would conceivably store long term data repositories while warding off degradation to integrity. Based on current stability tests on existing dye couplers, this could span up to a couple of centuries. For the last twenty years, the motion picture industry use (or have used) optical-digital methods on a gelatine emulsion base, shared with the motion picture itself, to deliver high fidelity digital audio for cinema projection. Digital audio formats like the Dolby Digital system, is a lossy compression scheme, while Sony’s SDDS is not. Either way, each digital audio system uses only one side of the film rebate (where the sprocket holes are located on on side). These optical-digital formats rely on a colour dye upon which most conventional colour films rely. With SDDS, for instance, a red LED reads the digital information imprinted with blue dyes in the film.

Expanding on this basic idea, using the entire width of a 35mm Kodachrome film reel (a product which, as of 2008, is still available), significantly extends the amount of storable data. By using the full width of the frame, and by employing a visual encoding system with conventional error-correction schemes (md5, etc.), digital data can be written optically, developed chemically and placed into permanent storage. Error correction helps to mitigate any emulsion anomalies (e.g., a scratch or speck of dust).

Without changing the emulsion spec, Kodachrome, with its three separate emulsion layers, stands unique in that a data writing algorithm could to write data in different wavelengths, or “colours”, on each of the sensitized different emulsion layers: red, green, and blue. The result of this method would produce a film stock with many variations of spectral colours. To the human eye, it would appear as a “noise” of colour, not unlike the similarly named effect in Adobe Photoshop. The optical reader, similar to SDDS, would use specially tuned LEDs in red, green, and blue (whose outputs would be adjusted to the same peak wavelengths inherent to each Kodachrome layer). This reader could interpret a data bit written to the blue layer even in situations where that blue bit overlaps with a green or red bit layer. The reader would parse only the layer it needs to read from archive. An alternate algorithm could employ a kind of matrix using the red, green, and blue layers along with black (an activation of all three layers) or transparent (a de-activation, or zeroing of all layers). Storage capacity would be determined by the minimum chemical film grains necessary to produce a reliable binary bit distinction.

Storage of a completed archive could be done over the course of years without demonstrable fading or loss of resolution. The ability to read the optical-digital Kodachrome media would not be hampered by obsolescence, unless encoding/error-correction schemes were deprecated. Even so, such conventions would merely require a software interpretation layer to parse the legacy encoding with a contemporary encoding standard.

The media writer would employ an LED-based laser writing technology — each LED tuned to the same specs as the reader.

Principal application

Kodachrome digital storage is intended for industrial and institutional-scale preservation of content which absolutely cannot be lost to degradation. Paradoxically, as vintage analogue media, such as original motion picture prints and other analogue repositories, are digitized for posterity, a way to assure that the digital transfer is not lost is to rely on a medium which is impervious to electromagnetic radiation, bit rot, and chemical instability. While miniaturization of writing capabilities will in the near future occur at the molecular and even atomic level, less emphasis on permanency of that data is addressed. By developing a deep-storage fallback, retrievable in the case of disaster or massive data loss, companies and institutions can assure that their digital repositories have a failsafe which employs a time-tested stability which reaches back well before the inception of the Digital Age. These deep-storage repositories could be converted salt mines, vaults, and other secure locations.