Vesicular Film - Description

Description

In the production process, a solution of Saran dissolved in an organic solvent and containing a small amount of a diazo compound was coated onto a substrate of Mylar. The film was then run through a drying oven to drive off the solvents. The film at this point was clear with a yellow cast from the diazo. In a second process called “cycling,” it was run through a tank of very hot water which created a fog of small holes in the Saran layer. This very greatly increased the photographic speed (light sensitivity) of the film. Several different formulations were in commercial use designed to optimize different photographic parameters for various customers.

Kalvar film consisted of a diazo compound, diazonium salt, suspended in a saran plastic film. When exposed to ultraviolet (UV) light one of the chemical bonds in the diazo is broken, leaving an isolated nitrogen molecule. The plastic softens when heated, allowing the nitrogen to collect into tiny bubbles, the "vesicles". When the film cools again, the bubbles are trapped in place. The bubbles strongly scatter light, making them appear white in reflected light, but opaque when backlit. Since the bubbles form where the UV light went through the original, the copy is a negative. Because the film was sensitive to UV only, it could be easily handled under normal incandescent lamps with no need for a darkroom, although for archival storage it was placed in UV-protective boxes.

The film was remarkably durable and indeed trying to effectively destroy the image was a serious problem when sensitive material needed to be destroyed. Simply heating the film damaged the image, eventually, but usually left it largely intact. Eventually a patented process was developed that accomplished this in a reasonable amount of time.

Since Kalvar film used no chemicals for processing, the equipment needed to use it was limited only by the speed of the transport mechanism and the power of the ultraviolet light. Used as duplication film for photoreconnaissance on carriers during the Vietnam War, processing speeds in excess of 1000 feet per minute were routinely done. A recon plane would make its run over the selected area, and on its way back to the carrier, the silver film used was developed in the aircraft. When it landed on the carrier, the silver film cassette was taken from the plane for duplication and distribution to the various specialists.

The diazo-copying process is not unique to Kalvar; the basic process was already widely used in other copying processes, including the "whiteprint" paper copying system and a number of commercial microfilm copying systems. However, Kalvar was unique in using photo-excited bubbles as the "printing" medium and heat "fixing", which made it much less expensive than the other diazo systems (at least at the time).

Kalvar had been in use for some time when a serious problem was discovered - when the saran plastic broke down it gave off hydrochloric acid. The gas would not corrode the Kalvar film, but would any normal film stored nearby, and especially the storage containers. This was cause for serious concern, as the New York Times had invested heavily in Kalvar copying and had distributed copies of the newspaper on Kalvar microfilm to libraries around the world where they co-mingled with normal films. They had to provide free replacements of their microfilm version when the problem was noticed. Versions of the Kalvar stock using improved supporting film were developed that fixed this problem.