Thirty years after its birth, a light-insensitive film has found its niche as the key element in Bell & Howell Co.'s Microx microfilm system, enabling the company to go after nontraditional markets.

The film's insensitivity to light gives it two advantages, Bell & Howell representatives say. First, it requires no special handling, permitting persons not trained in photoprocessing to transfer documents to film. Second, images can be erased and replaced, permitting updating of microfilm files.

Because of these advantages, the company is selling the system not only to traditional microfilm users -- banks, insurance companies, and the federal government -- but to others such as medical institutions, newspapers and embassies.

The Microx system uses microfiche, 4-by-6-inch film cards, instead of film rolls. A microfiche will hold 98 tiny rectangles, each containing the image of a standard sheet of paper. This means that the information on 10,000 pages of paper -- which would fill a five-drawer file cabinet -- will fit on a stack of microfiche two inches thick.

The film will accept images only when it is heated to 212 degrees and an electrostatic charge is passed through it. The camera then engraves the image on the surface of the plastic film. If the film is heated to 240 degrees, the plastic flows back into the engraved lines, erasing the image.

A company could record 17 pages on a microfiche one day, add 20 more the next, erase and replace 2 pages on the third day, and overlay a notation such as "superceded" on a page one week later, sales representative Kenneth Mitnick indicated in an interview.

The company sells machines to record documents on microfiche, to read the microfiche, and to produce either microfiche duplicates or full-size paper copies.

Some customers would find the ability to erase a document from the microfilm file a disadvantage, according to George Lettis, national sales manager for AB Dick Co., which markets a competing product, the System 200 Record Processor.

Among AB Dick customers who need microfiche that cannot be erased, and thus are admissable in court, are the Army and Marine Corps, Lettis said. Although System 200 microfiche can be updated, they are light-sensitive and cannot be erased and reused.

The circumstances under which the Microx system film found its way to Bell & Howell are as unusual as the film's properties.

It was developed by General Electric Co. in the 1950s to record from cathode ray tube displays in military aircraft, but it wasn't used much, according to Philip Sperin, the Microx system's national product manager.

GE couldn't find a practical use for the special film and was seeking to sell the technology in 1971 when a friend passed a roll of it on to Sperin, who then was vice president of marketing for a two-person company formed to develop a microfilm system that could be updated.

The company was named GLI Inc. after its president, Gordon Lysle, an expert in airborne reconaissance.

Sperin recalls that he and Lysle "charged the film with an electrostatic gun and we actually formed an image. . . . Then I sold a development contract to the federal government.

"We said, 'For $20,000 we can provide you with a breadboard' " -- an engineering term for a device that proves the feasibility of a product -- "which they bought and we delivered to them."

That process took six months and primarily was the result of Lysle's expertise, "with myself as the slave," according to Sperin.

By 1974, GLI had developed the supporting recorders, readers and duplicators and had patented them. The company was buying the film from GE, which "saw two fellows that had given new life to a technology they hadn't found a market for," Sperin said.

"Then we went with GE's technical ventures group -- they were great support -- to find investors," he recalled. "At that time, we entered into a joint venture with Bell & Howell and formed a corporation called Microx."

Microx developed prototypes of its microfiche system as original-equipment manufacturers for Bell & Howell, which bought all rights, technology and patents for the film and equipment from Microx, GE and GLI in December 1978.

At that point, Sperin went to work for Bell & Howell, closing down GLI's small manufacturing facility in eastern Long Island and moving to Chicago, where Bell & Howell is based. Sperin later returned to the Washington area, where he had been based in the 1960s when he worked for Fairchild Camera Inc.

Microx went on the market in 1979, and Sperin will say only that sales are "substantial."