It was on Dec. 19, 1977 - the day that Lyndal Shaneyfelt and Curtis Thompson walked into the U.S. prosecutors' office in Baltimore and set up their cameras - that the government's bribery case against former Congressman Edward A. Garmatz began to fall apart.

Shaneyfelt and Thompson, widely regarded as two of the country's top handwriting and document analysts, had been called into the case by defense attorney Arnold Weiner, who for weeks had suspected that there was something wrong with a crucial piece of prosecution evidence: the datebooks of Garmatz's chief accuser.

Within a week, the pair of document experts had amassed overwhelming evidence indicating that Weiner's suspicions were correct. The datebooks apparently had been altered.

Edward Heine, the principal prosecution witness, had told federal prosecutors a very intricate tale about how Garmatz had extorted bribes from Heine's shipping firm, United States Lines. This tale was bolstered with countless notations in Heine's datebooks, notations indicating that he met Garmatz at such and such a time, that they had a hpone conversation on such and such a day - and that they talked about "campaign contributions."

Shaneyfelt and Thompson took infrared photographs of page after page of these datebooks and, according to a source familiar with the case, found that most of the key notations had been written in ink that did not match the other ink on the pages.

The inference drawn by both Weiner and the federal prosecutor was that the datebooks had been altered long after they were originally written, and that the only purpose of this apparent alteration was to back up Heine's story.

"All we're trying to do is detect different kinds of inks," explained Shaneyfelt, who also was one of the key experts involved in proving that Howard Hughes' so-called "Mormon Will" a forgery.

"There are certain chemicals in some inks" that become visible under infrared light, continued Shaneyfelt, who did this kind of work for the FBI for 25 years before branching out on his own.

Other inks, he said, don't contain these chemicals. The result is that words written in some inks will show up clearly in infrared pictures; words written in others will not.

When Shaneyfelt and Thompson developed the pictures they took of the Heine datebooks, some of the key notations were invisible, while the remainder of the writing could easily be read. "That," Shaneyfelt said, "means a positive determination that the inks are different."

According to defense attorney Weiner, the pair of experts - who each charge about $50 an hour - used the photographic technique to determine possible variations in the inks used because another popular method of document analysis would have required that they use a solvent to take actual samples of the ink from the documents. That would have entailed the unacceptable risk of altering the documents themselves.

Before his involvement in the Garmatz and the Hughes cases. Shaneyfelt won national recognition for his work analyzing key photographic evidence of the Kennedy assassination and determining that one bullet had struck both the president and former Texas gov. John B. Connally.

This analysis was an important factor in the Warren Commission's determination that Lee Harvey Oswald could have fired all the bullets that struck the occupents of the Presidential limousine.