The crowds enter the Church of Santa Maria delle Grazie, blinking back the sunlight to adjust to the dim interior of this 500-year-old chapel. Their vision becomes clearer and the noisy clusters quiet down as they begin to see Leonardo da Vinci's masterpiece, "The Last Supper," at the far end of the darkened hall. The psychological effect of the ruined painting still brings the observer to silence.

In the last five years, however, restorers have begun to battle the ravages of time by beginning to clean and restore the painting. To document the changes brought about by their work, they are creating an exhibit that is likely to begin its American tour at the National Gallery of Art. It will combine original sketches Leonardo used in preparing to paint the picture in 1495-98 with unique photographs taken by researchers at Polaroid Corp.

Plans for "The Last Supper Exhibit" are not complete, but Pinin Brambilla Barcilon, who has led the restoration efforts, said the Polaroid images will join 20 Leonardo sketches coming to the National Gallery in 1985 from the queen of England's Windsor Castle collection. (The Windsor sketches served as the basis for the 12 Apostles and Christ in the painting.) A spokesman for the gallery confirmed there had been discussions about exhibiting "The Last Supper" material.

Since 1978, Brambilla, a historian and restorer of Renaissance art for museums around the world, has climbed the scaffolding in the church almost daily and pored over the painting with scalpel and microscope, trying to rescue what remains of the original.

"There have been times up here when I've trembled while trying to save a few tiny flakes of crumbling paint that remain from Leonardo's hand," said Brambilla. "It's as much terror as anything else--the responsibility of trying to erase the many interventions and intrusions that have crept into the painting since it was created centuries ago," she said.

Almost too well known today--usually through prettified versions that trivialize the original--"The Last Supper" is one of the enigmas of Renaissance art. Leonardo painted it directly on dry plaster and labored nearly three years to do it perfectly. He'd stand for hours at a time without moving, studying the relationship among color, texture, line and proportion. Literary accounts speak of the painting's sensitivity to the slightest throb of light, every silkened curl and flesh tone perfectly realized.

Yet the artist himself probably was as much to blame for its destruction as later calamities. Always a perfectionist, he dreamed up a kind of special oil paint whose exact composition is not yet known. And instead of painting on wet plaster, which requires quick work but lasts for centuries, he dallied with a See LEONARDO, K8, Col. 1 Pinin Brambilla Barcilon and Paolo Viti, of the sponsoring Olivetti Corporation, and portions of "The Last Supper" under restoration Photos by Raymond M. Lane Leonardo LEONARDO, From K1 dry technique that was already crumbling in his lifetime because of Milan's humidity. A generation later, in fact, students found it almost indecipherable.

"And that's when real trouble began," Brambilla explained, for restorers and charlatans stepped in to "save" the masterpiece. Twice repainted in the 18th century alone, the painting was further brutalized by soldiers during the Napoleanic wars, bombed by the British in 1943 (escaping destruction thanks to heavy sandbagging), and nicked and battered by careless caretakers throughout its history.

"Look," Brambilla commanded, pointing to nail holes as big as a finger in the wall, nicks where soldiers must have lobbed stones at the seated group, and scratches dug into the eyes of all 13 figures. As if that weren't bad enough, she said, "dust, air pollution, molds, waxes, old paint and plaster have diminished the picture even more."

Despite all the destruction, the miracle of Leonardo's creation survives. Its precision long gone, the ghostly painting nevertheless comes to life for today's visitor, especially in the area Brambilla has restored. What one sees on the right side, in the perhaps 20 percent she has cleaned and restored, is a glowing reflection of what it once must have been, not only the special brilliance of Leonardo's coloring but his faultless line.

After cleaning, for example, "you find on close examination that St. Matthew is speaking and has beautifully realized hair, a strong chin and manly neck," Brambilla explained, "all captured by the master at the moment of surprise and despair when Matthew learns that Christ is to be betrayed."

Restorers over the years had thickened and coarsened the features of the saint, closing his mouth and dulling his colors--completely negating Leonardo's intent. Similar revelations are found with St. Simon in the restored portion of the 25-by-15-foot painting, and even more hidden treasures are expected, she explained, when the entire mural is cleaned in the next three to five years.

The Polaroid Corp. is donating services to Brambilla's effort. This month it sent a team of experts to record restoration so far, as well as the work that remains to be done. Using a 200-pound wooden camera that instantly develops highly sensitive film, the team of Peter Bass and Denise Dunn-Ryan are creating lifesize "didactic images" of the figures in the painting, which would be included in an American exhibit. The Polaroid process, in fact, not only captures the nuances of color, but replicates precisely Leonardo's every stroke.

Moreover, if the bulky camera is moved close to the painting, larger-than-life photographs of almost microscopic clarity are possible. They will enable exhibitgoers to see closely and well-lighted what until now only Brambilla and a few other lucky souls have been able to enjoy of the master's work. At the same time, archival photos dating back to 1904 will be included in the exhibition, graphically illustrating the effect of industrial pollution on the delicate work.

"Not only will it be the occasion to rediscover the original," Brambilla said, "but any profit we realize from sales of catalogues and posters of the show will be used to further the restoration process and to buy an air conditioning and crowd control system for the chapel--a must if the deterioration is to be halted."