One cold night last December, as he prowled the Mall, Washington sculptor Rockne Krebs began to plan a work of art vast enough to change the way this city looks.

It would be made of lines of light -- and monuments and landscape. Entitled "The Source," it would take unto itself the Jefferson Memorial, the Lincoln Memorial, three miles of 16th Street, traffic lights and street lamps, the obelisk, the White House, the dark trees, the night sky . . .

At nightfall here next Wednesday -- as Abram Lerner, director of the Hirshhorn, speaking at the foot of the Washington Monument, inaugurates the Eleventh International Sculpture Conference -- Krebs will turn it on. The laser beams of green and white that organize "The Source" will suddenly appear in the sky above the Mall.

Three lasers will be used: two at the Lincoln Memorial, one at the Jefferson.

The two at the Lincoln -- one an 800 milliwatt krypton laser that emits a bright white light, the other a six-watt argon laser whose beam is emerald green -- will be installed together atop the carved urn that stands at the right of the monument's broad stair.

The Lincoln's krypton laser will send out one white ray of light. The beam will hit a mirror installed in the reflecting pool, and then, glancing upward, terminate in a bright spot, "a white circle ringed with colors," halfway up the Washington Monument. The argon laser's green beam will be split in two. The parallel lines produced will float above the Mall, one passing to the south, the other to the north of the central obelisk before they terminate, more than a mile away, in a grove of trees at the foot of Capitol Hill.

The single line emitted by the krypton laser at the Jefferson Memorial also will be white. Krebs will use twomirrors to turn its single beam into a "Z" of light.

The beam will be reflected twice by the mirrors before it floats across the Mall, past the Washington Monument, over the White House and up 16th Street toward Silver Spring.

"The spaces involved are huge," says Krebs. "It's two miles from the Lincoln to the Capitol, a mile and a quarter from the Jefferson to the White House, and three miles from the Jefferson Memorial to the high ground at 16th Street and Columbia Road. A work of art that deals with vistas of such scale is more than a line drawing. It becomes, instead, a three-dimensional collage.

"Standing on the high ground, looking to the south, you will see it all at once. You will see the street lights marching down 16th Street while the white beam floats above. That vantage point is special. The space seems oddly stacked. The Jefferson Memorial seems to hover over the White House.

"When you take a line from that distance, and bring it into your body space, its beginnings appear tiny. The first two miles, from the Jefferson Memorial past the White House to the foot of the hill, will not seem that long. But by the time it reaches you, the line will have accumulated all that lies below. When it reaches your space, in that last half mile, it will seem to fill the sky."

The lasers that Krebs uses do not always work on site; the technology is delicate.But it all goes as expected, his piece will be on view each night -- from 9 p.m. to midnight -- until July 5.

Like the sculptor's other urban works of art, "The Source," despite its scale and its daring, will be a strangely modest work. Though enormous, it is weightless: Pull the plug and it's gone.

"The name, "The Source," says Krebs, "refers, of course, to George Washington, to Jefferson's ideas and Lincoln's example -- but it has a personal meaning, too. This sort of art was first seen here. I began to work with lasers here in 1967. In 1969, I made a laser peice outside in the back yard of Phil and Leni Stern's private house on S. Street. In 1974, I did a more ambitious piece for the 'Art Now' festival at the Kennedy Center. I called it 'Irish Light.'

"I've done some 40 laser pieces all together -- most of them are gone now. And as I plan each one, I call to mind the public vistas of this town. I'm a Washington artist. I've always been bothered by sculptures that in no way excite the spaces they inhabit."

Krebs, who shares a studio at 14th and U streets NW with Euclid, his green parrot, often has made works of art of laser beams and sunlight -- two materials that despite their differing reputations are very much the same.

William F. Ruback, superintendent of the National Capital Parks' central region, one of the many bureaucrats involved with letting Krebs put an art work on the Mall says, "Everybody thinks of 'Star Wars' or Buck Rogers when they hear the word 'laser' -- but it's just a beam of light. That's all."

What makes laser light seem special is that it is coherent. Unlike a flashlight's beam, for instance, a laser beam won't spread. Some lasers can be dangerous, powerful enough to burn through plates of steel. But those employed by Krebs -- because they're bounced off mirrors or spread out by lenses -- are considerably weakened before they have a chance to harm.

"Looking at these beams in space is perfectly safe," Krebs says. "'The Source' is designed to be in complete compliance with very conservative federal regulations. No one will be hurt."

Most artists, when they make their art, do just as they please, but Krebs is an exception. Because he works with lasers, cautious bureaucrats must lend their approval to the laser pieces he installs outside. Half-a-dozen agencies have been dealing with "The Source." Is it ugly? The Commission of Fine Arts has offered no objection. Will it damage monuments? The Park Service thinks not. Is it safe? Dave Billings, chief of safety of the National Park Service's National Capital Region, does not, at this writing, think it will be banned. The Bureau of Radiological Health will examine Krebs' devises. The FAA will have to be advised that a beam of laser light will be put in the air. Because it will pass over the antennae of the White House, the Secret Service will have to be advised as well. "These pieces look so simple -- but they are not," says Krebs.

The lasers that Krebs uses now cost some $15,000 each. Though he has rented two -- from a group that puts on light shows at rock concerts -- he has had to purchase one. The National Endowment for the Arts has provided $10,000 to help pay for "The Source."

Dozens of sculptors will be exhibiting their works outside in this city during the four-day International Sculpture Conference, which opens here June 4. Most of them will show heavy, opaque things placed either on the ground or on the sides of buildings. Krebs, who has done major laser pieces in Philadelphia, New Orleans, St. Petersburg and other cities, has, in recent years, gained a national reputation in part because his works of art are peculiarly at peace with the visual confusion and the scale of the city. They deal with whole landscapes, they put color into space, and they lend a sense of order to the chaos of the night.

The artist's reputation has spread beyond this city, and beyond this country, in part because he has helped make monumental public sculpture viable again.