She was standing at a bus stop on Park Avenue, all white-blond hair and pale, pale skin and giggly smiles. She had to be about 13, or not much older. Rick Guidotti, high-fashion photographer, a man who spent his days snapping Cindy and Claudia and Kate, remembers looking at the girl and thinking: "Now, that is beauty. So much life. So much happiness. That girl is gorgeous."

That girl — who disappeared into the bus while Guidotti, camera in hand, chased after her image — had albinism. The white hair and white skin were a result of a genetic condition that can cause blindness or even skin cancer. Guidotti knew enough to recognize that. But he didn't know much else beyond the clinical name.

Four years, four continents and thousands of rolls of film later, Guidotti is an expert on albinism — and many other genetic conditions — and he no longer shoots pictures for Revlon or Elle. What he shoots now, he says, is "a celebration of difference" that he hopes will help define a new "beauty standard." His work will be on display at the National Museum of Natural History today as part of the People's Genome Celebration, a free-to-the-public event co-sponsored by the Smithsonian Institution and the Genetic Alliance, an international coalition of more than 300 organizations that promote genetic-related issues.

The exhibit, completed in conjunction with Diane McLean, a former epidemiologist and current medical student, is titled "Positive Exposure: The Spirit of Difference." It includes 40 of Guidotti's photographs of people with albinism and a variety of other genetic conditions, along with a videotaped presentation Guidotti and McLean will present at the museum today at 4:30 p.m.

"Albinism is the best metaphorical lens that I have," says Guidotti. "But this is not just for people with albinism, not just for people with genetic conditions, but for everyone. It's about genetics. It's about anyone who feels, 'I don't fit in.' It's about teenagers! It's about tolerance. It's about accepting your identity and finding the beauty in that."

Shortly after he spotted that girl at the bus stop, Guidotti looked up albinism in a medical textbook and what he saw deeply disturbed him. The photographs consisted of a series of children, all dressed in their underwear and standing against a blank wall in a doctor's office.

"It was very negative," he says. "These kids looked scared, unhappy, unloved. Like they had no life, no friends, no family. . . . It was such a strong contrast to that beautiful girl I saw waiting for the bus, laughing with her friends."

Already, Guidotti had begun to chafe under the restrictions of his glamorous job, which took him to London and Paris and Milan to photograph supermodels for print advertisements and international magazines such as Harper's Bazaar and Elle. He'd try to introduce "models" at shoots for different products who had what he called "a unique beauty." It didn't take.

"I was always pushing the envelope," Guidotti says. "I got into lots of trouble with clients because of that."

Spurred by what he had seen in the medical text, he began doing more research, and that led him to the National Organization for Albinism and Hypopigmentation (NOAH). After the organization's initial leeriness — understandable, given that people with albinism generally are portrayed in pop culture as freaks or demons — he began a photographic project in his New York studio that resulted in a five-page spread in the June 1998 issue of Life. It was titled "Redefining Beauty."

The experience, Guidotti says, was life-, and career-changing. He began declining work, finding it impossible to do his old shoots.

"I was so in love with this project and so excited by it," he says, "that I kept thinking, 'Why did I do fashion photography for so long?' But I realized that everything I'd done, I was in training for this."

He remembers one of the first individuals he photographed, a teenage girl who had hesitated to agree to the project because she'd been enduring terrible teasing at school. When she first arrived at his studio, Guidotti says, she was shy, gave one-word answers to his questions, hunched up her shoulders like one who had almost no self-esteem. He turned on the wind machine, the lights, the pulsing music — all the things he used back when he shot high-fashion models — and started yelling out the cliches — cliches that he thought were truth in this case. "You're beautiful, give me that smile, look at yourself, gorgeous!" And he watched the girl blossom in front of his lens.

"At one point, you could see her look in the mirror and think, 'I am beautiful,' " says Guidotti, who blames his Italian background for his tendency — even in hindsight — to get all teary-eyed and thick-voiced about the experience. "She had the ammunition she needed. She had the armor she needed. And it was the armor of self-esteem."

The Life spread led Guidotti to reconnect with one of his old New York roommates, McLean, who was now attending Cornell's Weill Medical College in Manhattan. She looked at her old friend's work and convinced him that one thing was missing: the voices of his subjects. So together they launched the second phase of the project, which included traveling throughout the United States, the South Pacific, New Zealand and other areas so that Guidotti could take photographs and McLean could tape direct-to-the-camera interviews of the subjects talking about their lives. The project has been financed by a variety of sources, including the Genetic Alliance, Weill Medical College, NOAH and Guidotti, who jokes that he pretends to be in a coma when the bill collectors call. Guidotti and McLean hope to turn their project into a traveling exhibit that will visit all the countries where the photographs have been taken, and also can be used as an educational tool in medical schools.

"It's an opportunity for people to define themselves and not to be defined by their condition," McLean says.

And for Randy Stuart, an 18-year-old from South Hampton, N.J., who has albinism and who is featured in one of the photographs, it's an opportunity to be cool. Stuart, who is about to graduate from high school, rented a tuxedo for his trip to Washington for the celebration. After years of getting teased — and getting into fights — because of his appearance, he's found a community of friends in his high school who, he says, "accept me for who I am." Not that those friends don't give him grief. It's just on another subject.

"All my friends are like, 'Yeah, don't let it go to your head,' " Stuart says. "Or they tell me to remember them when I get famous."

Call the Genetic Alliance at 202-966-5557 for free reservations to view the exhibit.