He arrived at the coal mines, textile mills and industrial factories dressed in a three-piece suit. He wooed those in charge, asking to be let in. He was just a humble Bible salesman, he claimed, who wanted to spread the good word to the laborers inside.

What Lewis Hine actually wanted was to take photos of those laborers — and show the world what it looked like when children were put to work.

Around the turn of the 20th century, at least 18 percent of children between the ages of 10 and 15 were employed. Hine's searing images of those children remade the public perception of child labor and inspired the laws to ban it.

Now, conservative lawmakers in a handful of states are seeking to relax child labor protections, amid what activists say is a surge in child labor, mostly by undocumented immigrant children, at meatpacking plants, auto factories and other dangerous job sites.

The Library of Congress maintains a collection of more than 5,000 of Hine's photographs, including the thousands he took for the National Child Labor Committee, known as the NCLC.

"It was Lewis Hine who made sure that millions of children are not working today," Jeffrey Newman, a former president of the New York-based nonprofit, said in 2018.

A century ago, NCLC's mission wasn't about showing the public that children were being used for financial gain — that was already a well-known fact. At the time, many believed the practice had substantial benefits. Youths could learn the value of hard work. Businesses could increase their productivity and decrease the hourly pay. Parents could depend on their children to support the family, meaning the adults could work less or not at all.

As one mother remarked to the NCLC in 1907: "I am really tired of seeing so many big children 10 years old playing in the streets."

Hine's photos showed the price: unsafe working conditions, dangerous machinery and business owners who refused to educate the children or limit their working hours.

Though there had been investigations that attempted to expose these circumstances in the past, "The industry simply dismissed those reports as — the term they would use today is — 'fake news,' " said Hugh Hindman, a historian of child labor. "When Hine comes along and supplements the investigations with pictures, it creates a set of facts that can't be denied anymore."

Taken with a heavy Graflex camera, Hine's photos were paired with captions and stories from his interviews with the children, who would tell him their ages, backgrounds and working conditions.

If they didn't know their own age, Hine would estimate it by measuring them. As a Bible salesman or in one of his other disguises — he posed as a postcard salesman and a machinery photographer — Hine could hardly be seen whipping out a measuring tape. That's why he wore a three-piece suit. He could measure the children against the buttons on his vest.

Hine's affinity for telling the stories of the downtrodden probably came from his own start in life. At 18, he began working at a Wisconsin furniture factory after the death of his father. It was up to Hine to keep his family financially afloat.

According to the International Photography Hall of Fame, Hine worked 13 hours a day, six days a week, until he could move on to a seemingly better job — as a janitor in a bank. He began taking college courses on the side to become a teacher.

One of Hine's mentors encouraged him to move to Manhattan and begin his teaching career in one of the city's private schools. It was there that Hine picked up photography. In the hope of teaching his students to respect the new wave of immigrants coming into the city, he began visiting Ellis Island and photographing the new arrivals.

Hine's work attracted the attention of the NCLC, which had been founded in 1904 with the mission of ending child labor. The organization had a particular project in mind for Hine.

Today, the use of photography as a tool to expose wrongdoing is hardly revolutionary. But in Hine's time, when newspapers were just beginning to incorporate photos into their daily product, it was nearly unheard of. Hine is credited with inventing the term "photo story" and for popularizing a style of portraiture in which the subject looks straight into the camera.

His images demand that the viewers look into the children's eyes. In many, the children are looking right back.

The National Child Labor Committee published Hine's photos in its publicity material, trying to influence lawmakers and power players to address the injustice being done. Exhibits, newspapers and progressive media outlets picked up his outrage-inspiring work, ensuring it was seen across the country.

But there was no such thing as going viral in the early 1900s. The spread of Hine's photos and the reform that they inspired was extremely slow.

The Fair Labor Standards Act, the federal law that would prohibit most employment of minors, wasn't passed until 1938. Hine died two years later — long before his work would be recognized for the impact it had. Now Hine's photos appear in museum exhibitions, are sold at auctions for upward of $5,000 apiece and are credited with influencing generations of documentary photographers.

In the NCLC's final years, the threat of child labor didn't have the fundraising power it once did. In a rare instance in which an organization dedicated to a social ill had worked itself out of a job, the NCLC board decided in 2017 to "declare victory and just move out," Newman said the next year.

But, he predicted, "there may well come a time when the NCLC may need to be reinvented and started up again."

You can explore the archive of Hine's photos on the Library of Congress digital archive.

A version of this story first published on Sept. 3, 2018, with the headline "The searing photos that helped end child labor in America."